java学习惊奇之路

惊奇之一

import com.bruceeckel.simpletest.*;public class Equivalence {  static Test monitor = new Test();  public static void main(String[] args) {    Integer n1 = new Integer(47);    Integer n2 = new Integer(47);    System.out.println(n1 == n2);    System.out.println(n1 != n2);    monitor.expect(new String[] {      "false",      "true"    });  }}

我一开始以为输出的结果错了，后来看了书才晓得：尽管对象的内容是相同的，但是对象的引用是不同的，而==和！=比较的都是对象的引用，所以才会出现上述的输出结果。如果要比较两个对象的实际内容需要使用特殊的方法equals。n1.euqals(n2)

 

惊奇之二

//: c06:FinalOverridingIllusion.java// It only looks like you can override// a private or private final method.// From 'Thinking in Java, 3rd ed.' (c) Bruce Eckel 2002// www.BruceEckel.com. See copyright notice in CopyRight.txt.import com.bruceeckel.simpletest.*;class WithFinals {  // Identical to "private" alone:  private final void f() {    System.out.println("WithFinals.f()");  }  // Also automatically "final":  private void g() {    System.out.println("WithFinals.g()");  }}class OverridingPrivate extends WithFinals {  private final void f() {    System.out.println("OverridingPrivate.f()");  }  private void g() {    System.out.println("OverridingPrivate.g()");  }}class OverridingPrivate2 extends OverridingPrivate {  public final void f() {    System.out.println("OverridingPrivate2.f()");  }  public void g() {    System.out.println("OverridingPrivate2.g()");  }}public class FinalOverridingIllusion {  private static Test monitor = new Test();  public static void main(String[] args) {    OverridingPrivate2 op2 = new OverridingPrivate2();    op2.f();    op2.g();    // You can upcast:    OverridingPrivate op = op2;    // But you can't call the methods:    //! op.f();    //! op.g();    // Same here:    WithFinals wf = op2;    //! wf.f();    //! wf.g();    monitor.expect(new String[] {      "OverridingPrivate2.f()",      "OverridingPrivate2.g()"    });  }} ///:~

在OverridingPrivate2方法中此时并不是覆盖方法，而是生成了一个新的方法。由于private方法无法触及而且能够隐藏。所以在OverridingPrivate2中定义f、g时并不知道其他f、g的存在，所以这时候是生成一个新的方法。

 惊奇之三

//: c07:PolyConstructors.java// Constructors and polymorphism// don't produce what you might expect.// From 'Thinking in Java, 3rd ed.' (c) Bruce Eckel 2002// www.BruceEckel.com. See copyright notice in CopyRight.txt.import com.bruceeckel.simpletest.*;abstract class Glyph {  abstract void draw();  Glyph() {    System.out.println("Glyph() before draw()");    draw();    System.out.println("Glyph() after draw()");  }}class RoundGlyph extends Glyph {  private int radius = 1;  RoundGlyph(int r) {    radius = r;    System.out.println(      "RoundGlyph.RoundGlyph(), radius = " + radius);  }  void draw() {    System.out.println(      "RoundGlyph.draw(), radius = " + radius);  }}public class PolyConstructors {  private static Test monitor = new Test();  public static void main(String[] args) {    new RoundGlyph(5);    monitor.expect(new String[] {      "Glyph() before draw()",      "RoundGlyph.draw(), radius = 0",      "Glyph() after draw()",      "RoundGlyph.RoundGlyph(), radius = 5"    });  }} ///:~

java中复杂对象调用构造器要遵照以下顺序：

radius的初始值是0而不是1，主要是因为：上面的初始化顺序并不完整，