Java/Class/class object

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A changeable wrapper class

   <source lang="java">

// : appendixa:MutableInteger.java // A changeable wrapper class. // From "Thinking in Java, 3rd ed." (c) Bruce Eckel 2002 // www.BruceEckel.ru. See copyright notice in CopyRight.txt. import java.util.ArrayList; import java.util.List; class IntValue {

 private int n;
 public IntValue(int x) {
   n = x;
 }
 public int getValue() {
   return n;
 }
 public void setValue(int n) {
   this.n = n;
 }
 public void increment() {
   n++;
 }
 public String toString() {
   return Integer.toString(n);
 }

} public class MutableInteger {

 public static void main(String[] args) {
   List v = new ArrayList();
   for (int i = 0; i < 10; i++)
     v.add(new IntValue(i));
   System.out.println(v);
   for (int i = 0; i < v.size(); i++)
     ((IntValue) v.get(i)).increment();
   System.out.println(v);
 }

} ///:~


      </source>
   
  
 
  



A companion class to modify immutable objects

   <source lang="java">

// : appendixa:Immutable2.java // A companion class to modify immutable objects. // From "Thinking in Java, 3rd ed." (c) Bruce Eckel 2002 // www.BruceEckel.ru. See copyright notice in CopyRight.txt. class Mutable {

 private int data;
 public Mutable(int initVal) {
   data = initVal;
 }
 public Mutable add(int x) {
   data += x;
   return this;
 }
 public Mutable multiply(int x) {
   data *= x;
   return this;
 }
 public Immutable2 makeImmutable2() {
   return new Immutable2(data);
 }

} public class Immutable2 {

 private int data;
 public Immutable2(int initVal) {
   data = initVal;
 }
 public int read() {
   return data;
 }
 public boolean nonzero() {
   return data != 0;
 }
 public Immutable2 add(int x) {
   return new Immutable2(data + x);
 }
 public Immutable2 multiply(int x) {
   return new Immutable2(data * x);
 }
 public Mutable makeMutable() {
   return new Mutable(data);
 }
 public static Immutable2 modify1(Immutable2 y) {
   Immutable2 val = y.add(12);
   val = val.multiply(3);
   val = val.add(11);
   val = val.multiply(2);
   return val;
 }
 // This produces the same result:
 public static Immutable2 modify2(Immutable2 y) {
   Mutable m = y.makeMutable();
   m.add(12).multiply(3).add(11).multiply(2);
   return m.makeImmutable2();
 }
 public static void main(String[] args) {
   Immutable2 i2 = new Immutable2(47);
   Immutable2 r1 = modify1(i2);
   Immutable2 r2 = modify2(i2);
   System.out.println("i2 = " + i2.read());
   System.out.println("r1 = " + r1.read());
   System.out.println("r2 = " + r2.read());
 }

} ///:~


      </source>
   
  
 
  



Create Object Demo

   <source lang="java">

/* From http://java.sun.ru/docs/books/tutorial/index.html */ /*

* Copyright (c) 2006 Sun Microsystems, Inc. All Rights Reserved.
*
* Redistribution and use in source and binary forms, with or without
* modification, are permitted provided that the following conditions are met:
*
* -Redistribution of source code must retain the above copyright notice, this
*  list of conditions and the following disclaimer.
*
* -Redistribution in binary form must reproduce the above copyright notice,
*  this list of conditions and the following disclaimer in the documentation
*  and/or other materials provided with the distribution.
*
* Neither the name of Sun Microsystems, Inc. or the names of contributors may
* be used to endorse or promote products derived from this software without
* specific prior written permission.
*
* This software is provided "AS IS," without a warranty of any kind. ALL
* EXPRESS OR IMPLIED CONDITIONS, REPRESENTATIONS AND WARRANTIES, INCLUDING
* ANY IMPLIED WARRANTY OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE
* OR NON-INFRINGEMENT, ARE HEREBY EXCLUDED. SUN MIDROSYSTEMS, INC. ("SUN")
* AND ITS LICENSORS SHALL NOT BE LIABLE FOR ANY DAMAGES SUFFERED BY LICENSEE
* AS A RESULT OF USING, MODIFYING OR DISTRIBUTING THIS SOFTWARE OR ITS
* DERIVATIVES. IN NO EVENT WILL SUN OR ITS LICENSORS BE LIABLE FOR ANY LOST
* REVENUE, PROFIT OR DATA, OR FOR DIRECT, INDIRECT, SPECIAL, CONSEQUENTIAL,
* INCIDENTAL OR PUNITIVE DAMAGES, HOWEVER CAUSED AND REGARDLESS OF THE THEORY
* OF LIABILITY, ARISING OUT OF THE USE OF OR INABILITY TO USE THIS SOFTWARE,
* EVEN IF SUN HAS BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGES.
*
* You acknowledge that this software is not designed, licensed or intended
* for use in the design, construction, operation or maintenance of any
* nuclear facility.
*/

public class CreateObjectDemo {

 public static void main(String[] args) {
   // create a point object and two rectangle objects
   Point origin_one = new Point(23, 94);
   Rectangle rect_one = new Rectangle(origin_one, 100, 200);
   Rectangle rect_two = new Rectangle(50, 100);
   // display rect_one"s width, height, and area
   System.out.println("Width of rect_one: " + rect_one.width);
   System.out.println("Height of rect_one: " + rect_one.height);
   System.out.println("Area of rect_one: " + rect_one.area());
   // set rect_two"s position
   rect_two.origin = origin_one;
   // display rect_two"s position
   System.out.println("X Position of rect_two: " + rect_two.origin.x);
   System.out.println("Y Position of rect_two: " + rect_two.origin.y);
   // move rect_two and display its new position
   rect_two.move(40, 72);
   System.out.println("X Position of rect_two: " + rect_two.origin.x);
   System.out.println("Y Position of rect_two: " + rect_two.origin.y);
 }

} class Point {

 public int x = 0;
 public int y = 0;
 // a constructor!
 public Point(int x, int y) {
   this.x = x;
   this.y = y;
 }

} class Rectangle {

 public int width = 0;
 public int height = 0;
 public Point origin;
 // four constructors
 public Rectangle() {
   origin = new Point(0, 0);
 }
 public Rectangle(Point p) {
   origin = p;
 }
 public Rectangle(int w, int h) {
   this(new Point(0, 0), w, h);
 }
 public Rectangle(Point p, int w, int h) {
   origin = p;
   width = w;
   height = h;
 }
 // a method for moving the rectangle
 public void move(int x, int y) {
   origin.x = x;
   origin.y = y;
 }
 // a method for computing the area of the rectangle
 public int area() {
   return width * height;
 }

}

      </source>
   
  
 
  



Demonstrates Reference objects

   <source lang="java">

// : c11:References.java // Demonstrates Reference objects // From "Thinking in Java, 3rd ed." (c) Bruce Eckel 2002 // www.BruceEckel.ru. See copyright notice in CopyRight.txt. import java.lang.ref.PhantomReference; import java.lang.ref.Reference; import java.lang.ref.ReferenceQueue; import java.lang.ref.SoftReference; import java.lang.ref.WeakReference; class VeryBig {

 private static final int SZ = 10000;
 private double[] d = new double[SZ];
 private String ident;
 public VeryBig(String id) {
   ident = id;
 }
 public String toString() {
   return ident;
 }
 public void finalize() {
   System.out.println("Finalizing " + ident);
 }

} public class References {

 private static ReferenceQueue rq = new ReferenceQueue();
 public static void checkQueue() {
   Object inq = rq.poll();
   if (inq != null)
     System.out
         .println("In queue: " + (VeryBig) ((Reference) inq).get());
 }
 public static void main(String[] args) {
   int size = 10;
   // Or, choose size via the command line:
   if (args.length > 0)
     size = Integer.parseInt(args[0]);
   SoftReference[] sa = new SoftReference[size];
   for (int i = 0; i < sa.length; i++) {
     sa[i] = new SoftReference(new VeryBig("Soft " + i), rq);
     System.out.println("Just created: " + (VeryBig) sa[i].get());
     checkQueue();
   }
   WeakReference[] wa = new WeakReference[size];
   for (int i = 0; i < wa.length; i++) {
     wa[i] = new WeakReference(new VeryBig("Weak " + i), rq);
     System.out.println("Just created: " + (VeryBig) wa[i].get());
     checkQueue();
   }
   SoftReference s = new SoftReference(new VeryBig("Soft"));
   WeakReference w = new WeakReference(new VeryBig("Weak"));
   System.gc();
   PhantomReference[] pa = new PhantomReference[size];
   for (int i = 0; i < pa.length; i++) {
     pa[i] = new PhantomReference(new VeryBig("Phantom " + i), rq);
     System.out.println("Just created: " + (VeryBig) pa[i].get());
     checkQueue();
   }
 }

} ///:~


      </source>
   
  
 
  



Examination of the way the class loader works

   <source lang="java">

// : c10:SweetShop.java // Examination of the way the class loader works. // From "Thinking in Java, 3rd ed." (c) Bruce Eckel 2002 // www.BruceEckel.ru. See copyright notice in CopyRight.txt. class Candy {

 static {
   System.out.println("Loading Candy");
 }

} class Gum {

 static {
   System.out.println("Loading Gum");
 }

} class Cookie {

 static {
   System.out.println("Loading Cookie");
 }

} public class SweetShop {

 public static void main(String[] args) {
   System.out.println("inside main");
   new Candy();
   System.out.println("After creating Candy");
   try {
     Class.forName("Gum");
   } catch (ClassNotFoundException e) {
     System.out.println("Couldn"t find Gum");
   }
   System.out.println("After Class.forName(\"Gum\")");
   new Cookie();
   System.out.println("After creating Cookie");
 }

} ///:~


      </source>
   
  
 
  



Objects that cannot be modified are immune to aliasing

   <source lang="java">

// : appendixa:Immutable1.java // Objects that cannot be modified are immune to aliasing. // From "Thinking in Java, 3rd ed." (c) Bruce Eckel 2002 // www.BruceEckel.ru. See copyright notice in CopyRight.txt. public class Immutable1 {

 private int data;
 public Immutable1(int initVal) {
   data = initVal;
 }
 public int read() {
   return data;
 }
 public boolean nonzero() {
   return data != 0;
 }
 public Immutable1 multiply(int multiplier) {
   return new Immutable1(data * multiplier);
 }
 public static void f(Immutable1 i1) {
   Immutable1 quad = i1.multiply(4);
   System.out.println("i1 = " + i1.read());
   System.out.println("quad = " + quad.read());
 }
 public static void main(String[] args) {
   Immutable1 x = new Immutable1(47);
   System.out.println("x = " + x.read());
   f(x);
   System.out.println("x = " + x.read());
 }

} ///:~


      </source>
   
  
 
  



Passing objects to methods may not be what you"re used to.

   <source lang="java">

//: c03:PassObject.java // From "Thinking in Java, 3rd ed." (c) Bruce Eckel 2002 // www.BruceEckel.ru. See copyright notice in CopyRight.txt. class Letter {

 char c;

} public class PassObject {

 static void f(Letter y) {
   y.c = "z";
 }
 public static void main(String[] args) {
   Letter x = new Letter();
   x.c = "a";
   System.out.println("1: x.c: " + x.c);
   f(x);
   System.out.println("2: x.c: " + x.c);
 }

} ///:~


      </source>