Primitive Types vs. Reference Types

Variables of primitive types hold the values of the primitive types directly. Variables of reference types hold references (pointers) to objects, which resides in the garbage-collected heap.

Garbage Collection

Programmers are not responsible for deallocating memory.When an object is no longer accessible from anywhere in the program, it becomes garbage.The garbage-collector in the Java virtual machine automatically reclaims the memory occupied by garbage.System.gc()

suggest to the Java virtual machine to do a garbage-collection

finalize() invoked by the Java runtime just before the object is

garbage-collected

Null Reference

null

A reference does not refer to any object.The followings are not the same: String s1 = null; String s2 = "";

Alias of Objects

Two or more reference variables refer to the same object.Example:ChessPiece bishop1 = new ChessPiece();ChessPiece bishop2 = new ChessPiece();

bishop1 bishop2

Alias of ObjectsAssignment of Reference Types

Before

bishop1 bishop2

After

bishop1 bishop2

bishop2 = bishop1;

Assignment of Primitive Types

Before

num1

5

num2

12

After

num1

5

num2

5

num2 = num1;

Parameter Passing

Parameters in a Java method are passed by valueThe actual parameters (the values passed in) are assigned to the formal parameters (declared in the method header)For a parameter of primitive types, the formal parameter is a copy of the actual parameter.For a parameter of reference types (objects), the formal parameter is an alias of the actual parameter.

Example: Num.java

public class Num { private int value;

public Num(int update) { value = update; } public void setValue(int update) { value = update; } public String toString() { return value + ""; }}

Example: ParameterTester.javapublic class ParameterTester { public void changeValues (int f1, Num f2, Num f3) { System.out.println("Before changing the values:"); System.out.println("f1\tf2\tf3"); System.out.println(f1 + "\t" + f2 + "\t" + f3 + "\n"); f1 = 999; f2.setValue(888); f3 = new Num (777); System.out.println("After changing the values:"); System.out.println("f1\tf2\tf3"); System.out.println(f1 + "\t" + f2 + "\t" + f3 + "\n"); }}

Example: ParameterPassing.java

public class ParameterPassing { public static void main (String[] args) { ParameterTester tester = new ParameterTester(); int a1 = 111; Num a2 = new Num (222); Num a3 = new Num (333); System.out.println("Before calling changeValues:"); System.out.println("a1\ta2\ta3"); System.out.println(a1 + "\t" + a2 + "\t" + a3 + "\n"); tester.changeValues(a1, a2, a3); System.out.println("After calling changeValues:"); System.out.println("a1\ta2\ta3"); System.out.println(a1 + "\t" + a2 + "\t" + a3 + "\n"); }}

Example: The OutputC:\Courses\CSC224\Examples>java ParameterPassingBefore calling changeValues:a1 a2 a3111 222 333

Before changing the values:f1 f2 f3111 222 333

After changing the values:f1 f2 f3999 888 777

After calling changeValues:a1 a2 a3111 888 333

Static Fields and Methods Static fields are per class, instead of per instance.Static methods can only use the static fields of the class.Static fields can be used by all methods.Static fields and static methods can be used without creating instances of the class:ClassName.staticFieldClassName.staticMethod(parameters, ...)

Static fields are shared among all the instances of the class.

Initialization of Static Fields

Static fields can be initialized in a static block: 

public class MyClass { private static int value;   static {     value = ...;   }   // ... }

InterfaceA Java interface consists of declarations of abstract methods and constants.An abstract method declaration is a method header without a method body.An interface defines a contact without an implementation.An interface cannot be instantiated, since it has no implementation.

Interface and Class

Interfaces are implemented by classes. A class that implements an interface is responsible for providing implementations of all the abstract methods declared in the interface.An interface provides a client's view of a software component, and a class implementing the interface provides the implementer's view of the software component.

Interface Examples

public interface Comparable {   public int compareTo(Object o); }

public interface Iterator {   public boolean hasNext();   public Object next();   public void remove(); }

Java Applet

Java programs that run in web browsers.Embedded into an HTML file using the <applet> tag.No main() methodMust extend the Applet classThere are several special methods that serve specific purposes

Drawing

public void paint(Graphics page) Automatically executed to draw the

applets contents A Graphics object defines a graphics

context on which we can draw shapes and text

The Graphics class has several methods for drawing shapes

Drawing a LineX

Y

10

20

150

45

page.drawLine (10, 20, 150, 45);

page.drawLine (150, 45, 10, 20);

oror

Drawing a RectangleX

Y

page.drawRect (50, 20, 100, 40);

50

20

100

40

Drawing an OvalX

Y

page.drawOval (175, 20, 50, 80);

175

20

50

80

boundingboundingrectanglerectangle

Example: Snowman.javaimport java.applet.Applet;import java.awt.*;public class Snowman extends Applet { public void paint (Graphics page) { final int MID = 150; final int TOP = 50; setBackground (Color.cyan); page.setColor (Color.blue); page.fillRect (0, 175, 300, 50); // ground page.setColor (Color.yellow); page.fillOval (-40, -40, 80, 80); // sun page.setColor (Color.white); page.fillOval (MID-20, TOP, 40, 40); // head page.fillOval (MID-35, TOP+35, 70, 50); // torso page.fillOval (MID-50, TOP+80, 100, 60);// torso

Example: Snowman.java page.setColor (Color.black); page.fillOval (MID-10, TOP+10, 5, 5);// left eye page.fillOval (MID+5, TOP+10, 5, 5); // right eye page.drawArc (MID-10, TOP+20, 20, 10, 190, 160); // smile page.drawLine (MID-25, TOP+60, MID-50, TOP+40); // left arm page.drawLine (MID+25, TOP+60, MID+55, TOP+60); // right arm page.drawLine (MID-20, TOP+5, MID+20, TOP+5); // brim of hat page.fillRect (MID-15, TOP-20, 30, 25); // top of hat }}

Example: HTML Page

<HTML><HEAD><TITLE>The Snowman Applet</TITLE></HEAD><BODY><center><H3>The Snowman Applet</H3>

<applet code="Snowman.class" width=300 height=225></applet>

</center>

</BODY></HTML>

Compile and Run Applets

Compile: javac Snowman.java

Run: appletviewer Snowman.html

In browser: File -> Open page -> Snowman.html