Java

“ A simple, object-oriented, distributed, interpreted, robust, secure, architecture neutral, portable, high-performance, multithreaded, and dynamic language.”

— Sun

Greg J. Badros

badros@cs.washington.edu

University of Washington, Seattle

CSE-341, Summer 1999

(C) 1999, Greg J. Badros—All Rights Reserved

Java: A Timeline

Patrick Naughton, James Gosling, et al. found the “Green Team” within Sun. The language “Oak” is born, targeting embedded applications

NCSA introduces Mosaic browser, and the World Wide Web takes off

Oak retargeted to web,

WebRunner browser is born

1991

1993

23-May-1995

Public launch of Oak, renamed “Java”

23-Jan-1996

Sun ships JDK 1.0

29-May-1996

1st JavaOne Conference

18-February-1997

Sun ships JDK 1.1

1995

1997

Latest version: Java 1.2—JDK 1.2 shipped October 1998

Java: The Language

Ref: Sun- What is Java? page

Byte codes

Hello World!

/** Application HelloWorld

Just output "Hello World!" */

public class HelloWorld {

public static void main(String[] args) { System.out.println("Hello World!");

}

}

% javac HelloWorld.java

% java HelloWorld

Hello World!

HelloWorld.java

Java vs. C++

/** Application HelloWorld */

public class HelloWorld {

public static void main(String[] args) { System.out.println("Hello World!");

}

}

// Application HelloWorld

#include <iostream.h>

int main(int argc, char* argv[]) {

cout << “Hello World!” << endl;

}

Unlike C++, Java has….

• No global functions — everything is in a class!

• Real String objects — not just char[]

• No pointers — everything is a reference

• No operator-overloading

• No preprocessor — cpp is not as necessary

Brewing Java

Java

basic operators, primitive types,

control structures

object model,

dynamic features,

rich class hierarchy

package system

object syntax,

object types,

exceptions

Java vs. Smalltalk

int x = 50, y = 50;

Ball ball = new Ball(x, y);

PinballAnimationPane pap = new PinballAnimationPane();

pap.addObject(ball);

ball.animate();

| ball pap x y |

x := 50. y := 50.

ball := Ball newX: x centerY: y.

pap := PinballAnimationPane new.

pap addObject: ball.

ball animate

Unlike Smalltalk, Java...

• Specifies types for all variables

• Permits primitive types such as int

• Has new keyword for creating objects

• Does not have keyword arguments

• Uses C operators
  • = is assignment
  • . operator for message sends

Java vs. C++, Revisited

Ball *pball = new Ball(50,50);

PinballAnimationPane *pxpap = new PinballAnimationPane();

pxpap->addObject(pball);

pball->animate();

Ball ball = new Ball(50, 50);

PinballAnimationPane pap = new PinballAnimationPane();

pap.addObject(ball);

ball.animate();

Ball ball(50,50); // creates ball on stack

PinballAnimationPane xpap(); // creates xpap on stack

xpap.addObject(ball); // calls: addObject(Ball &b);

ball.animate();

(pointer-style)

(reference-style)

Java’s Hybrid Object Model

• Primitive types on stack
  • May be wrapped or boxed into a real object Integer anInteger = new Integer(43);(useful for storing in java.util.*’s collections)
  • Unboxed primitives very similar to in C++

• All object instances live in the heap (not stack)
  • all object creation is done with new
  • No “delete” — Java uses garbage collection like Smalltalk, but also provides finalize() method

Java’s Class Hierarchy

Object

String

Number

Byte

Long

arrays

Boolean

Component

Container

Panel

Applet

Compiler

final class

abstract class

wrapped primitives

concrete class

Java Documentation

HelloWorld Applet

import java.applet.*; import java.awt.*;

public class HelloWorldApplet extends Applet {

static final String message = “Hello World”;

private Font font;

public void init() { // one-time initialization

font = new Font(“Helvetica”, Font.BOLD, 48); }

public void paint(Graphics g) {

g.setColor(Color.purple); g.fillOval(10, 10, 330, 100);

g.setColor(Color.red);

g.drawOval(10, 10, 330, 100); g.drawOval(9, 9, 332, 102);

g.drawOval(8, 8, 334, 104); g.drawOval(7, 7, 336, 106);

g.setColor(Color.black); g.setFont(font);

g.drawString(message, 40, 75);

} }

HelloWorldApplet.java

Ref: Java In a Nutshell, O’Reilly

Running the HelloWorld Applet

<APPLET code="HelloWorldApplet.class"

width=350 height=120> Java Missing

</APPLET>

HelloWorldApplet.html

Add "." to your $CLASSPATH, then% appletviewer HelloWorldApplet.html

Ref: Java In a Nutshell, O’Reilly

Run on the

.html file

Methods:A Closer Look

• this is implicit on instance fields and methods
  • can be explicit if the field is hidden by a local or formal
  • analogous to self in Smalltalk (though self is necessarily explicit)

• also super keyword, as in Smalltalk (no C++ :: operator)
  • also used for constructor chaining with arguments

public class Point {

. . .

public void move(int dx) {

x += dx;

moved();

}

private void moved() { . . }

private int x, y;

}

public class Point {

. . .

public void move(int dx) {

this.x += dx;

this.moved();

}

private void moved() { . . }

private int x, y;

}

More on Methods

• Instance methods (no static keyword)
  • have implicit this argument
  • can use super keyword
  • no need to use “->” operator as in C++just . operator since this, super are references

• static (class) methods
  • do not have implicit this argument
  • cannot use the super keyword

Default Arguments

• No language support—must use overloading instead

public class Point {

public Point() { this(0,0); }

public Point(int x, int y) { this.x = this.y; }

public void move() { move(1); }

public void move(int dx) { x += dx; }

private int x, y;

}

special use

of "this"

“Override” vs. “Overload”

• Override
  • replace a superclass’s method with a specialized version
  • signatures must match(including return type; C++ permits narrowing of return types, Java does not)

• Overload
  • write several methods for a given classwith the same name
  • language can disambiguatebased on number or types of arguments

Java’s Class Hierarchy

Object

String

Number

Byte

Long

arrays

Boolean

Component

Container

Panel

Applet

Compiler

wrapped primitives

final class

abstract class

concrete class

What can anObject do for you today?

• Object clone() Return a duplicate copy of self

• boolean equals(Object obj) Return true if and only if self is value-equal to obj

• String toString() Return printable representation of self

• int hashCode() Return a reasonable hash code for self

• Class getClass() Return the class object for self

Objects and Identities

Ball ball = new Ball(50,50);

Ball sameBall = ball;

instance of Ball

Class Ball

Ball(int, int)

animate()

…..

x: 50

y: 50

…..

Heap

Test object identity:

ball == sameBall ? true

Test object value’s equality:

ball.equals(sameBall) ? true

Cloning Objects

Ball ball = new Ball(50,50);

Ball sameBall = ball;

Ball anotherBall = (Ball) ball.clone();

instance of Ball

Class Ball

Ball(int, int)

animate()

…..

x: 50

y: 50

…..

Heap

instance of Ball

x: 50

y: 50

…..

creates new

instance

Test object identity:

ball == anotherBall ? false

Test object value’s equality:

ball.equals(anotherBall) ? true

Changing a Ball

ball.setPosition(20,35);

sameBall

anotherBall

instance of Ball

Class Ball

Ball(int, int)

animate()

…..

x: 20

y: 35

…..

Heap

instance of Ball

x: 50

y: 50

…..

Test object identity:

ball == sameBall ? ???

ball == anotherBall ? ???

Test object value’s equality:

ball.equals(sameBall) ? ???

ball.equals(anotherBall) ? ???

Inequality in Balls!

ball.setPosition(20,35);

sameBall

anotherBall

instance of Ball

Class Ball

Ball(int, int)

animate()

…..

x: 20

y: 35

…..

Heap

instance of Ball

x: 50

y: 50

…..

Test object identity:

ball == sameBall ? true

ball == anotherBall ? false

Test object value’s equality:

ball.equals(sameBalll) ? true

ball.equals(anotherBall) ? false

Assignmentjust changes the pointer

ball

sameBall = anotherBall;

anotherBall

instance of Ball

Class Ball

Ball(int, int)

animate()

…..

x: 20

y: 35

…..

Heap

instance of Ball

x: 50

y: 50

…..

Test object identity:

ball == sameBall ? false

ball == anotherBall ? false

Test object value’s equality:

ball.equals(sameBalll) ? false

ball.equals(anotherBall) ? false

Java variables hold...

• primitive boolean foo; // boolean, not bool as in C++ char aChar = 'a'; // 16 bit char (unicode)

• Object reference (may be null) ColoredBall cball = new Ball(); Ball ball = cball;

• Array reference int[] intArray = { 1, 2, 3, 4, 5, }; String[] strArray = { "Hello", "World", }; // same as String[] strArray = new String[2]; strArray[0] = new String("Hello"); strArray[1] = new String("World");

String literals actually

invoke constructor

e.g., new String("World")

Arrays

• Java arrays are 1st-class Objects

• 0-indexed

• Bounds checking is performed

• Store/Retrieve using [] operator strArray[0] = strArray[1];

• Have implicit length field strArray.length ? 2

Similar to:

A field, not a method!

Identifiers

• Everything has a globally-unique name Java.lang.String Java.util.Hashtable Java.applet.Applet EDU.Washington.grad.gjb.cassowary.Variable.toString()

• Pretty wordy, so...

Package name

Class name

Method name

import statement

• Two forms:
  • import java.util.HashTable;Just make the HashTable class available from package java.util
  • import EDU.Washington.grad.gjb.cassowary.*;Make all classes from package available on demand

• Always an implicit "import java.lang.*"

• Permits using simple (short) names
  • Not like C++'s "#include"
  • More like C++'s "using namespace "

How JavaFinds a Class...

• Package names mirror the directory structure

• package statement informs the compiler

package EDU.Washington.grad.gjb.cassowary;

public class Variable extends AbstractVariable {

. . .

}

class Helper { … }

./EDU/Washington/grad/gjb/cassowary/Variable.java

Compilationof Source File

% ls

Variable.java

% javac Variable.java

% ls

Variable.java

Variable.class

Helper.class

One java source file

may create multiple

.class files containing

the byte-compiled code

Class Access Protection

package EDU.Washington.grad.gjb.cassowary;

public class Variable extends AbstractVariable {

. . .

}

class Helper { … }

• Only one public class per file

• No specifier ? package protection visible to all classes in the package no “package” keyword — remember it is a statement

Not even friends cantouch Java’s private parts

public class Point {

private int x, y;

void setXY(int x, int y) {

this.x = x; this.y = y;

}

protected void move(int x, int y) {

setXY(this.x+x, this.y+y);

}

public int getX() { return x; }

public int getY() { return y; }

}

Y N N N private

Y Y N N package

Y Y Y N protected

Y Y Y Y public

same class

class in same package

subclass in different package

non-subclass,

different package

Ref: Java In a Nutshell, O’Reilly

Java Accessibility vs. C++

• No “friend” keyword

• Every field or method has an access specifier (no “public:” sections)

• Default is package-visibility which has no associated keyword (not private)

No Need forForward Declarations

public class Point {

private PointColor c;

// setXY(int,int) used below before its definition in the source

protected void move(int x, int y) { setXY(this.x+x, this.y+y); }

void setXY(int x, int y) { this.x = x; this.y = y; }

private int x, y;

} // no trailing semicolon (C++ requires one)

// PointColor already used above before this definition

class PointColor {

byte red, green, blue;

}

Definition

Use

Final Fields

public final class Circle {

private final double MY_PI = 3.1415;

public double area() { return MY_PI * r*r; }

}

• final fields correspond to C++’s “const”

• final fields cannot be changed once initialized

• final on formal function parameters is not part of the function signature (just implementation detail)

Ball and CBall Example

package BallExample;

public class Ball implements Bounceable {

private int x, y;

public Ball(int x_, int y_) {

x = x_; y = y_;

}

public void Bounce() {

System.err.println("Ball bounces");

}

static public void ClassFn() {

System.err.println("Ball.ClassFn()");

}

}

package BallExample;

public class CBall extends Ball {

private int colorSelector;

public CBall(int x, int y) {

super(x,y); // chain constructors

colorSelector = 0; // for black

}

public void Bounce() {

System.err.println(”CBall bounces");

}

static public void ClassFn() {

System.err.println(”CBall.ClassFn()");

}

}

BallExample/Ball.java

BallExample/CBall.java

Inheritance Mechanisms

• extends superclass
  • similar to “: public” in C++
  • for expressing an “is-a” relation

• implements superinterface
  • similar in use to C++’s multiple inheritance
  • for expressing an “is-capable-of” or “knows-how-to” relation

Java Interfaces

public interface Bounceable {

public void Bounce();

private void BounceNow(); // error

}

public interface BounceDropable

extends Bounceable {

public void Drop();

}

• Interfaces can only specify public methods

• Similar to protocols in Smalltalk

• May be used as a type for a variable

• Can specify sub-interfaces and can extend multiple interfaces at a time

Bounceable Interface

package BallExample;

public interface Bounceable {

public void Bounce();

}

package BallExample;

public class BallTest {

public static void main(String[] args) {

Ball b1 = new Ball(10,10);

Ball b2 = new CBall(20,20);

Bounceable b3 = new Ball(30,30);

Bounceable b4 = new CBall(40,40);

b1.Bounce(); b2.Bounce();

b3.Bounce(); b4.Bounce();

b1.ClassFn(); b2.ClassFn();

b3.ClassFn(); b4.ClassFn();

CBall cb1 = (CBall) b1;

CBall cb2 = (CBall) b2;

cb2.ClassFn();

} } // end class

BallExample/Bounceable.java

BallExample/BallTest.java

Errors?

Output?

Ball ExampleOutput and Errors

package BallExample;

public class BallTest {

public static void main(String[] args) {

Ball b1 = new Ball(10,10);

Ball b2 = new CBall(20,20);

Bounceable b3 = new Ball(30,30);

Bounceable b4 = new CBall(40,40);

b1.Bounce(); b2.Bounce();

b3.Bounce(); b4.Bounce();

b1.ClassFn(); b2.ClassFn();

// compile time errors

// b3.ClassFn(); b4.ClassFn();

// CBall cb1 = (CBall) b1; ClassCastException

CBall cb2 = (CBall) b2; // ok

cb2.ClassFn();

} } // end class

BallExample/BallTest.java

% java BallExample.BallTest

Ball bounces

CBall bounces

Ball bounces

CBall bounces

Ball.ClassFn()

Ball.ClassFn()

CBall.ClassFn()

Types vs. Classes

• Types are a compile-time notion
  • variables have types
  • used for checking validity of method invocations
  • may be an interface

• Classes are a run-time notion
  • objects (i.e. instances) have classes
  • used for dynamic dispatch(binding of non-static function call)
  • Each class has a corresponding type —that hierarchy of types mirrors the class hierarchy

Multiple Inheritance in Java

• A Java class can extend (subclass) another class and implement multiple interfaces

public class TopLevelWindow extends Window

implements Drawable, Cloneable, Streamable

{ . . . }

Drawable

Cloneable

Streamable

Window

TopLevelWindow

is-a

Abstract Methodsand Abstract Classes

// Note abstract keyword is used for the class, too

public abstract class Shape {

public abstract void rotate(int); // no definition

public abstract double area(); // no definition

}

• abstract methods correspond to C++’s “pure virtual functions”(But C++ uses “=0” syntax, and permits an implementation)

• abstract methods must be overridden in concrete subclasses

• Only abstract classes can have abstract methods(C++ infers abstract classes, Java requires you mark the class explicitly)

Final Methods

public class Circle {

….

public final double area() { return Math.PI * r*r; }

double r; // radius

}

• final methods cannot be overriden

• final methods may be inlined (no “inline” keyword)

• similar to non-virtual member functions in C++(but those can be overriden, they just do not dispatch dynamically)

Final Classes

public final class Circle {

….

public double area() { return Math.PI * r*r; }

double r; // radius

}

• final classes cannot be subclassed — they are leafs in the class hierarchy

• methods in final classes are implicitly final

• provides compiler with optimization opportunities

try { throw } andcatch, finally (exceptions)

class ExceptionExample {

static public void main(String args[]) {

try {

// allocate some resource (besides memory)

doSomething();

if (!FThingsAreOkay()) {

throw new RuntimeException("Things not ok");

}

doSomethingElse();

} catch (RuntimeException e) {

System.err.println("Runtime Exception: " + e);

} catch (Exception e) { // similar to "catch (..)" in C++

System.err.println("Exception: " + e);

} finally { // finally is not in C++

// cleanup resource

} }

Exception Hierarchy

Object

Throwable

Exception

ClassNotFoundException

RuntimeException

InterruptedException

NoSuchMethodException

CloneNotSupportedException

IllegalAccessException

InstantiationException

NoSuchFieldException

ArithmeticException

NullPointerException

IndexOutOfBoundsException

ClassCastException

Threads

public class Pendulum extends Applet implements Runnable {

private Thread myThread;

public void start() {

if (myThread == null) {

myThread = new Thread(this,"Pendulum");

myThread.start();

}

}

public void run() {

while (myThread != null) {

try { myThread.sleep(100); }

catch (InterruptedException e) { /* do nothing */ }

myRepaint();

}

}

public void stop() { myThread.stop(); myThread = null; }

}

set thread's target to this Pendulum class, and use its run() method

Ref: Boone's Java Essentials for C and C++ Programmers

Summary:What Java Left Out from C++

• No stack objects, only heap objects

• No destructors, only finalize() method

• No pointers, everything is a reference

• No delete, garbage collector instead

• No const, only final (methods, fields, classes)

• No templates, no preprocessor

• No operator overloading

• No multiple inheritance of classes

• No enumerations or typedefs

Summary:What Java Put In (vs. C++)

• Garbage collector

• Object-rooted, rich class hierarchyStrings, first-class arrays with bounds checking

• Package system with import

• interface, implements, extends, abstract

• finally blocks, static/instance initializers

• Secure and portable JavaVM, threads

• Dynamic reflection capabilities, inner classes

• JavaDoc system