Java包装器相等性测试 [英] Java Wrapper equality test

问题描述

  public class WrapperTest {

    public static void main(String[] args) {

        Integer i = 100;
        Integer j = 100;

        if(i == j)
            System.out.println("same");
        else
            System.out.println("not same");
    }

   }

上面的代码在运行时给出了same的输出，但是，如果我们将i和j的值更改为1000，则输出将更改为not same.在为SCJP进行准备时，需要弄清楚这一概念.有人可以解释这种行为.谢谢.

The above code gives the output of same when run, however if we change the value of i and j to 1000 the output changes to not same. As I'm preparing for SCJP, need to get the concept behind this clear. Can someone explain this behavior.Thanks.

推荐答案

在Java中，-128至127(含)之间的整数通常由相同的Integer对象实例表示.这可以通过使用称为IntegerCache的内部类(包含在Integer类中，并在例如调用Integer.valueOf()时或在自动装箱期间使用)来处理:

In Java, Integers between -128 and 127 (inclusive) are generally represented by the same Integer object instance. This is handled by the use of a inner class called IntegerCache (contained inside the Integer class, and used e.g. when Integer.valueOf() is called, or during autoboxing):

private static class IntegerCache {
    private IntegerCache(){}

    static final Integer cache[] = new Integer[-(-128) + 127 + 1];

    static {
        for(int i = 0; i < cache.length; i++)
            cache[i] = new Integer(i - 128);
    }
}

另请参见: http://www.owasp.org/index.php/Java_gotchas

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