Java有意思的知识点

今天在论坛上看到的几个题，有几个貌似还挺有意思的。

 
public static void test() {  
    String x = "hello";  
    String y = "world";  
    String z = new String("helloworld");  
    String a = "helloworld"; 
    System.out.println("x == hello:" + (x == "hello"));  
    System.out.println("a == helloworld:" + (a == "hello" + "world"));  
    System.out.println("a == x+y:" + (a == (x + y)));  
}  

x == hello:true
a == helloworld:true
a == x+y:false
请看翻译过的源码：

public static void test();
      0  ldc <String "hello"> [15]
      2  astore_0 [x]
      3  ldc <String "world"> [17]
      5  astore_1 [y]
      6  ldc <String "helloworld"> [19]
      8  astore_2 [a]
      9  getstatic java.lang.System.out : java.io.PrintStream [21]
     12  new java.lang.StringBuilder [27]
     15  dup
     16  ldc <String "x == hello:"> [29]
     18  invokespecial java.lang.StringBuilder(java.lang.String) [31]
     21  aload_0 [x]
     22  ldc <String "hello"> [15]
     24  if_acmpne 31
     27  iconst_1
     28  goto 32
     31  iconst_0
     32  invokevirtual java.lang.StringBuilder.append(boolean) : java.lang.StringBuilder [34]
     35  invokevirtual java.lang.StringBuilder.toString() : java.lang.String [38]
     38  invokevirtual java.io.PrintStream.println(java.lang.String) : void [42]
     41  getstatic java.lang.System.out : java.io.PrintStream [21]
     44  new java.lang.StringBuilder [27]
     47  dup
     48  ldc <String "a == helloworld:"> [47]
     50  invokespecial java.lang.StringBuilder(java.lang.String) [31]
     53  aload_2 [a]
     54  ldc <String "helloworld"> [19]
     56  if_acmpne 63
     59  iconst_1
     60  goto 64
     63  iconst_0
     64  invokevirtual java.lang.StringBuilder.append(boolean) : java.lang.StringBuilder [34]
     67  invokevirtual java.lang.StringBuilder.toString() : java.lang.String [38]
     70  invokevirtual java.io.PrintStream.println(java.lang.String) : void [42]
     73  getstatic java.lang.System.out : java.io.PrintStream [21]
     76  new java.lang.StringBuilder [27]
     79  dup
     80  ldc <String "a == x+y:"> [49]
     82  invokespecial java.lang.StringBuilder(java.lang.String) [31]
     85  aload_2 [a]
     86  new java.lang.StringBuilder [27]
     89  dup
     90  aload_0 [x]
     91  invokestatic java.lang.String.valueOf(java.lang.Object) : java.lang.String [51]
     94  invokespecial java.lang.StringBuilder(java.lang.String) [31]
     97  aload_1 [y]
     98  invokevirtual java.lang.StringBuilder.append(java.lang.String) : java.lang.StringBuilder [57]
    101  invokevirtual java.lang.StringBuilder.toString() : java.lang.String [38]
    104  if_acmpne 111
    107  iconst_1
    108  goto 112
    111  iconst_0
    112  invokevirtual java.lang.StringBuilder.append(boolean) : java.lang.StringBuilder [34]
    115  invokevirtual java.lang.StringBuilder.toString() : java.lang.String [38]
    118  invokevirtual java.io.PrintStream.println(java.lang.String) : void [42]
    121  return

对于String常量，包括“hello”+“world”这种方式的的值（第54行），都被设置为常量。然而对于x+y这种方式，其实首先是创建了一个StringBuilder，然后把两个变量都加到StringBuilder内，再转换成String。此时，就形成了一个新的String对象了。equals方法也就不适用了。

三、Override覆盖

public class Parent {  
  
    public static String say() {  
        return "parent static say";  
    }  
  
    public String say2() {  
        return "parent say";  
    }  
}  
  
public class Child extends Parent {  
    public static String say() {  
        return "child static say";  
    }  
  
    public String say2() {  
        return "child say";  
    }  
}  

public class OverrideTest {  
  
    public static void main(String[] args) {  
        Parent p = new Child();  
        System.out.println(p.say());  
        System.out.println(p.say2());  
  
    }  
  
}  

parent static say
child say

所谓静态方法，并不仅仅指该方法在所有实例中只有一份，同时也指该方法是“静态”加载的，即在编译期就已决定其行为。此处p的静态类型为Parent，所以它所调用的方法也在编译期和Parent的say()方法绑定：

  // Method descriptor #15 ([Ljava/lang/String;)V
  // Stack: 2, Locals: 2
  public static void main(java.lang.String[] args);
     0  new com.ibm.oneteam.Child [16]
     3  dup
     4  invokespecial com.ibm.oneteam.Child() [18]
     7  astore_1 [p]
     8  getstatic java.lang.System.out : java.io.PrintStream [19]
    11  invokestatic com.ibm.oneteam.Parent.say() : java.lang.String [25]
    14  invokevirtual java.io.PrintStream.println(java.lang.String) : void [31]
    17  getstatic java.lang.System.out : java.io.PrintStream [19]
    20  aload_1 [p]
    21  invokevirtual com.ibm.oneteam.Parent.say2() : java.lang.String [37]
    24  invokevirtual java.io.PrintStream.println(java.lang.String) : void [31]
    27  return

六、提前引用

public class ForwardReference {  
  
    static int first = test();  
    static int second = 2;  
  
    static int test() {  
        return second;  
    }  
  
    public static void main(String[] args) {  
        System.out.println("first = " + first);  
    }  
  
}  

first = 0

这段代码有点诡异，但明白万变不离其宗，只要牢记静态方法&变量是“静态”加载的，即编译时就决定的即可。当然，对于静态变量的这种先定义后赋值的行为，还是需要小心：

public class com.ibm.oneteam.Main {
  
  // Field descriptor #6 I
  static int first;
  
  // Field descriptor #6 I
  static int second;
  
  // Method descriptor #9 ()V
  // Stack: 1, Locals: 0
  static {};
     0  invokestatic com.ibm.oneteam.Main.test() : int [11]
     3  putstatic com.ibm.oneteam.Main.first : int [15]
     6  iconst_2
     7  putstatic com.ibm.oneteam.Main.second : int [17]
    10  return
      Line numbers:
        [pc: 0, line: 5]
        [pc: 6, line: 6]
        [pc: 10, line: 3]
  
  // Method descriptor #9 ()V
  // Stack: 1, Locals: 1
  public Main();
    0  aload_0 [this]
    1  invokespecial java.lang.Object() [22]
    4  return
      Line numbers:
        [pc: 0, line: 3]
      Local variable table:
        [pc: 0, pc: 5] local: this index: 0 type: com.ibm.oneteam.Main
  
  // Method descriptor #14 ()I
  // Stack: 1, Locals: 0
  static int test();
    0  getstatic com.ibm.oneteam.Main.second : int [17]
    3  ireturn
      Line numbers:
        [pc: 0, line: 9]

看这段代码也许更加直观点：

public class Main {

    static int first = test();    
    static int second = 2;    
    
    static int test() {    
        return second;    
    } 
    
    static int third = 3;
    
    public static void main(String[] args) {
        System.out.println("first = " + first);
    }
}

编译后的代码：

// Compiled from Main.java (version 1.6 : 50.0, super bit)
public class com.ibm.oneteam.Main {
  
  // Field descriptor #6 I
  static int first;
  
  // Field descriptor #6 I
  static int second;
  
  // Field descriptor #6 I
  static int third;
  
  // Method descriptor #10 ()V
  // Stack: 1, Locals: 0
  static {};
     0  invokestatic com.ibm.oneteam.Main.test() : int [12]
     3  putstatic com.ibm.oneteam.Main.first : int [16]
     6  iconst_2
     7  putstatic com.ibm.oneteam.Main.second : int [18]
    10  iconst_3
    11  putstatic com.ibm.oneteam.Main.third : int [20]
    14  return
      Line numbers:
        [pc: 0, line: 5]
        [pc: 6, line: 6]
        [pc: 10, line: 12]
        [pc: 14, line: 3]
  
  // Method descriptor #10 ()V
  // Stack: 1, Locals: 1
  public Main();
    0  aload_0 [this]
    1  invokespecial java.lang.Object() [25]
    4  return
      Line numbers:
        [pc: 0, line: 3]
      Local variable table:
        [pc: 0, pc: 5] local: this index: 0 type: com.ibm.oneteam.Main
  
  // Method descriptor #15 ()I
  // Stack: 1, Locals: 0
  static int test();
    0  getstatic com.ibm.oneteam.Main.second : int [18]
    3  ireturn
      Line numbers:
        [pc: 0, line: 9]
  
  // Method descriptor #30 ([Ljava/lang/String;)V
  // Stack: 4, Locals: 1
  public static void main(java.lang.String[] args);
     0  getstatic java.lang.System.out : java.io.PrintStream [31]
     3  new java.lang.StringBuilder [37]
     6  dup
     7  ldc <String "first = "> [39]
     9  invokespecial java.lang.StringBuilder(java.lang.String) [41]
    12  getstatic com.ibm.oneteam.Main.first : int [16]
    15  invokevirtual java.lang.StringBuilder.append(int) : java.lang.StringBuilder [44]
    18  invokevirtual java.lang.StringBuilder.toString() : java.lang.String [48]
    21  invokevirtual java.io.PrintStream.println(java.lang.String) : void [52]
    24  return
      Line numbers:
        [pc: 0, line: 15]
        [pc: 24, line: 16]
      Local variable table:
        [pc: 0, pc: 25] local: args index: 0 type: java.lang.String[]
}

七、对象引用

public class TestRef {  
  
    public static void main(String[] args) {  
        StringBuffer a = new StringBuffer("a");  
        StringBuffer b = new StringBuffer("b");  
        append(a, b);  
        System.out.println(a.toString() + "," + b.toString());  
        b = a;  
        System.out.println(a.toString() + "," + b.toString());  
    }  
  
    public static void append(StringBuffer a, StringBuffer b) {  
        a.append(b);  
        b = a;  
    }  
}  

ab,b
ab,ab
这段代码就十分简单了，明白了引用拷贝就知道原因了。

评论

1 楼 daly1987 2011-12-18  

不错啊，加油