如何！=和==运算符在Java中使用整数？ [英] How != and == operators work on Integers in Java?

问题描述

以下代码似乎让我感到困惑，因为它提供了两个不同的输出。代码在jdk 1.7上测试。

The following code seemed really confusing to me since it provided two different outputs.The code was tested on jdk 1.7.

public class NotEq {

public static void main(String[] args) {

    ver1();
    System.out.println();
    ver2();
}

public static void ver1() {
    Integer a = 128;
    Integer b = 128;

    if (a == b) {
        System.out.println("Equal Object");
    }

    if (a != b) {
        System.out.println("Different objects");
    }

    if (a.equals(b)) {
        System.out.println("Meaningfully equal.");
    }
}

public static void ver2() {
    Integer i1 = 127;
    Integer i2 = 127;
    if (i1 == i2) {
        System.out.println("Equal Object");
    }

    if (i1 != i2){
        System.out.println("Different objects");
    }
    if (i1.equals(i2)){
        System.out.println("Meaningfully equal");
    }
}

}

输出：

Output:

[ver1输出]

不同的对象< br>
有意义相等。

[ver1 output]
Different objects
Meaningfully equal.

[ver2输出]

等于对象

有意义相等

[ver2 output]
Equal Object
Meaningfully equal

为什么==和！=测试会为ver1（）和ver2（）产生不同的结果，因为相同的数字远小于Integer.MAX_VALUE？可以断定==检查大于127的数字（对于代码中显示的整数等包装类）是完全浪费时间吗？

Why the == and != testing produces different results for ver1() and ver2() for same number much less than the Integer.MAX_VALUE? Can it be concluded that == checking for numbers greater than 127 (for wrapper classes like Integer as shown in the code) is totally waste of time?

推荐答案

为-128到127之间的值缓存整数，因此 Integer i = 127 将始终返回相同的引用。 整数j = 128 不一定会这样做。然后，您需要使用 equals 来测试基础 int 的相等性。

Integers are cached for values between -128 and 127 so Integer i = 127 will always return the same reference. Integer j = 128 will not necessarily do so. You will then need to use equals to test for equality of the underlying int.

这是的一部分Java语言规范：

如果框中的值p为true，false，字节或范围内的char \ u0000到\ u007f，或-128到127（含）之间的int或short数，然后让r1和r2为p的任意两次装箱转换的结果。总是这样的情况是r1 == r2。

If the value p being boxed is true, false, a byte, or a char in the range \u0000 to \u007f, or an int or short number between -128 and 127 (inclusive), then let r1 and r2 be the results of any two boxing conversions of p. It is always the case that r1 == r2.

但是2次调用整数j = 128 可能会返回相同的引用（不保证）：

But 2 calls to Integer j = 128 might return the same reference (not guaranteed):

较少内存限制的实现可能会缓存所有char和短值，以及-32K到+ 32K范围内的int和long值。

Less memory-limited implementations might, for example, cache all char and short values, as well as int and long values in the range of -32K to +32K.

这篇关于如何！=和==运算符在Java中使用整数？的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！