Java/Class/class object
Содержание
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>