CS221Week 1 - Friday

Last time

What did we talk about last time? Basic programming model Other Java stuff

References Static Inner classes Exceptions

Questions?

Exceptions

Exceptions

Java handles errors with exceptions Code that goes wrong throws an

exception The exception propagates back up

through the call stack until it is caught

If the exception is never caught, it crashes the thread it's running on, often crashing the program

Kinds of exceptions

There are two kinds of exceptions: Checked Unchecked

Checked exceptions can only be thrown if the throwing code is: Surrounded by a try block with a catch block

matching the kind of exception, or Inside a method that is marked with the keyword throws as throwing the exception in question

Unchecked exceptions can be thrown at any time (and usually indicate unrecoverable runtime errors)

Examples of exceptions

Checked exceptions FileNotFoundException IOException InterruptedException Any exception you write that extends Exception

Unchecked exceptions ArrayIndexOutOfBoundsException NullPointerException ArithmeticException Any exception you write that extends Error or RuntimeException

Exceptions in code

Scanner in = null;try {in = new Scanner( file );while( in.hasNextInt() )

process( in.nextInt() );}catch( FileNotFoundException e ) {

System.out.println ("File " + file.getName () + " not

found !");}finally { if( in != null ) in.close (); }

Threads

Threads

In Java, concurrency and parallelism are achieved primarily through threads

A thread is a path of code execution that runs independently of others But threads can share memory with each

other Some other languages use message

passing semantics and no direct memory sharing

Creating threads in Java

To create a customized thread in Java, there are two routes: Create a class which is a subclass of Thread

Create a class which implements the Runnable interface

If you subclass Thread, you can't subclass some other object

If you implement Runnable, there's an extra bit of syntax to create a new thread

Sample thread to sum things

public class Summer extends Thread {private double[] array;private int start;private int end;private double result = 0.0;

public Summer(double[] array,int start, int end) {this.array = array;this.start = start;this.end = end;}

public void run() {for( int i = start; i < end; i++ )result += array[i];}

public double getResult() { return result; }}

Using Summer

public double findSum( double[] array, int threads )throws InterruptedException {Summer[] summers = new Summer[threads];double result = 0.0;int step = array.length / threads;if( array.length % threads > 0 ) step++;

for( int i = 0; i < threads; i++ ) {summers[i] = new Summer(array, i*step,

Math.min((i+1)*step, array.length));summers[i].start();

}

for( int i = 0; i < threads; i++ ) {summers[i].join();result += summers[i].getResult();

}

return result;}

Tips for concurrency

Keep it simple Do not share data via static

variables Be careful with load balancing

If the work is not evenly divisible by n (the number of threads), give the first n – 1 threads one extra piece of work

Why? Be aware that threading out a

program will not necessarily make it faster

OOP

What is an object?

Members Methods Why are they useful?

Monolitihic main() function

• Code and data together

Work divided

into functions

• Code separated, but data shared

Objects

• Code and data separated

What is a class?

A template or prototype for an object

Class: Human

Object: David Bowie

Object: Barack Obama

Class: Company

Object: Microsoft

Object: Boeing

Object Oriented Programming

Encapsulation Dynamic dispatch Polymorphism Inheritance Self-reference

Encapsulation

Information hiding We want to bind operations and data tightly

together Consequently, we don't want you to touch

our privates Encapsulation in Java is provided by the private and protected keywords (and also by default, package level access)

Hardcore OOP people think that all data should be private and most methods should be public

Encapsulation example

public class A{private int a;

public int getA(){

return a;}

public void setA(int value){

a = value;}

}

Inheritance

Allows code reuse Is thought of as an is-a relationship Java does not allow multiple

inheritance, but some languages do Deriving a subclass usually means

creating a "refined" or "more specific" version of a superclass

Inheritance example

public class B extends A{ //has member and methods from A}

public class C extends A{//has A stuff and moreprivate int c;public int getC(){ return c; }void increment() { c++; }

}

Polymorphism

A confusing word whose underlying concept many programmers misunderstand

Polymorphism is when code is designed for a superclass but can be used with a subclass

If BMW235i is a subtype of Car, then you can use an BMW235i anywhere you could use a Car

Polymorphism example

//defined somewherepublic void drive( Car car ){…}

public class BMW235i extends Car{…}

Car car = new Car();BMW235i bimmer = new BMW235i();drive( bimmer ); //okaydrive( car ); //okay

Dynamic dispatch

Polymorphism can be used to extend the functionality of an existing method using dynamic dispatch

In dynamic dispatch, the method that is actually called is not known until run time

Dynamic dispatch example

public class A {public void print() { System.out.println("A");}

}

public class B extends A {public void print() { System.out.println("B");}

}

Dynamic dispatch example

A a = new A();B b = new B(); // B extends AA c;

a.print(); // Ab.print(); // B

c = a;c.print(); // Ac = b;c.print(); // B

Self-reference

Objects are able to refer to themselves

This can be used to explicitly reference variables in the class

Or, it can be used to provide the object itself as an argument to other methods

Self reference example

public class Stuff{private int things;

public void setThings(int things){

this.things = things;}

}

Self reference example

public class SelfAdder{public void addToList(List list){

list.add(this);}

}

Constructor syntax

Java provides syntax that allows you to call another constructor from the current class or specify which superclass constructor you want to call

The first line of a constructor is a call to the superclass constructor

If neither a this() or a super() constructor are the first line, an implicit default super() constructor is called

Constructor example

public class A {private double half;public A(int value) {

half = value / 2.0;}

}

public class B extends A {public B(int input) {

super(input); // calls super constructor}

public B() {this(5); // calls other constructor

}}

Interfaces

Interface basics

An interface is a set of methods which a class must have

Implementing an interface means making a promise to define each of the listed methods

It can do what it wants inside the body of each method, but it must have them to compile

Unlike superclasses, a class can implement as many interfaces as it wants

Interface definition

An interface looks a lot like a class, but all its methods are empty

Interfaces have no members except for (static final) constants

public interface Guitarist {void strumChord(Chord chord);void playMelody(Melody notes);

}

Interface use

public class RockGuitarist extends RockMusician implements Guitarist {

public void strumChord( Chord chord ) {System.out.print("Totally wails on that " +chord.getName() + " chord!");

}

public void playMelody( Melody notes ) {System.out.print("Burns through the notes " +notes.toString() +" like Jimmy Page!" );

}}

Usefulness

A class has an is-a relationship with interfaces it implements, just like a superclass it extends

Code that specifies a particular interface can use any class that implements it

public static void perform(Guitarist guitarist, Chord chord, Melody notes) {

System.out.println("Give it up " + "for the next guitarist!");

guitarist.strumChord( chord );guitarist.playMelody( notes );

}

Generics

Generics

Allow classes, interfaces, and methods to be written with a generic type parameter, then bound later

Java does the type checking (e.g. making sure that you only put String objects into a List<String>)

After typechecking, it erases the generic type parameter This works because all classes extend Object in Java

Appears to function like templates in C++, but works very differently under the covers

Most of the time you will use generics, not create them

Generic classes

You can make a class using generics The most common use for these is

for container classes e.g. you want a List class that can be a

list of anything JCF is filled with such generics

Generic class examplepublic class Pair<T> {

private T x;private T y;public Pair(T a, T b ) {

x = a;y = b;

}

public T getX() { return x; }

public T getY() { return y; }

public void swap() {T temp = x;x = y;y = temp;

}

public String toString() {return "( " + x + ", " + y + " )";

}}

Generic class use

public class Test {public static void main(String[] args) {

Pair<String> pair1 = new Pair<String>("ham", "eggs");

Pair<Integer> pair2 =new Pair<Integer>( 5, 7 );

pair1.swap();System.out.println( pair1 );System.out.println( pair2 );

}}

JCFJava Collections Framework

Container interfaces

CollectionIterableListQueueSet

SortedSetMap

SortedMap

Container classes

LinkedListArrayListStackVectorHashSetTreeSetHashMapTreeMap

Tools

Collections sort() max() min() replaceAll() reverse()

Arrays binarySearch() sort()

Upcoming

Next time…

Computational complexity Read section 1.4

Reminders

Read section 1.4 Work on Assignment 1

Due next Friday by 11:59pm Start on Project 1