java比较相等符

public class Test1 {

    /**
     * @param args
     */
    public static void main(String[] args){
        int a = 1000, b = 1000;
        System.out.println(a == b);

        Integer c = 1000, d = 1000;
        System.out.println(c==d);

        Integer e = 100, f = 100;
        System.out.println(e == f);         
    }
}

运行结果：

true

false

true

结果分析：

查看Test1.class文件如下：

public class Test1 extends java.lang.Object{
public Test1();
  Code:
   0:   aload_0
   1:   invokespecial   #1; //Method java/lang/Object."<init>":()V
   4:   return

public static void main(java.lang.String[]);
  Code:
   0:   sipush  1000
   3:   istore_1
   4:   sipush  1000
   7:   istore_2
   8:   getstatic       #2; //Field java/lang/System.out:Ljava/io/PrintStream;
   11:  iload_1
   12:  iload_2
   13:  if_icmpne       20
   16:  iconst_1
   17:  goto    21
   20:  iconst_0
   21:  invokevirtual   #3; //Method java/io/PrintStream.println:(Z)V
   24:  sipush  1000
   27:  invokestatic    #4; //Method java/lang/Integer.valueOf:(I)Ljava/lang/Integer;
   30:  astore_3
   31:  sipush  1000
   34:  invokestatic    #4; //Method java/lang/Integer.valueOf:(I)Ljava/lang/Integer;
   37:  astore  4
   39:  getstatic       #2; //Field java/lang/System.out:Ljava/io/PrintStream;
   42:  aload_3
   43:  aload   4
   45:  if_acmpne       52
   48:  iconst_1
   49:  goto    53
   52:  iconst_0
   53:  invokevirtual   #3; //Method java/io/PrintStream.println:(Z)V
   56:  bipush  100
   58:  invokestatic    #4; //Method java/lang/Integer.valueOf:(I)Ljava/lang/Integer;
   61:  astore  5
   63:  bipush  100
   65:  invokestatic    #4; //Method java/lang/Integer.valueOf:(I)Ljava/lang/Integer;
   68:  astore  6
   70:  getstatic       #2; //Field java/lang/System.out:Ljava/io/PrintStream;
   73:  aload   5
   75:  aload   6
   77:  if_acmpne       84
   80:  iconst_1
   81:  goto    85
   84:  iconst_0
   85:  invokevirtual   #3; //Method java/io/PrintStream.println:(Z)V
   88:  return
}

由上述字节码我们可以看出 int a = 1000, b = 1000;System.out.println(a == b);执行的是if_icmpne命令，比较的是2个int值是否相等。在此返回为true。

而 Integer c = 1000, d = 1000;System.out.println(c==d);执行的是if_acmpne命令，比较的是对象引用地址。查看jdk源码

    public static Integer valueOf(int i) {
    final int offset = 128;
    if (i >= -128 && i <= 127) { // must cache
        return IntegerCache.cache[i + offset];
    }
        return new Integer(i);
    }

此时的值为1000，不在-128~127范围内。所以变量c和d是2个不同的对象。因此System.out.println(c==d);返回结果为false。

而 Integer e = 100, f = 100;System.out.println(e == f);  用的是Integer缓存中的同一对象，因此返回结果为true。

今天又是学习了。。。以前没有注意这个问题

转载与：http://blog.csdn.net/jeamking/article/details/6883623