autumn collections : from iterable to lambdas, streams and collectors
DESCRIPTION
These are the slides of my talk at Devoxx 2013. This presentation introduces the lambda expressions, how we can write them, and what they bring to us developers. The second part presents in details the new patterns introduced by the Stream and Collector APIs. These new APIs will change the way we process large collections content, enabling concurrent and parallel access, with a very simple programming model, and amazing patterns. Complex examples will show how powerful those APIs are, and how they can help us write efficient and clean code.TRANSCRIPT
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Autumn Collections
From Iterable to Lambdas,
Streams and Collectors
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Why were they introduced ?
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Why were they introduced ?
Why are they so important ?
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Why were they introduced ?
Why are they so important ?
How will they change the way we build
applications ?
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Questions ?
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Let’s introduce the lambdas
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Let’s introduce the lambdas on a simple example
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A very simple example
public class Person { private String name ; private int age ; // constructors // getters / setters }
List<Person> list = new ArrayList<>() ;
A plain old bean…
… and a good old list
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int sum = 0 ; // I need a default value in case // the list is empty int average = 0 ; for (Person person : list) { sum += person.getAge() ; } if (!list.isEmpty()) { average = sum / list.size() ; }
Average of the ages
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int sum = 0 ; int n = 0 ; int average = 0 ; for (Person person : list) { if (person.getAge() > 20) { n++ ; sum += person.getAge() ; } } if (n > 0) { average = sum / n ; }
Trickier : average of the ages of people older than 20
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Trickier : average of the ages of people older than 20
« imperative programming »
int sum = 0 ; int n = 0 ; int average = 0 ; for (Person person : list) { if (person.getAge() > 20) { n++ ; sum += person.getAge() ; } } if (n > 0) { average = sum / n ; }
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select avg(age) from Person where age > 20
… it does not have to be like that !
This is a description of the result
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select avg(age) from Person where age > 20
… it does not have to be like that !
This is a description of the result
In that case, the DB server is free to compute the
result the way it sees fit
© SQL language, 1974
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Person age age > 20 sum
1st step : mapping
map
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Person age age > 20 sum
1st step : mapping
Mapping :
- takes a list of a given type
- gives another list of a different type
- same number of elements
map
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Person age age > 20 sum
2nd step : filtering
filter map
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Person age age > 20 sum
2nd step : filtering
Filtering :
- takes a list of a given type
- gives another list of a the same type
- less elements
filter map
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Person age age > 20 sum
3rd step : reduction
reduce filter map
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Person age age > 20 sum
3rd step : reduction
Reduction : agregation of all the elements
in a single one
Ex : average, sum, min, max, etc…
reduce filter map
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How can I model that ?
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The JDK 7 way
Create an interface to model the mapper…
public interface Mapper<T, V> { public V map(T t) ; }
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public class PersonToAgeMapper implements Mapper<Person, Integer> { public Integer map(Person p) { return p.getAge() ; } }
The JDK 7 way
… and create an implementation …
public interface Mapper<T, V> { public V map(T t) ; }
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Mapper<Person, Integer> mapper = new Mapper<Person, Integer>() { public Integer map(Person p) { return p.getAge() ; } }
The JDK 7 way
… that could be anonymous
public interface Mapper<T, V> { public V map(T t) ; }
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The JDK 7 way
We can do the same for the filtering
public interface Predicate<T> { public boolean filter(T t) ; }
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public class AgePredicate implements Predicate<Integer> { public boolean filter(Integer i) { return i > 20 ; } }
The JDK 7 way
We can do the same for the filtering
public interface Predicate<T> { public boolean filter(T t) ; }
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AgePredicate predicate = new Predicate<Integer>() { public boolean filter(Integer i) { return i > 20 ; } }
The JDK 7 way
We can do the same for the filtering
public interface Predicate<T> { public boolean filter(T t) ; }
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The JDK 7 way
And for the reduction
public interface Reducer<T> { public T reduce(T t1, T t2) ; }
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public class Sum implements Reducer<Integer> { public Integer reduce(Integer i1, Integer i2) { return i1 + i2 ; } }
The JDK 7 way
And for the reduction
public interface Reducer<T> { public T reduce(T t1, T t2) ; }
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Reducer<Integer> reduction = new Reducer<Integer>() { public Integer reduce(Integer i1, Integer i2) { return i1 + i2 ; } }
The JDK 7 way
And for the reduction
public interface Reducer<T> { public T reduce(T t1, T t2) ; }
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The JDK 7 way
So the whole map / filter / reduce looks like this
1) Create 3 interfaces public interface Mapper<T, V> { public V map(T t) ; }
public interface Predicate<T> { public boolean filter(T t) ; }
public interface Reducer<T> { public T reduce(T t1, T t2) ; }
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The JDK 7 way
So the whole map / filter / reduce looks like this
1) Create 3 interfaces
2) Apply the pattern
List<Person> persons = ... ; int sum = persons.map( new Mapper<Person, Integer>() { public Integer map(Person p) { return p.getAge() ; } }) .filter( new Filter<Integer>() { public boolean filter(Integer age) { return age > 20 ; } }) .reduce(0, new Reducer<Integer>() { public Integer recude(Integer i1, Integer i2) { return i1 + i2 ; } } }) ;
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The JDK 7 way
So the whole map / filter / reduce looks like this
1) Create 3 interfaces
2) Apply the pattern
List<Person> persons = ... ; int sum = persons.map( new Mapper<Person, Integer>() { public Integer map(Person p) { return p.getAge() ; } }) .filter( new Filter<Integer>() { public boolean filter(Integer age) { return age > 20 ; } }) .reduce(0, new Reducer<Integer>() { public Integer recude(Integer i1, Integer i2) { return i1 + i2 ; } } }) ;
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The JDK 8 way
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mapper = new Mapper<Person, Integer>() { public Integer map(Person person) { return person.getAge() ; } }
The JDK 8 way
Let’s rewrite our mapper
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mapper = new Mapper<Person, Integer>() { public Integer map(Person person) { // 1 method return person.getAge() ; } }
The JDK 8 way
Let’s rewrite our mapper
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mapper = new Mapper<Person, Integer>() { public Integer map(Person person) { // 1 method return p.getAge() ; } }
The JDK 8 way
mapper = Person person ;
we take a
person p
Let’s rewrite our mapper
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mapper = new Mapper<Person, Integer>() { public Integer map(Person person) { // 1 method return person.getAge() ; } }
The JDK 8 way
mapper = Person person -> ;
and then …
Let’s rewrite our mapper
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mapper = new Mapper<Person, Integer>() { public Integer map(Person person) { // 1 method return person.getAge() ; } }
The JDK 8 way
mapper = Person person -> person.getAge() ;
… return the age of p
Let’s rewrite our mapper
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Let’s rewrite our mapper
mapper = new Mapper<Person, Integer>() { public Integer map(Person person) { // 1 method return person.getAge() ; } }
The JDK 8 way
mapper = Person person -> person.getAge() ;
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Let’s rewrite our mapper
The compiler can recognize this as an implementation of
Mapper
mapper = new Mapper<Person, Integer>() { public Integer map(Person person) { // 1 method return person.getAge() ; } }
The JDK 8 way
mapper = Person person -> person.getAge() ;
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What if …
… there is more than one statement ?
The return has to be explicit
mapper = Person person -> { System.out.println("Mapping " + person) ; return person.getAge() ; }
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What if …
…I have no return value ?
consumer = Person person -> p.setAge(p.getAge() + 1) ;
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What if …
…I have more than one argument ?
Or :
reducer = (int i1, int i2) -> { return i1 + i2 ; }
reducer = (int i1, int i2) -> i1 + i2 ;
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The JDK 8 way
How can the compiler recognize the implementation of
map() ?
mapper = Person person -> person.getAge() ;
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The JDK 8 way
How can the compiler recognize the implementation of
map() ?
1) There’s only one method in Mapper
mapper = Person person -> person.getAge() ;
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The JDK 8 way
How can the compiler recognize the implementation of
map() ?
1) There’s only one method in Mapper
2) Both the parameters and the return types are
compatible
mapper = Person person -> person.getAge() ;
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The JDK 8 way
How can the compiler recognize the implementation of
map() ?
1) There’s only one method in Mapper
2) Both the parameters and the return types are
compatible
3) Thrown exceptions (if any) are compatible
mapper = Person person -> person.getAge() ;
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More lambdas
Writing more lambdas becomes natural :
mapper = Person person -> person.getAge() ; // mapper filter = int age -> age > 20 ; // filter reducer = (int i1, int i2) -> i1 + i2 ; // reducer
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More lambdas
And most of the time, the compiler understands this :
The « parameter types » can be omitted
mapper = person -> person.getAge() ; // mapper filter = age -> age > 20 ; // filter reducer = (i1, i2) -> i1 + i2 ; // reducer
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Just a remark on the reduction
How does it really work ?
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Reduction
2 examples :
Caveat :
the result is always reproductible in serial
it’s not in parallel
Reducer r1 = (i1, i2) -> i1 + i2 ; // Ok Reducer r2 = (i1, i2) -> i1*i1 + i2*i2 ; // Oooops
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So far
A lambda expression is an alternative
to write instances of anonymous inner classes
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Other syntaxes
Very often one writes
This syntax is also possible :
mapper = person -> person.getAge() ;
mapper = Person::getAge ; // non static method
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Other syntaxes
Other example :
Or :
sum = (i1, i2) -> i1 + i2 ; sum = Integer::sum ; // static method, new !
max = (i1, i2) -> i1 > i2 ? i1 : i2 ; max = Integer::max ; // static method, new !
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Other syntaxes
Other example :
toLower = String::toLowerCase ;
// !!!! NO NO NO !!!! toLowerFR = String::toLowerCase(Locale.FRANCE) ;
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More on the lambdas
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Questions :
What model for a lambda ?
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Questions :
What model for a lambda ?
Can I put a lambda in a variable ?
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Questions :
What model for a lambda ?
Can I put a lambda in a variable ?
Is a lambda an object ?
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Modelization
A lambda is an instance of a « functional interface »
@FunctionalInterface public interface Consumer<T> { public void accept(T t) ; }
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Modelization
A lambda is an instance of a « functional interface »
- only one abstract method (methods from Object dont
count)
- can be annotated by @FunctionalInterface (optional)
@FunctionalInterface public interface Consumer<T> { public void accept(T t) ; }
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Modelization
A lambda is an instance of « functional interface »
@FunctionalInterface public interface Consumer<T> { public void accept(T t) ; public default Consumer<T> chain(Consumer<? super T> other) { Objects.requireNonNull(other); return (T t) -> { accept(t); other.accept(t); }; } }
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Putting a lambda in a variable
Example of a consumer
So :
Consumer<String> c = new Consumer<String>() { @Override public void accept(String s) { System.out.println(s) ; } } ;
Consumer<String> c = s -> System.out.println(s) ;
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Putting a lambda in a variable
Example of a consumer
So :
Consumer<String> c = new Consumer<String>() { @Override public void accept(String s) { System.out.println(s) ; } } ;
Consumer<String> c = s -> System.out.println(s) ;
Question : what is s ?
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Putting a lambda in a variable
Example of a consumer
So :
Consumer<String> c = new Consumer<String>() { @Override public void accept(String s) { System.out.println(s) ; } } ;
Consumer<String> c = s -> System.out.println(s) ;
Answer : the compiler
infers that it is a String
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Putting a lambda in a variable
Side question : can one write this ?
int i = ... ; Consumer<String> c = new Consumer<String>() { @Override public void accept(String s) { System.out.println("i = " + i) ; } } ;
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Putting a lambda in a variable
Side question : can one write this ?
JDK 7 : i should be final
int i = ... ; Consumer<String> c = new Consumer<String>() { @Override public void accept(String s) { System.out.println("i = " + i) ; } } ;
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Putting a lambda in a variable
Side question : can one write this ?
JDK 8 : Yes ! (no final needed)
int i = ... ; Consumer<String> c = new Consumer<String>() { @Override public void accept(String s) { System.out.println("i = " + i) ; } } ;
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Putting a lambda in a variable
But this code …
does not compile
int i = ... ; Consumer<String> c = new Consumer<String>() { @Override public void accept(String s) { System.out.println("i = " + i) ; } } ; i = i + 1 ;
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Putting a lambda in a variable
Because i is an effectively final variable
final int i = ... ; Consumer<String> c = new Consumer<String>() { @Override public void accept(String s) { System.out.println("i = " + i) ; } } ; i = i + 1 ; // Cant modify
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And what about this ?
JDK 7 : this is the anonymous instance
Calling accept() will print « I’m in c »
Consumer<String> c = new Consumer<String>() { @Override public void accept(String s) { System.out.println("this = " + this) ; } public String toString() { return "I’m in c" ; } } ;
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And what about this ?
JDK 8 : no change, calling accept() still prints « I’m in c »
But …
Consumer<String> c = new Consumer<String>() { @Override public void accept(String s) { System.out.println("this = " + this) ; } public String toString() { return "I’m in c" ; } } ;
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And what about this ?
Consumer can also be instanced with a lambda
Here this is the enclosing instance
Consumer<String> c = s -> System.out.println(this) ;
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Questions :
What model for a lambda ?
answer : with a functional interface
Can I put a lambda in a variable ?
answer : yes
Is a lambda an object ?
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Is a lambda an objet ?
Let’s play the game of 7 errors (with only 1 error)
Consumer<String> c = new Consumer<String>() { @Override public void accept(String s) { System.out.println(s) ; } } ;
Consumer<String> c = s -> System.out.println(s) ;
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Is a lambda an objet ?
Let’s play the game of 7 errors (with only 1 error)
Consumer<String> c = new Consumer<String>() { @Override public void accept(String s) { System.out.println(s) ; } } ;
Consumer<String> c = s -> System.out.println(s) ;
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Questions :
What model for a lambda ?
answer : with a functionnal interface
Can I put a lambda in a variable ?
answer : yes
Is a lambda an object ?
answer : no
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Plenty of lambdas :
Java.util.functions
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Java.util.functions
This new package holds the functionnal interfaces
There are 43 of them
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Java.util.functions
Supplier : alone in its kind
Consumer / BiConsumer
Function / BiFunction (UnaryOperator / BinaryOperator)
Predicate / BiPredicate
Plus the primitive types versions
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Supplier
A supplier supplies an object
public interface Supplier<T> { T get() ; }
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Consumer
A consumer just … accepts an object
public interface Consumer<T> { void accept(T t) ; }
Consumer<String> c1 = s -> System.out.println(s) ; Consumer<String> c2 = ... ; Consumer<String> c3 = c1.andThen(c2) ; persons.stream().forEach(c3) ;
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BiConsumer
Takes 2 arguments instead of one
Can be chained
Primitive types :
- ObjIntConsumer
- ObjLongConsumer
- ObjDoubleConsumer
ObjIntConsumer takes an object and an int as arguments
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Function
A function takes an object and returns another one
Can be chained and / or composed
public interface Function<T, R> { R apply(T t) ; }
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BiFunction
A function that takes 2 arguments
Can be chained
Primitive type versions :
- ToIntBiFunction
- ToLongBiFunction
- ToDoubleBiFunction
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UnaryOperator
A UnaryOperator is a Function that operates on one type
Other UnaryOperators :
- IntUnaryOperator
- LongUnaryOperator
- DoubleUnaryOperator
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BinaryOperator
A BinaryOperator is a BiFunction that operates on one type
Other BinaryOperators :
- IntBinaryOperator
- LongBinaryOperator
- DoubleBinaryOperator
IntBinaryOperator takes 2 int, and returns an int
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Predicate
A Predicate takes an object and returns a boolean
Can be negated, composed with and / or
public interface Predicate<T> { boolean test(T t) ; }
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BiPredicate
A BiPredicate takes two object and returns a boolean
public interface BiPredicate<T, U> { boolean test(T t, U u) ; }
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Predicate & BiPredicate
Primitive types :
- IntPredicate
- LongPredicate
- DoublePredicate
Not for BiPredicate
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Questions ?
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Back to the
map / filter / reduce
pattern
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The JDK 7 way
How to apply map / filter / reduce
to List<Person> ?
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The JDK 7 way
How to apply map / filter / reduce
to List<Person> ?
The legal way is to iterate over the elements
and apply the pattern
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The JDK 7 way
How to apply map / filter / reduce
to List<Person> ?
The legal way is to iterate over the elements
and apply the pattern
One can create a helper method
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Applying map to a List
With a helper method, JDK 7
List<Person> persons = new ArrayList<>() ; List<Integer> ages = // ages is a new list Lists.map( persons, new Mapper<Person, Integer>() { public Integer map(Person p) { return p.getAge() ; } } ) ;
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Applying map to a List
With a helper method, JDK 8 & lambdas
List<Person> persons = new ArrayList<>() ; List<Integer> ages = Lists.map( persons, person -> person.getAge() ) ;
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Applying map to a List
With a helper method, JDK 8 & lambdas
List<Person> persons = new ArrayList<>() ; List<Integer> ages = Lists.map( persons, Person::getAge ) ;
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Applying map to a List
Caveats : beware the generics : what about managers ?
We want that to compile !
List<Manager> managers = new ArrayList<>() ; List<Integer> ages = Lists.map( managers, person -> person.getAge() ) ;
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Applying map to a List
Caveats : beware the generics
// in Lists public static <T, V> List<V> map( List<? extends T> list, Mapper<T, V> mapper) { List<V> result = new ArrayList<>() ; for (T t : list) { V v = mapper.map(t) ; result.add(v) ; } return result ; }
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Putting things together
The map / filter / reduce pattern would look like this
// applying the map / filter / reduce pattern List<Person> persons = ... ; List<Integer> ages = Lists.map( persons, p -> p.getAge()) ; List<Integer> agesGT20 = Lists.filter(ages, a -> a > 20) ; int sum = Lists.reduce(agesGT20, (i1, i2) -> i1 + i2) ;
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Putting things together
The map / filter / reduce pattern would look like this
The idea is to push lambdas to the API, and let it apply
them on its content
// applying the map / filter / reduce pattern List<Person> persons = ... ; List<Integer> ages = Lists.map( persons, p -> p.getAge()) ; List<Integer> agesGT20 = Lists.filter(ages, a -> a > 20) ; int sum = Lists.reduce(agesGT20, (i1, i2) -> i1 + i2) ;
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Putting things together
The map / filter / reduce pattern would look like this
Pro : the API can be optimized
without touching the code : great !
// applying the map / filter / reduce pattern List<Person> persons = ... ; List<Integer> ages = Lists.map( persons, p -> p.getAge()) ; List<Integer> agesGT20 = Lists.filter(ages, a -> a > 20) ; int sum = Lists.reduce(agesGT20, (i1, i2) -> i1 + i2) ;
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Putting things together
The map / filter / reduce pattern would look like this
Pro : the API can be optimized
without touching the code
Cons …
// applying the map / filter / reduce pattern List<Person> persons = ... ; List<Integer> ages = Lists.map( persons, p -> p.getAge()) ; List<Integer> agesGT20 = Lists.filter(ages, a -> a > 20) ; int sum = Lists.reduce(agesGT20, (i1, i2) -> i1 + i2) ;
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Putting things together
1) Suppose persons is a really BIG list
// applying the map / filter / reduce pattern List<Person> persons = ... ; List<Integer> ages = Lists.map( persons, p -> p.getAge()) ; List<Integer> agesGT20 = Lists.filter(ages, a -> a > 20) ; int sum = Lists.reduce(agesGT20, (i1, i2) -> i1 + i2) ;
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Putting things together
1) Suppose persons is a really BIG list
2 duplications : ages & agesGT20
// applying the map / filter / reduce pattern List<Person> persons = ... ; List<Integer> ages = Lists.map( persons, p -> p.getAge()) ; List<Integer> agesGT20 = Lists.filter(ages, a -> a > 20) ; int sum = Lists.reduce(agesGT20, (i1, i2) -> i1 + i2) ;
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Putting things together
1) Suppose persons is a really BIG list
2 duplications : ages & agesGT20
What do we do with them ? Send them to the GC
// applying the map / filter / reduce pattern List<Person> persons = ... ; List<Integer> ages = Lists.map( persons, p -> p.getAge()) ; List<Integer> agesGT20 = Lists.filter(ages, a -> a > 20) ; int sum = Lists.reduce(agesGT20, (i1, i2) -> i1 + i2) ;
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Putting things together
2) Suppose the algorithms to compute the map / filter /
reduce are optimal
// applying the map / filter / reduce pattern List<Person> persons = ... ; List<Integer> ages = Lists.map( persons, p -> p.getAge()) ; List<Integer> agesGT20 = Lists.filter(ages, a -> a > 20) ; int sum = Lists.reduce(agesGT20, (i1, i2) -> i1 + i2) ;
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Putting things together
2) Suppose the algorithms to compute the map / filter /
reduce are optimal
… but I need to go even faster !
// applying the map / filter / reduce pattern List<Person> persons = ... ; List<Integer> ages = Lists.map( persons, p -> p.getAge()) ; List<Integer> agesGT20 = Lists.filter(ages, a -> a > 20) ; int sum = Lists.reduce(agesGT20, (i1, i2) -> i1 + i2) ;
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Putting things together
2) Suppose the algorithms to compute the map / filter /
reduce are optimal
Only solution : go parallel !
// applying the map / filter / reduce pattern List<Person> persons = ... ; List<Integer> ages = Lists.map( persons, p -> p.getAge()) ; List<Integer> agesGT20 = Lists.filter(ages, a -> a > 20) ; int sum = Lists.reduce(agesGT20, (i1, i2) -> i1 + i2) ;
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Putting things together
2) Suppose we want to go parallel
ages & agesGT20 would better be concurrent aware !
Because they will be accessed concurrently
// applying the map / filter / reduce pattern List<Person> persons = ... ; List<Integer> ages = Lists.map( persons, p -> p.getAge()) ; List<Integer> agesGT20 = Lists.filter(ages, a -> a > 20) ; int sum = Lists.reduce(agesGT20, (i1, i2) -> i1 + i2) ;
#DV13LBDS @JosePaumard
Putting things together
3) Suppose we have a allMatch() reducer
allMatch is true if all the names are shorter than 20 chars
let’s see a possible implementation of allMatch()
// applying the map / filter / reduce pattern List<Person> persons = ... ; List<String> names = Lists.map(persons, p -> p.getName()) ; boolean allMatch = Lists.allMatch(names, n -> n.length() < 20) ;
#DV13LBDS @JosePaumard
How could one write allMatch ?
Here is a basic implementation of allMatch
public static <T> boolean allMatch( List<? extends T> list, Filter<T> filter) { for (T t : list) { if (!filter.filter(t)) { return false ; } } return true ; }
#DV13LBDS @JosePaumard
How could one write allMatch ?
Here is a basic implementation of allMatch
public static <T> boolean allMatch( List<? extends T> list, Filter<T> filter) { for (T t : list) { if (!filter.filter(t)) { return false ; } } return true ; }
No need to iterate
over the whole list
#DV13LBDS @JosePaumard
How could one write allMatch ?
Here is a basic implementation of allMatch
public static <T> boolean allMatch( List<? extends T> list, Filter<T> filter) { for (T t : list) { if (!filter.filter(t)) { return false ; } } return true ; }
But…
#DV13LBDS @JosePaumard
Putting things together
When we apply the allMatch() reducer…
… the list names has already been evaluated !
// applying the map / filter / reduce pattern List<Person> persons = ... ; List<String> names = Lists.map(persons, p -> p.getName()) ; boolean allMatch = Lists.allMatch(names, n.length() < 20) ;
#DV13LBDS @JosePaumard
Putting things together
When we apply the allMatch() reducer…
… the list names has already been evaluated !
We lost a big opportunity for optimization !
// applying the map / filter / reduce pattern List<Person> persons = ... ; List<String> names = Lists.map(persons, p -> p.getName()) ; boolean allMatch = Lists.allMatch(names, n.length() < 20) ;
#DV13LBDS @JosePaumard
Putting things together
When we apply the allMatch() reducer…
… we should have wait to apply the filter
A good way to do that : apply the filter step lazily !
// applying the map / filter / reduce pattern List<Person> persons = ... ; List<String> names = Lists.map(persons, p -> p.getName()) ; boolean allMatch = Lists.allMatch(names, n.length() < 20) ;
#DV13LBDS @JosePaumard
Conclusion
Pro : 1
Cons : 3 (at least)
// applying the map / filter / reduce pattern List<Person> persons = ... ; List<Integer> ages = Lists.map( persons, p -> p.getAge()) ; List<Integer> agesGT20 = Lists.filter(ages, a -> a > 20) ; int sum = Lists.reduce(agesGT20, (i1, i2) -> i1 + i2) ;
#DV13LBDS @JosePaumard
// applying the map / filter / reduce pattern List<Person> persons = ... ; List<Integer> ages = Lists.map( persons, p -> p.getAge()) ; List<Integer> agesGT20 = Lists.filter(ages, a -> a > 20) ; int sum = Lists.reduce(agesGT20, (i1, i2) -> i1 + i2) ;
Conclusion
Pro : 1
Cons : 3 (at least)
#DV13LBDS @JosePaumard
Conclusion
And the same goes for this one
// applying the map / filter / reduce pattern List<Person> persons = ... ; List<Integer> ages = persons.map(p -> p.getAge()) ; List<Integer> agesGT20 = ages.filter(a -> a > 20) ; int sum = agesGT20.reduce(ages, (i1, i2) -> i1 + i2) ;
#DV13LBDS @JosePaumard
What about « in place » operations ?
Not possible for the map step, because the type of the list
changes
Could be evaluated for the filter, with concurrency issues
Doesnt make sense for the reduction
#DV13LBDS @JosePaumard
Conclusion (again)
We need a new concept to handle BIG lists efficiently
#DV13LBDS @JosePaumard
Conclusion (again)
We need a new concept to handle BIG lists efficiently
The Collection framework is not the right one…
#DV13LBDS @JosePaumard
Conclusion (again)
We need a new concept to handle BIG lists efficiently
The Collection framework is not the right one
We need something else !
#DV13LBDS @JosePaumard
So what pattern
to choose ?
#DV13LBDS @JosePaumard
Introduction
Putting map / filter / reduce methods on Collection
would have led to :
And even if it doesnt lead to viable patterns,
it is still a pleasant way of writing things
// map / filter / reduce pattern on collections int sum = persons .map(p -> p.getAge()) .filter(a -> a > 20) .reduce(0, (a1, a2) -> a1 + a2) ;
#DV13LBDS @JosePaumard
Introduction
Let’s keep the same kind of pattern and add a stream()
// map / filter / reduce pattern on collections int sum = persons.stream() .map(p -> p.getAge()) .filter(a -> a > 20) .reduce(0, (a1, a2) -> a1 + a2) ;
#DV13LBDS @JosePaumard
Introduction
Collection.stream() returns a Stream : new interface
New interface = our hands are free !
// map / filter / reduce pattern on collections int sum = persons.stream() .map(p -> p.getAge()) .filter(a -> a > 20) .reduce(0, (a1, a2) -> a1 + a2) ;
#DV13LBDS @JosePaumard
New Collection interface
So we need a new method on Collection
public interface Collection<E> { // our good old methods Stream<E> stream() ; }
#DV13LBDS @JosePaumard
New Collection interface
Problem : ArrayList doesnt compile anymore…
Something has to be done !
public interface Collection<E> { // our good old methods Stream<E> stream() ; }
#DV13LBDS @JosePaumard
Interfaces
Problem : ArrayList doesnt compile anymore…
Something has to be done !
Something that does not need to change or recompile all
the existing implementations of Collection
#DV13LBDS @JosePaumard
Questions ?
#DV13LBDS
#DV13LBDS @JosePaumard
New interfaces
#DV13LBDS @JosePaumard
Java 8 interfaces
Problem : ArrayList doesnt compile anymore…
Something has to be done !
Something that does not need to change or recompile all
the existing implementations of Collection
Problem : how to add methods to an interface, without
changing any of the implementations ?
#DV13LBDS @JosePaumard
Java 8 interfaces
Problem : how to add methods to an interface, without
changing any of the implementations ?
Solution : change the way interfaces work in Java
#DV13LBDS @JosePaumard
Java 8 interfaces
ArrayList needs to know the implementation of stream()…
public interface Collection<E> { // our good old methods Stream<E> stream() ; }
#DV13LBDS @JosePaumard
Java 8 interfaces
Let’s put it in the interface !
public interface Collection<E> { // our good old methods default Stream<E> stream() { return ... ; } }
#DV13LBDS @JosePaumard
Java 8 interfaces
Let’s introduce default methods in Java
Default methods are about interface evolution
Allows the extension of old interfaces
public interface Collection<E> { // our good old methods default Stream<E> stream() { return ... ; } }
#DV13LBDS @JosePaumard
Default methods
Does it bring multiple inheritance to Java ?
#DV13LBDS @JosePaumard
Default methods
Does it bring multiple inheritance to Java ?
Yes ! But we already have it
#DV13LBDS @JosePaumard
Default methods
Does it bring multiple inheritance to Java ?
Yes ! But we already have it
public class String implements Serializable, Comparable<String>, CharSequence { // ... }
#DV13LBDS @JosePaumard
Default methods
What we have so far is multiple inheritance of type
#DV13LBDS @JosePaumard
Default methods
What we have so far is multiple inheritance of type
What we get in Java 8 is multiple inheritance of behavior
#DV13LBDS @JosePaumard
Default methods
What we have so far is multiple inheritance of type
What we get in Java 8 is multiple inheritance of behavior
What we dont have is multiple inheritance of state
#DV13LBDS @JosePaumard
Default methods
What we have so far is multiple inheritance of type
What we get in Java 8 is multiple inheritance of behavior
What we dont have is multiple inheritance of state
… and this is where the trouble is !
#DV13LBDS @JosePaumard
Default methods
Will we have conflicts ?
#DV13LBDS @JosePaumard
Default methods
Will we have conflicts ?
Oooh yes…
#DV13LBDS @JosePaumard
Default methods
Will we have conflicts ?
Oooh yes…
So we need rules to handle them
#DV13LBDS @JosePaumard
Default methods
public class C implements A, B { // ... }
#DV13LBDS @JosePaumard
Default methods
public class C implements A, B { // ... }
public interface A { default String a() { ... } }
public interface B { default String a() { ... } }
#DV13LBDS @JosePaumard
Default methods
Example #0
Class wins !
No default if an implementation is provided
public class C implements A, B { public String a() { ... } }
public interface A { default String a() { ... } }
public interface B { default String a() { ... } }
#DV13LBDS @JosePaumard
Default methods
Example #1
public class C implements A, B { // ... }
public interface A { default String a() { ... } }
public interface B { default String a() { ... } }
#DV13LBDS @JosePaumard
Default methods
Example #1
Compile time error :
class C inherits unrelated defaults for a() from types A and B
public class C implements A, B { // ... }
public interface A { default String a() { ... } }
public interface B { default String a() { ... } }
#DV13LBDS @JosePaumard
Default methods
Example #2
A is more specific than B : A.a() is chosen
public class C implements A, B { // ... }
public interface A extends B { default String a() { ... } }
public interface B { default String a() { ... } }
#DV13LBDS @JosePaumard
Default methods
Example #2
public class C implements A { // ... }
public interface A extends B { default String a() { ... } }
public interface B { default String a() { ... } }
public class D extends C implements B { // ... }
#DV13LBDS @JosePaumard
Default methods
Example #2
A is still more specific than B : A.a() is chosen
public class C implements A { // ... }
public interface A extends B { default String a() { ... } }
public interface B { default String a() { ... } }
public class D extends C implements B { // ... }
#DV13LBDS @JosePaumard
Default methods
Back to example #1
We can also make an explicit call
public interface A { default String a() { ... } }
public interface B { default String a() { ... } }
public class C implements A, B { public String a() { return B.super.a() ; } }
#DV13LBDS @JosePaumard
3 simple rules
To handle the conflict of multiple inheritance of behavior
#DV13LBDS @JosePaumard
3 simple rules
To handle the conflict of multiple inheritance of behavior
1) Class wins !
#DV13LBDS @JosePaumard
3 simple rules
To handle the conflict of multiple inheritance of behavior
1) Class wins !
2) More specific interfaces wins over less specific
#DV13LBDS @JosePaumard
3 simple rules
To handle the conflict of multiple inheritance of behavior
1) Class wins !
2) More specific interfaces wins over less specific
3) In fact there’s no rule #3
#DV13LBDS @JosePaumard
An example
Everybody who writes an implementation of Iterator
writes this :
public class MyIterator implements Iterator<E> { // some critical business code public void remove() { throw new UnsupportedOperationException("You shouldnt do that") ; } ; }
#DV13LBDS @JosePaumard
An example
Thanks to Java 8 :
No silly implementation of remove() anymore !
public interface Iterator<E> { default void remove() { throw new UnsupportedOperationException("remove") ; } ; }
#DV13LBDS @JosePaumard
Java 8 interfaces
So the concept of « interface » changed in Java 8
public class HelloWorld { // souvenir souvenir public static void main(String[] args) { System.out.println("Hello world!") ; } ; }
#DV13LBDS @JosePaumard
Java 8 interfaces
So the concept of « interface » changed in Java 8
I mean, really
public interface HelloWorld { // souvenir souvenir public static void main(String[] args) { System.out.println("Hello world!") ; } ; }
#DV13LBDS @JosePaumard
Java 8 interfaces
1) Default methods, multiple behavior inheritance
Rules to handle conflicts, solved at compile time
2) Static methods in interfaces
Will bring new patterns and new ways to write APIs
#DV13LBDS @JosePaumard
Where are we ?
1) We have a new syntax
2) We have a new pattern to implement
3) We have new interfaces to keep the backward
compatibily
#DV13LBDS @JosePaumard
Where are we going to ?
1) The Stream API
2) Reductions
3) Performances
4) Collectors
But for now…
#DV13LBDS @JosePaumard
#DV13LBDS @JosePaumard
The Stream API
#DV13LBDS @JosePaumard
What is a Stream
From a technical point of view : a typed interface
« a stream of String »
#DV13LBDS @JosePaumard
What is a Stream
From a technical point of view : a typed interface
Other classes for primitive types :
« a stream of String »
IntStream, LongStream, DoubleStream
#DV13LBDS @JosePaumard
It’s a new notion
It‘ looks like a collection, but…
- It is built on a « source », that can be a collection
- No data needs to be provided at build time
- No limit on what the source can produce
#DV13LBDS @JosePaumard
It’s a new notion
A stream can be built on :
- A collection or an array
- An iterator
- An I/O source
Some of those can be « infinite »
#DV13LBDS @JosePaumard
It’s a new notion
1) A stream doesnt hold any data
It’s just an object on which one can declare operations
Streams are about « specifying computations »
#DV13LBDS @JosePaumard
It’s a new notion
1) A stream doesnt hold any data
2) A stream cant modify its source
Consequence : it can use parallel processing
#DV13LBDS @JosePaumard
It’s a new notion
1) A stream doesnt hold any data
2) A stream cant modify its source
3) A source can be infinite
So we need a way to guarantee that computations
will be conducted in finite time
#DV13LBDS @JosePaumard
It’s a new notion
1) A stream doesnt hold any data
2) A stream cant modify its source
3) A source can be infinite
4) A stream works lazily
Then optimizations can be made among the different
operations
We need a way / convention to trigger the computations
#DV13LBDS @JosePaumard
Is a Stream the same as a collection ?
Answer is no
#DV13LBDS @JosePaumard
Is a Stream the same as a collection ?
Answer is no
A Collection provides means to access the elements one
by one
#DV13LBDS @JosePaumard
Is a Stream the same as a collection ?
Answer is no
A Collection provides means to access the elements one
by one
A Stream does not
#DV13LBDS @JosePaumard
Is a Stream the same as a collection ?
Answer is no
A Collection does not provide any mean to process its
elements with lambdas
#DV13LBDS @JosePaumard
Is a Stream the same as a collection ?
Answer is no
A Collection does not provide any mean to process its
elements with lambdas
A Stream does
#DV13LBDS @JosePaumard
How can we build a stream ?
Many ways…
1) From a collection : method Collection.stream
Collection<String> collection = ... ; Stream<String> stream = collection.stream() ;
#DV13LBDS @JosePaumard
How can we build a stream ?
Many ways…
1) From a collection : method Collection.stream
2) From an array : Arrays.stream(Object [])
Stream<String> stream2 = Arrays.stream(new String [] {"one", "two", "three"}) ;
#DV13LBDS @JosePaumard
How can we build a stream ?
Many ways…
1) From a collection : method Collection.stream
2) From an array : Arrays.stream(Object [])
3) From factory methods in Stream, IntStream, …
Stream<String> stream1 = Stream.of("one", "two", "three") ;
#DV13LBDS @JosePaumard
How can we build a stream ?
Some last patterns
Stream.empty() ; // returns an empty Stream Stream.of(T t) ; // a stream with a single element Stream.generate(Supplier<T> s) ; Stream.iterate(T seed, UnaryOperator<T> f) ;
#DV13LBDS @JosePaumard
How can we build a stream ?
Other ways scattered in the JDK
string.chars() ; // returns a IntStream lineNumberReader.lines() ; // returns a Stream<String> random.ints() ; // return a IntStream
#DV13LBDS @JosePaumard
How can we build a stream ?
Factory methods : a closer look
A stream builder is a consumer
- one sends it elements, that are accepted
- it builds back a stream on the build() call with the
accepted elemts
- it has a life cycle
Stream.build() ; // returns a Stream.Builder
#DV13LBDS @JosePaumard
How can we build a stream ?
And as a last resort : StreamSupport + Spliterator
« This class is mostly for library writers
presenting stream views of data structures;
most static stream methods intended for end
users are in the various Stream classes »
#DV13LBDS @JosePaumard
A 1st example
Back to our map / filter / reduce example
// map / filter / reduce pattern on collections int sum = persons.stream() .map(p -> p.getAge()) .filter(a -> a > 20) .reduce(0, (a1, a2) -> a1 + a2) ;
Build a stream
on a List
#DV13LBDS @JosePaumard
A 1st example
Back to our map / filter / reduce example
// map / filter / reduce pattern on collections int sum = persons.stream() .map(p -> p.getAge()) .filter(a -> a > 20) .reduce(0, (a1, a2) -> a1 + a2) ;
Declare
operations
#DV13LBDS @JosePaumard
A 1st example
Back to our map / filter / reduce example
// map / filter / reduce pattern on collections int sum = persons.stream() .map(p -> p.getAge()) .filter(a -> a > 20) .reduce(0, (a1, a2) -> a1 + a2) ;
Triggers the
computation
#DV13LBDS @JosePaumard
A 1st example
Back to our map / filter / reduce example
// map / filter / reduce pattern on collections int sum = persons.stream() .map(p -> p.getAge()) .filter(a -> a > 20) .reduce(0, (a1, a2) -> a1 + a2) ;
Default value, in case the
stream is empty
#DV13LBDS @JosePaumard
Two types of operations
One can declare operations on a Stream
Two types of operations :
1) Intermediate operations : declarations, processed lazily
ex : map, filter
#DV13LBDS @JosePaumard
Two types of operations
One can declare operations on a Stream
Two types of operations :
1) Intermediate operations : declarations, processed lazily
ex : map, filter
2) Terminal operations : trigger the computations
ex : reduce
#DV13LBDS @JosePaumard
State of a stream
The implementation of Stream has a state :
- SIZED : the number of elements is known
- ORDERED : the order matters (List)
- DISTINCT : all elements are unique (Set)
- SORTED : all elements have been sorted (SortedSet)
#DV13LBDS @JosePaumard
State of a stream
Some operations change that :
- filtering removes the SIZED
- mapping removes DISTINCT and SORTED
#DV13LBDS @JosePaumard
State of a stream
And it allows nice optimizations !
- a HashSet cant have doubles so…
#DV13LBDS @JosePaumard
State of a stream
And it allows nice optimizations !
- a HashSet cant have doubles so…
- distinct() is NOPed on a stream built on a HashSet
#DV13LBDS @JosePaumard
State of a stream
And it allows nice optimizations !
- a HashSet cant have doubles so…
- distinct() is NOPed on a stream built on a HashSet
- A TreeSet is sorted so…
#DV13LBDS @JosePaumard
State of a stream
And it allows nice optimizations !
- a HashSet cant have doubles so…
- distinct() is NOPed on a stream built on a HashSet
- A TreeSet is sorted so…
- sorted() is NOPed on a stream built on a TreeSet
#DV13LBDS @JosePaumard
A 2nd example
Sorting a list of String
List<String> strings = new ArrayList<>() ; // populate the list with strings // then somewhere else in the code : List<String> result = strings.stream() .sorted() .collect(Collector.toList()) ;
#DV13LBDS @JosePaumard
A 2nd example
Sorting a list of String
// then some smart guy changes the implementation SortedSet<String> strings = new TreeSet<>() ; // populate the list with strings // then somewhere else in the code : List<String> result = strings.stream() .sorted() .collect(Collector.toList()) ;
#DV13LBDS @JosePaumard
A 2nd example
Sorting a list of String
In this case, no sorting is required, and executed !
// then some smart guy changes the implementation SortedSet<String> strings = new TreeSet<>() ; // populate the list with strings // then somewhere else in the code : List<String> result = strings.stream() .sorted() .collect(Collector.toList()) ;
#DV13LBDS @JosePaumard
Stateless / statefull operations
Stateless / statefull operations
Some operations are stateless :
It means that they dont need more information than the
one held by their parameters
persons.stream().map(p -> p.getAge()) ;
#DV13LBDS @JosePaumard
Stateless / statefull operations
Stateless / statefull operations
Some operations are stateless :
Some need to retain information :
persons.stream().map(p -> p.getAge()) ;
stream.limit(10_000_000) ; // select the 10M first elements
#DV13LBDS @JosePaumard
Example
Let’s sort an array of String
Random rand = new Random() ; String [] strings = new String[10_000_000] ; for (int i = 0 ; i < strings.length ; i++) { strings[i] = Long.toHexString(rand.nextLong()) ; }
#DV13LBDS @JosePaumard
Example
Let’s sort an array of String
Soooo Java 7…
Random rand = new Random() ; String [] strings = new String[10_000_000] ; for (int i = 0 ; i < strings.length ; i++) { strings[i] = Long.toHexString(rand.nextLong()) ; }
#DV13LBDS @JosePaumard
Example
Let’s sort an array of String
Better ?
Random rand = new Random() ; Stream<String> stream = Stream.generate( () -> Long.toHexString(rand.nextLong()) ) ;
#DV13LBDS @JosePaumard
Example
Let’s sort an array of String
Better !
// Random rand = new Random() ; Stream<String> stream = Stream.generate( () -> Long.toHexString(ThreadLocalRandom.current().nextLong()) ) ;
#DV13LBDS @JosePaumard
Example
Let’s sort an array of String
// other way Stream<String> stream = ThreadLocalRandom .current() .longs() // returns a LongStream .mapToObj(l -> Long.toHexString(l)) ;
#DV13LBDS @JosePaumard
Example
Let’s sort an array of String
// other way Stream<String> stream = ThreadLocalRandom .current() .longs() .mapToObj(Long::toHexString) ;
#DV13LBDS @JosePaumard
Example
Let’s sort an array of String
// other way Stream<String> stream = ThreadLocalRandom .current() .longs() .mapToObj(Long::toHexString) .limit(10_000_000) .sorted() ;
T = 4 ms
#DV13LBDS @JosePaumard
Example
Let’s sort an array of String
// other way Stream<String> stream = ThreadLocalRandom .current() .longs() .mapToObj(Long::toHexString) .limit(10_000_000) .sorted() ; Object [] sorted = stream.toArray() ;
#DV13LBDS @JosePaumard
Example
Let’s sort an array of String
// other way Stream<String> stream = ThreadLocalRandom .current() .longs() .mapToObj(Long::toHexString) .limit(10_000_000) .sorted() ; Object [] sorted = stream.toArray() ;
T = 14 s
#DV13LBDS @JosePaumard
Example
Let’s sort an array of String
// other way Stream<String> stream = ThreadLocalRandom .current() .longs() .mapToObj(Long::toHexString) .limit(10_000_000) .sorted() ; Object [] sorted = stream.toArray() ;
T = 14 s
Intermediate calls !
#DV13LBDS @JosePaumard
More on stream operations
1) There are intermediate and terminal operations
2) A stream is processed on a terminal operation call
#DV13LBDS @JosePaumard
More on stream operations
1) There are intermediate and terminal operations
2) A stream is processed on a terminal operation call
3) Only one terminal operation is allowed
4) It cannot be processed again
If needed another stream should be built on the source
#DV13LBDS @JosePaumard
Parallel Streams
#DV13LBDS @JosePaumard
Optimization
The first optimization (well, laziness was first !) we need is
parallelism
The fork / join enable parallel programming since JDK 7
But writing tasks is hard and does not always lead to better
performances
#DV13LBDS @JosePaumard
Optimization
The first optimization (well, laziness was first !) we need is
parallelism
The fork / join enable parallel programming since JDK 7
But writing tasks is hard and does not always lead to better
performances
Using the parallel stream API is much more secure
#DV13LBDS @JosePaumard
How to build a parallel stream ?
Two patterns
1) Call parallelStream() instead of stream()
2) Call parallel() on an existing stream
Stream<String> s = strings.parallelStream() ;
Stream<String> s = strings.stream().parallel() ;
#DV13LBDS @JosePaumard
Can we decide the # of cores ?
Answer is no
#DV13LBDS @JosePaumard
Can we decide the # of cores ?
Answer is no
But… do we really need it ?
#DV13LBDS @JosePaumard
Is parallelism really that simple ?
Well… yes !
#DV13LBDS @JosePaumard
Is parallelism really that simple ?
Well… yes !
… and no
#DV13LBDS @JosePaumard
Is parallelism really that simple ?
Example 1 :
« returns the 10M first elements of the source »
persons.stream().limit(10_000_000) ;
#DV13LBDS @JosePaumard
Is parallelism really that simple ?
Example 1 :
« returns the 10M first elements of the source »
persons.parallelStream().limit(10_000_000) ;
#DV13LBDS @JosePaumard
Is parallelism really that simple ?
Example 1 :
« returns the 10M first elements of the source »
But how can I track the first 10M of the source on several
cores of my CPU ?
persons.parallelStream().limit(10_000_000) ;
#DV13LBDS @JosePaumard
Is parallelism really that simple ?
Example 1 : performances
Code 1
List<Long> list = new ArrayList<>(10_000_100) ; for (int i = 0 ; i < 10_000_000 ; i++) { list1.add(ThreadLocalRandom.current().nextLong()) ; }
#DV13LBDS @JosePaumard
Is parallelism really that simple ?
Example 1 : performances
Code 2
Stream<Long> stream = Stream.generate(() -> ThreadLocalRandom.current().nextLong()) ; List<Long> list1 = stream.limit(10_000_000).collect(Collectors.toList()) ;
#DV13LBDS @JosePaumard
Is parallelism really that simple ?
Example 1 : performances
Code 3
Stream<Long> stream = ThreadLocalRandom.current().longs(10_000_000).mapToObj(Long::new) ; List<Long> list = stream.collect(Collectors.toList()) ;
#DV13LBDS @JosePaumard
Is parallelism really that simple ?
Example 1 : performances
Serial Parallel
Code 1 (for) 270 ms
Code 2 (limit) 310 ms
Code 3 (longs) 250 ms
#DV13LBDS @JosePaumard
Is parallelism really that simple ?
Example 1 : performances
Serial Parallel
Code 1 (for) 270 ms
Code 2 (limit) 310 ms 500 ms
Code 3 (longs) 250 ms 320 ms
#DV13LBDS @JosePaumard
Is parallelism really that simple ?
Example 2 :
« returns a stream with all the elements of the first stream
followed by all the elements of the second stream »
Stream s3 = Stream.concat(stream1, stream2) ;
#DV13LBDS @JosePaumard
Parallelism
Parallelism implies overhead most of the time
Badly configured parallel operations can lead to unneeded
computations that will slow down the overall process
Unnecessary ordering of a stream will lead to performance
hits
#DV13LBDS @JosePaumard
Back to our sorting example
Parallel version
Stream<String> stream = ... ; // created before the test Object [] sorted = stream.parallel() .sorted() // or not .toArray() ;
#DV13LBDS @JosePaumard
Sorting performances
In fact it’s not that simple
# Serial Parallel Ratio Ratio n.log(n)
10 33 ms 32 ms
100 1 ms 1 ms
1K 2 ms 1 ms
10K 5 ms 7 ms
100K 65 ms 49 ms
1M 540 ms 153 ms
10M 9,7 s 2,2 s
20M 19,1 s 4,78 s
30M 31,0 s 8,13 s
#DV13LBDS @JosePaumard
Sorting performances
In fact it’s not that simple
# Serial Parallel Ratio Ratio n.log(n)
10 33 ms 32 ms 1
100 1 ms 1 ms 1
1K 2 ms 1 ms 2
10K 5 ms 7 ms 0,7
100K 65 ms 49 ms 1,3
1M 540 ms 153 ms 3,5
10M 9,7 s 2,2 s 4,4
20M 19,1 s 4,78 s 4,0
30M 31,0 s 8,13 s 3,8
#DV13LBDS @JosePaumard
Sorting performances
In fact it’s not that simple
# Serial Parallel Ratio Ratio n.log(n)
10 33 ms 32 ms 1 1
100 1 ms 1 ms 1 664
1K 2 ms 1 ms 2 4982
10K 5 ms 7 ms 0,7 26,6K
100K 65 ms 49 ms 1,3 25,6K
1M 540 ms 153 ms 3,5 36,9K
10M 9,7 s 2,2 s 4,4 23,9K
20M 19,1 s 4,78 s 4,0 25,3K
30M 31,0 s 8,13 s 3,8 24,1K
#DV13LBDS @JosePaumard
Questions ?
#DV13LBDS
#DV13LBDS @JosePaumard
Reductions
#DV13LBDS @JosePaumard
What is a reduction
2 kinds of reduction :
1) The reduction
« A reduction operation (also called a fold) takes a
sequence of input elements and combines them
into a single summary result by repeated
application of a combining operation »
#DV13LBDS @JosePaumard
What is a reduction
2 kinds of reduction :
2) The mutable reduction
« A mutable reduction operation accumulates
input elements into a mutable result container,
such as a Collection or StringBuilder, as it
processes the elements in the stream »
#DV13LBDS @JosePaumard
A simple reduction
Sum of ages
// map / filter / reduce pattern on collections int sum = persons.stream() .map(p -> p.getAge()) .filter(a -> a > 20) .reduce(0, (a1, a2) -> a1 + a2) ;
#DV13LBDS @JosePaumard
A simple reduction
It would be nice to be able to write :
But there’s no sum() on Stream<T>
What would be the sense of « adding persons » ?
// map / filter / reduce pattern on collections int sum = persons.stream() .map(p -> p.getAge()) .filter(a -> a > 20) .sum() ;
#DV13LBDS @JosePaumard
A simple reduction
It would be nice to be able to write :
But there’s no sum() on Stream<T>
There’s one on IntStream !
// map / filter / reduce pattern on collections int sum = persons.stream() .map(p -> p.getAge()) .filter(a -> a > 20) .sum() ;
#DV13LBDS @JosePaumard
A simple reduction
2nd version :
Value if persons is an empty list : 0
// map / filter / reduce pattern on collections int sum = persons.stream() .map(Person::getAge) .filter(a -> a > 20) .mapToInt(Integer::intValue) .sum() ;
#DV13LBDS @JosePaumard
A simple reduction
2nd version (even better) :
Value if persons is an empty list : 0
// map / filter / reduce pattern on collections int sum = persons.stream() .mapToInt(Person::getAge) .filter(a -> a > 20) // .mapToInt(Integer::intValue) .sum() ;
#DV13LBDS @JosePaumard
A simple reduction
What about max() and min() ?
How can one chose a default value ?
// map / filter / reduce pattern on collections ....... = persons.stream() .mapToInt(Person::getAge) .filter(a -> a > 20) .max() ;
#DV13LBDS @JosePaumard
Problem with the default value
The « default value » is a tricky notion
#DV13LBDS @JosePaumard
Problem with the default value
The « default value » is a tricky notion
1) The « default value » is the reduction of the empty set
#DV13LBDS @JosePaumard
Problem with the default value
The « default value » is a tricky notion
1) The « default value » is the reduction of the empty set
2) But it’s also the identity element for the reduction
#DV13LBDS @JosePaumard
#DV13LBDS @JosePaumard
#DV13LBDS @JosePaumard
#DV13LBDS @JosePaumard
#DV13LBDS @JosePaumard
#DV13LBDS @JosePaumard
#DV13LBDS @JosePaumard
#DV13LBDS @JosePaumard
#DV13LBDS @JosePaumard
Problem with the default value
The « default value » is a tricky notion
1) The « default value » is the reduction of the empty set
2) But it’s also the identity element for the reduction
#DV13LBDS @JosePaumard
Problem with the default value
Problem : max() and min() have no identity element
eg : an element e for which max(e, a) = a
#DV13LBDS @JosePaumard
Problem with the default value
Problem : max() and min() have no identity element
eg : an element e for which max(e, a) = a
0 cant be, max(0, -1) is not -1…
−∞ is not an integer
#DV13LBDS @JosePaumard
Problem with the default value
So what is the default value of max() and min() ?
#DV13LBDS @JosePaumard
Problem with the default value
So what is the default value of max() and min() ?
Answer : there is no default value for max() and min()
#DV13LBDS @JosePaumard
Problem with the default value
So what is the returned value of the max() reduction ?
// map / filter / reduce pattern on collections ....... = persons.stream() .mapToInt(Person::getAge) .filter(a -> a > 20) .max() ;
#DV13LBDS @JosePaumard
Problem with the default value
So what is the returned value of the max() reduction ?
If it’s int, then the default value will be 0…
// map / filter / reduce pattern on collections ....... = persons.stream() .mapToInt(Person::getAge) .filter(a -> a > 20) .max() ;
#DV13LBDS @JosePaumard
Optionals
Since there’s none, we need another notion
// map / filter / reduce pattern on collections OptionalInt optionalMax = persons.stream() .mapToInt(Person::getAge) .filter(a -> a > 20) .max() ;
« there might be no
result »
#DV13LBDS @JosePaumard
Optionals
What can I do with OptionalInt ?
1st pattern : test if it holds a value
OptionalInt optionalMax = ... ; int max ; if (optionalMax.isPresent()) { max = optionalMax.get() ; } else { max = ... ; // decide a « default value » }
#DV13LBDS @JosePaumard
Optionals
What can I do with OptionalInt ?
2nd pattern : read the held value, can get an exception
OptionalInt optionalMax = ... ; // throws NoSuchElementException if no held value int max = optionalMax.getAsInt() ;
#DV13LBDS @JosePaumard
Optionals
What can I do with OptionalInt ?
3rd pattern : read the held value, giving a default value
OptionalInt optionalMax = ... ; // get 0 if no held value int max = optionalMax.orElse(0) ;
#DV13LBDS @JosePaumard
Optionals
What can I do with OptionalInt ?
4th pattern : read the held value, or throw an exception
OptionalInt optionalMax = ... ; // exceptionSupplier will supply an exception, if no held value int max = optionalMax.orElseThrow(exceptionSupplier) ;
#DV13LBDS @JosePaumard
Available optionals
Optional<T>
OptionalInt, OptionalLong, OptionalDouble
#DV13LBDS @JosePaumard
Available reductions
On Stream<T> :
- reduce(), with different parameters
- count(), min(), max()
- anyMatch(), allMatch(), noneMatch()
- findFirst(), findAny()
- toArray()
- forEach(), forEachOrdered()
#DV13LBDS @JosePaumard
Available reductions
On IntStream, LongStream, DoubleStream :
- average()
- summaryStatistics()
#DV13LBDS @JosePaumard
Available reductions
SummaryStatistics is an object that holds :
- Count, Min, Max, Sum, Average
It’s a consumer, so it can be easily extended to compute
other statistics
#DV13LBDS @JosePaumard
Mutable reductions : example 1
Use of the helper class : Collectors
ArrayList<String> strings = stream .map(Object::toString) .collect(Collectors.toList()) ;
#DV13LBDS @JosePaumard
Mutable reductions : example 2
Concatenating strings with a helper
String names = persons .stream() .map(Person::getName) .collect(Collectors.joining()) ;
#DV13LBDS @JosePaumard
Mutable reductions
Common things :
- have a container : Collection or StringBuilder
- have a way to add an element to the container
- have a way to merge two containers (for parallelization)
#DV13LBDS @JosePaumard
Mutable reductions
The general form works with 3 objects :
- a supplier : supplies a new instance of the mutable result
container
Supplier<ArrayList<String>> supplier = () -> new ArrayList<String>() ;
#DV13LBDS @JosePaumard
Mutable reductions
The general form works with 3 objects :
- a supplier : supplies a new instance of the mutable result
container
- an accumulator : takes an partly filled container and
adds an element to it
BiConsumer<ArrayList<String>, ? super String> accumulator = (suppliedList, s) -> suppliedList.add(s) ;
#DV13LBDS @JosePaumard
Mutable reductions
The general form works with 3 objects :
- a supplier : supplies a new instance of the mutable result
container
- an accumulator : takes an partly filled container and
adds an element to it
- a combiner : takes two partly filled containers and
combines them
BiConsumer<ArrayList<String>, ArrayList<String>> combiner = (supplied1, supplied2) -> supplied1.addAll(supplied2) ;
#DV13LBDS @JosePaumard
Mutable reductions : example 1
Putting things together :
ArrayList<String> strings = stream .map(Object::toString) .collect( () -> new ArrayList<String>(), // the supplier (suppliedList, s) -> suppliedList.add(s), // the accumulator (supplied1, supplied2) -> supplied1.addAll(supplied2) // the combiner ) ;
#DV13LBDS @JosePaumard
Mutable reductions : example 1
Putting things together :
ArrayList<String> strings = stream .map(Object::toString) .collect( ArrayList::new, // the supplier ArrayList::add, // the accumulator ArrayList::addAll // the combiner ) ;
#DV13LBDS @JosePaumard
Collectors
#DV13LBDS @JosePaumard
The Collectors class
A rich toolbox (37 methods) for various types of reductions
- counting, minBy, maxBy
- summing, averaging, summarizing
- joining
- toList, toSet
And
- mapping, groupingBy, partionningBy
#DV13LBDS @JosePaumard
The Collectors class
Average, Sum, Count
persons .stream() .collect(Collectors.averagingDouble(Person::getAge)) ;
persons .stream() .collect(Collectors.counting()) ;
#DV13LBDS @JosePaumard
The Collectors class
Concatenating the names in a String
String names = persons .stream() .map(Person::getName) .collect(Collectors.joining(", ")) ;
#DV13LBDS @JosePaumard
The Collectors class
Accumulating in a List, Set
Set<Person> setOfPersons = persons .stream() .collect( Collectors.toSet()) ;
#DV13LBDS @JosePaumard
The Collectors class
Accumulating in a custom collection
TreeSet<Person> treeSetOfPersons = persons .stream() .collect( Collectors.toCollection(TreeSet::new)) ;
#DV13LBDS @JosePaumard
The Collectors class
Getting the max according to a comparator
Bonus : new API to build comparators
Optional<Person> optionalPerson = persons .stream() .collect( Collectors.maxBy( Comparator.comparing(Person::getAge)) ;
#DV13LBDS @JosePaumard
Building comparators
New API to build comparators in a declarative way
Comparator<Person> comp = Comparator.comparing(Person::getLastName) .thenComparing(Person::getFirstName) .thenComparing(Person:getAge) ;
#DV13LBDS @JosePaumard
The Collectors class : mapping
The mapping helper method takes 2 parameter :
- a function, that maps the elements of the stream
- a collector, called a « downstream », that is applied to
the mapped values
#DV13LBDS @JosePaumard
The Collectors class : mapping
Accumulating names in a set
Set<String> set = persons .stream() .collect( Collectors.mapping( Person::getLastName, Collectors.toSet())) ;
#DV13LBDS @JosePaumard
The Collectors class : mapping
Mapping the stream, then accumulating in a collection
TreeSet<String> set = persons .stream() .collect( Collectors.mapping( Person::getLastName, Collectors.toCollection(TreeSet::new)) ) ;
#DV13LBDS @JosePaumard
The Collector API : groupingBy
« Grouping by » builds hash maps
- must explain how the keys are built
- by default the values are put in a list
- may specify a downstream (ie a collector)
#DV13LBDS @JosePaumard
The Collector API : groupingBy
Grouping a list of persons by their age
Map<Integer, List<Person>> map = persons .stream() .collect( Collectors.groupingBy(Person::getAge)) ;
#DV13LBDS @JosePaumard
The Collector API : groupingBy
Grouping a list of persons by their age
Map<Integer, Set<Person>> map = persons .stream() .collect( Collectors.groupingBy( Person::getAge, Collectors.toSet() // the downstream ) ;
#DV13LBDS @JosePaumard
The Collector API : groupingBy
Grouping a list of persons names by their age
Map<Integer, Set<String>> map = persons .stream() .collect( Collectors.groupingBy( Person::getAge, Collectors.mapping( // Person::getLastName, // the downstream Collectors.toSet() // ) ) ;
#DV13LBDS @JosePaumard
The Collector API : groupingBy
Grouping a list of persons names by their age
Map<Integer, TreeSet<String>> map = persons .stream() .collect( Collectors.groupingBy( Person::getAge, Collectors.mapping( Person::getLastName, Collectors.toCollection(TreeSet::new) ) ) ;
#DV13LBDS @JosePaumard
The Collector API : groupingBy
Grouping a list of blah blah blah
TreeMap<Integer, TreeSet<String>> map = persons .stream() .collect( Collectors.groupingBy( Person::getAge, TreeMap::new, Collectors.mapping( Person::getLastName, Collectors.toCollection(TreeSet::new) ) ) ;
#DV13LBDS @JosePaumard
The Collector API : groupingBy
Example : creating an age histogram
Gives the # of persons by age
Map<Integer, Long> map = persons .stream() .collect( Collectors.groupingBy(Person::getAge, Collectors.counting()) ) ;
#DV13LBDS @JosePaumard
The Collector API : partionningBy
Creates a Map<Boolean, …> on a predicate
- the map has 2 keys : TRUE and FALSE
- may specify a downstream
#DV13LBDS @JosePaumard
The Collector API : partionningBy
Creates a Map<Boolean, …> on a predicate
map.get(TRUE) returns the list people older than 20
Map<Boolean, List<Person>> map = persons .stream() .collect( Collectors.partitioningBy(p -> p.getAge() > 20) ) ;
#DV13LBDS @JosePaumard
The Collector API : partionningBy
Can further process the list of persons
Map<Boolean, TreeSet<String>> map = persons .stream() .collect( Collectors.partitioningBy( p -> p.getAge() > 20, Collectors.mapping( Person::getLastName, Collectors.toCollection(TreeSet::new)) ) ) ) ;
#DV13LBDS @JosePaumard
The Collector API : partionningBy
Can further process the list of persons
Map<Boolean, TreeSet<String>> map = persons .stream() .collect( Collectors.partitioningBy( p -> p.getAge() > 20, Collectors.mapping( Person::getLastName, Collectors.toCollection(TreeSet::new)) ) ) ) ;
Partition the persons
on age > 20
#DV13LBDS @JosePaumard
The Collector API : partionningBy
Can further process the list of persons
Map<Boolean, TreeSet<String>> map = persons .stream() .collect( Collectors.partitioningBy( p -> p.getAge() > 20, Collectors.mapping( Person::getLastName, Collectors.toCollection(TreeSet::new)) ) ) ) ;
Gather their names…
#DV13LBDS @JosePaumard
The Collector API : partionningBy
Can further process the list of persons
Map<Boolean, TreeSet<String>> map = persons .stream() .collect( Collectors.partitioningBy( p -> p.getAge() > 20, Collectors.mapping( Person::getLastName, Collectors.toCollection(TreeSet::new)) ) ) ) ;
… in TreeSets
#DV13LBDS @JosePaumard
The Collector API : collectingAndThen
Collect data with a downstream
Then apply a function called a « finisher »
Useful for putting the result in a immutable collection
#DV13LBDS @JosePaumard
The Collector API : collectingAndThen
In this case « Map::entrySet » is a finisher
Set<Map.Entry<Integer, List<Person>>> set = persons .stream() .collect( Collectors.collectingAndThen( Collectors.groupingBy( Person::getAge), // downstream, builds a map Map::entrySet // finisher, applied on the map ) ;
#DV13LBDS @JosePaumard
Questions ?
#DV13LBDS
#DV13LBDS @JosePaumard
Some real examples
#DV13LBDS @JosePaumard
1st example
Optional<Entry<Integer, Long>> opt = movies.stream().parallel() .collect( Collectors.collectingAndThen( Collectors.groupingBy( movie -> movie.releaseYear(), Collectors.counting() ), Map::entrySet ) ) .stream() .max(Map.Entry.comparingByValue()) ;
#DV13LBDS @JosePaumard
1st example
Optional<Entry<Integer, Long>> opt = movies.stream().parallel() .collect( Collectors.collectingAndThen( Collectors.groupingBy( movie -> movie.releaseYear(), Collectors.counting() ), Map::entrySet ) ) .stream() .max(Map.Entry.comparingByValue()) ;
A Stream of movies
#DV13LBDS @JosePaumard
1st example
Optional<Entry<Integer, Long>> opt = movies.stream().parallel() .collect( Collectors.collectingAndThen( Collectors.groupingBy( movie -> movie.releaseYear(), Collectors.counting() ), Map::entrySet ) ) .stream() .max(Map.Entry.comparingByValue()) ;
Building a map
of year / # of movies
#DV13LBDS @JosePaumard
1st example
Optional<Entry<Integer, Long>> opt = movies.stream().parallel() .collect( Collectors.collectingAndThen( Collectors.groupingBy( movie -> movie.releaseYear(), Collectors.counting() ), Map::entrySet ) ) .stream() .max(Map.Entry.comparingByValue()) ;
Then getting the
entry set
#DV13LBDS @JosePaumard
1st example
Optional<Entry<Integer, Long>> opt = movies.stream().parallel() .collect( Collectors.collectingAndThen( Collectors.groupingBy( movie -> movie.releaseYear(), Collectors.counting() ), Map::entrySet ) ) .stream() .max(Map.Entry.comparingByValue()) ;
Then getting the
max by value
#DV13LBDS @JosePaumard
1st example
Optional<Entry<Integer, Long>> opt = movies.stream().parallel() .collect( Collectors.collectingAndThen( Collectors.groupingBy( movie -> movie.releaseYear(), Collectors.counting() ), Map::entrySet ) ) .stream() .max(Map.Entry.comparingByValue()) ;
Returns the
year with the most
movies released
#DV13LBDS @JosePaumard
Some thoughts on parallel
#DV13LBDS @JosePaumard
Going parallel
To ways to do it :
- create a parallel stream up front
- call the parallel() method on a given stream, or
sequential() to go sequential
When a terminal operation is initiated, the operations are
executed in parallel or not, depending on the mode of the
stream
#DV13LBDS @JosePaumard
Things that will go wrong
Some operations, like findAny() dont give reproductible
results in parallel
Changing the source (which is forbidden) can lead to
unpredictable results even if the source is concurrent
aware
#DV13LBDS @JosePaumard
Things that will not go wrong, but…
The parallel processing of a stream has to be freed from
constraints
- ordering is costly, relaxing ordering may lead to better
performances
- Collecting must be concurrent aware
#DV13LBDS @JosePaumard
Conclusion
#DV13LBDS @JosePaumard
Conclusion
Why are lambdas introduced in Java 8 ?
#DV13LBDS @JosePaumard
Conclusion
Why are lambdas introduced in Java 8 ?
Because it’s in the mood !
#DV13LBDS @JosePaumard
#DV13LBDS @JosePaumard
Conclusion
Why are lambdas introduced in Java 8 ?
Because it’s in the mood !
#DV13LBDS @JosePaumard
Conclusion
Why are lambdas introduced in Java 8 ?
Because it’s in the mood !
Because it allows to write more compact code !
#DV13LBDS @JosePaumard
Compact code is better !
An example of compact code (in C)
#include "stdio.h" main() { int b=0,c=0,q=60,_=q;for(float i=-20,o,O=0,l=0,j,p;j=O*O,p=l*l, (!_--|(j+p>4)?fputc(b?q+(_/3):10,(i+=!b,p=j=O=l=0,c++,stdout)), _=q:l=2*O*l+i/20,O=j-p+o),b=c%q,c<2400;o=-2+b*.05) ; }
http://www.codeproject.com/Articles/2228/Obfuscating-your-Mandelbrot-code
#DV13LBDS @JosePaumard
Compact code is better !
An example of compact code (in C)
#include "stdio.h" main() { int b=0,c=0,q=60,_=q;for(float i=-20,o,O=0,l=0,j,p;j=O*O,p=l*l, (!_--|(j+p>4)?fputc(b?q+(_/3):10,(i+=!b,p=j=O=l=0,c++,stdout)), _=q:l=2*O*l+i/20,O=j-p+o),b=c%q,c<2400;o=-2+b*.05) ; }
http://www.codeproject.com/Articles/2228/Obfuscating-your-Mandelbrot-code
++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ ++++++++++++++++++++++++++++++++++++++++++++ +++++++++++++++ ++++++++++++++++++++++++++++++++++++++++++ ++++++++++++++ ++++++++++++++++++++++++++++++++++++++++++ ++++++++++++++ ++++++++++++++++++++++++++++++++++++++++++ ++++++++++++++ ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ ++++++++++++++++++++++++++++++++++++ ++ +++++++++++ +++++++++++++++++++++++++++++++++++ +++++++ ++++++++++++++++++++++++++++++++++++ +++++++ +++++++++++++++++++++++++++++++++++ ++++++++ ++++++++++++++++++++++++++++++++++ +++++++ +++++++++++++++++++++++++++++++++ +++++ +++++++++++++++++++++++++++++++++ ++++++ +++++++++++++++++++++++ + +++++ ++++++ +++++++++++++++++++++++ ++ ++++++ ++++++++++++++++++++++ + ++++++ ++++++++++++++++++++++ + ++++++ ++++++++++++++++++++ + + +++++++ ++++++ ++++++++ ++++++++++++++++++++ + + +++++++ ++++++++++++++++++++++ + ++++++ ++++++++++++++++++++++ + ++++++ +++++++++++++++++++++++ ++ ++++++ +++++++++++++++++++++++ + +++++ ++++++ +++++++++++++++++++++++++++++++++ ++++++ +++++++++++++++++++++++++++++++++ +++++ ++++++++++++++++++++++++++++++++++ +++++++ +++++++++++++++++++++++++++++++++++ ++++++++ ++++++++++++++++++++++++++++++++++++ +++++++ +++++++++++++++++++++++++++++++++++ +++++++ ++++++++++++++++++++++++++++++++++++ ++ +++++++++++ ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ ++++++++++++++++++++++++++++++++++++++++++ ++++++++++++++ ++++++++++++++++++++++++++++++++++++++++++ ++++++++++++++ ++++++++++++++++++++++++++++++++++++++++++ ++++++++++++++ ++++++++++++++++++++++++++++++++++++++++++++ +++++++++++++++ ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
#DV13LBDS @JosePaumard
Conclusion
Why are lambdas introduced in Java 8 ?
Because it’s in the mood !
Because it allows to write more compact code !
#DV13LBDS @JosePaumard
Conclusion
Why are lambdas introduced in Java 8 ?
Because it brings new and efficient patterns,
that enable easy and safe parallelization,
- needed by modern applications
- needed by API writers
#DV13LBDS @JosePaumard
Conclusion
Java 8 is coming, it’s the biggest update in 15 years
#DV13LBDS @JosePaumard
Conclusion
Java 8 is coming, it’s the biggest update in 15 years
Moving to Java 8 means a lot of work for us developers !
- Self training
- Changing our habits
#DV13LBDS @JosePaumard
Conclusion
Java 8 is coming, it’s the biggest update in 15 years
Moving to Java 8 means a lot of work for us developers !
- Self training
- Changing our habits
- Convincing our bosses (sigh…)
#DV13LBDS @JosePaumard
Conclusion
Java 8 is coming, it’s the biggest update in 15 years
Download the develop preview version on Open JDK site
Release date : 18th march 2014 !
#DV13LBDS @JosePaumard
Conclusion
Java 8 is coming, it’s the biggest update in 15 years
Devoxx openning keynote
Brian Goetz : « Lamda a peek under the hood », Wed
12:00
Paul Sandoz : « In full flow : Java 8 lambdas in the
stream », Wed 14:00
#DV13LBDS @JosePaumard
Conclusion
Java 8 is coming, it’s the biggest update in 15 years
« Java is a blue collar language. It’s not PhD thesis
material but a language for a job » – James Gosling, 1997
#DV13LBDS @JosePaumard
Conclusion
Java 8 is coming, it’s the biggest update in 15 years
« Language features are not a goal unto themselves;
language features are enablers, encouraging or
discouraging certain styles and idioms » – Brian Goetz,
2013
#DV13LBDS @JosePaumard
Conclusion
Java 8 is coming, it’s the biggest update in 15 years
The good news is :
Java is still Java !
#DV13LBDS @JosePaumard
#DV13LBDS @JosePaumard