Java Generics(Under The Hood Of The Compiler)

By: Harmeet Singh (Taara)(Java EE Developer)

harmeetsingh.0013@gmail.comhttp://harmeetsingh13.blogspot.in

skype: harmeetsingh0013Contact: Via mail or Skype

Contents

1. Generic Type.2. WildCards in Generics.3. Unchecked Warnings.4. Heap Pollution.5. Type Eraser.6. Bridge method.7. Type Erasure of A Parameterized Type.8. Type Erasure of A Type Parameter.9. Type Erasure of A Generic Method.10.Reficiation.11. Reifilable Type.

Acknowledgement

● Special Thanks to “Angelika Langer” (Trainer & Author and Mentor). Everything about generics in this presentation is get from “Angelika Langer” Generics FAQ.

info@angelikaLanger.com

Generic Type● Type Parameter: Is a placeholder that will later be replace but a type

argument.● A parameterized or generic type is an instantiation of a generic type with

actual argument. ● Define Generics:

class Pair<S,T>{private S first;private T second;-----------------

}

Generic Type● Types that cannot have type parameter :

o Anonymous Inner Classeso Exception Typeso Enum Types.

● Generic Type Instantiated: o Pair<String, Long> pair = new Pair<String, Long>();

● Generics Declarations: o Collection<String> : Concrete Declaration.o Collection<?> : Wild Card Declaration.

WildCard in Generics● We are not create directly object whose types is wildcard parameterized

objectArrayList<?> list = new ArrayList<?>(); //error

● WildCard parameterized type is not declare in a supertypeclass WildCardParam<?>{ //error---------------------

}

WildCards In Generics● WildCard Types Family:

o “ ? “ : Unbounded WildCard o “ ? extends Type ” : WildCard With an upper boundo “ ? super Type “ : WildCard With an lower bound

WildCards in Generics● WildCards Bounds: A Reference type that is used to further describe the

family of types denoted by a wildcard. ● Difference between WildCard Bound and Type Parameter Bound:

o A WildCard can have only one bound, while a type parameter can have several bounds.

o A WildCard can have lower or an upper bound, while there is no such things as a lower bound for a types parameter.

WildCards In Generics● Does “extends” always means Inheritance ? : extends is an overloaded

keyword in java. it has different meanings, depending on the context it appears. The extends keyword appear in four different locations : o In the Definition of Class.o In the Definition of Interface.o In the Definition of Type Parameter bounds.o In the Definition of WildCards Bounds.

Unchecked Warnings● Compiler and the runtime system do not have enough type information to

perform all type checks that would be necessary to ensure type safety. example: TreeSet set = new TreeSet();

set.add(“abc”); //unchecked warnings. ● Xlint:unchecked enable “unchecked” warnings.● Xlint:-unchecked and @SuppressWarnings(“unchecked”) disable

“unchecked” warnings.

Heap Pollution● A Situation where a variable of parameterized types refer to an Object that

is not of that parameterized type. ● Reason of Heap Pollution Occur :-

1. Mixing Raw and Parameterized Type.2. Unwise Casting.3. Separate Compilation.

Type Erasure● How Compiler Translate Java Generics:

○ Compiler translate generic type or method(in any language, not just java) has in principle two choice. ■ Code Specialization : The compiler generates a new

representation of a generic type of method.■ Code Sharing : The compiler generates code for only one

representation of a generic type or method and maps all the instantiations of the generic type of method to the unique representation , perform type check and type conversion where needed.

Type Erasure● What is Type Erasure ?

o The compiler generates only one byte code representation of a generic type or method and maps all the instantiations of the generic type or method to the unique representation.

● The type Erasure process can be imagined as a translation from generic Java Source code back into regular Java Code.

● The Steps performed during type erasure : o Eliding Type Parameter : When the Compiler finds the definition of a

generic type or method, it removes all occurrences of the type parameters and replaces them by their leftmost, or type Object if no bounds specified. Ex: <T> var; into Object var;

Type Erasure ● The Steps performed during type erasure :

o Eliding Type argument: When the compiler finds a parameterized type, i.e an instantiation of a generic types, then it removes the type arguments. Like : List<String>, Set<Long> and Map<String, ?> are translated to List, Set and Map respectively.

Bridge Method● A Synthetic method that the compiler generates in the course of type

erasure. it is sometimes needed when a type extends or implements a parameterized class or interface.

Example Before type Erasure:

Bridge Method

Example After type Erasure:

● Side Effect of type erasure is that two methods have identical signature before type erasure and different signature after type erasure.

Bridge Method● Under which circumstances is a bridge method generated ?

o Bridge methods are necessary when a class implements a parameterized interface or extends a parametrized superclass and type erasure changes the arguments type of any of the inherited non-static method.

Bridge Method● The compiler must add the bridge methods even if the subclass does not

override the inherited method.

Type Erasure OF Parameterized Type

● The Erasure of a parameterized type is the type without any type argument (i.e the raw type). The definition extends to arrays and nested types.

Type Erasure Of Type Parameter● The type erasure of a type parameter is the erasure of its leftmost bound.

The type erasure of an unbounded type parameter is type Object. Examples :

Type Erasure Of Generic Type● The Erasure of a method signature consisting of the same name and the

erasures of all the formal method parameters types.Examples :

Reficiation● Representing type parameters and arguments of generic types and

method at runtime. Reficiation is the opposite of type Erasure.

Reifiable● A type whose type information is fully available at run time, that is, a type that does not lose

information in the course of type erasure. ● The following types are reifiable:

o primitive typeso non-generic (or non-parameterized) reference typeso unbounded wildcard instantiationso raw typeso arrays of any of the above

● The non-reifiable types, which lose type information as a side effect of type erasure, are:o instantiations of a generic type with at least one concrete type argumento instantiations of a generic type with at least one bounded wildcard as type argument

● Reifiable types are permitted in some places where non-reifiable types are disallowed.o as type in an instanceof expressiono as component type of an array

References

● http://www.angelikalanger.com/GenericsFAQ/JavaGenericsFAQ.html