Java Integer比较

代码

@Slf4j
public class IntegerCompare {
    public static void doCompare() {

        Integer a = 127; //Integer.valueOf(127)
        Integer b = 127; //Integer.valueOf(127)
        log.info("
Integer a = 127;
" +
                "Integer b = 127;
" +
                "a == b ? {}", a == b);    // true

        Integer c = 128; //Integer.valueOf(128)
        Integer d = 128; //Integer.valueOf(128)
        log.info("
Integer c = 128;
" +
                "Integer d = 128;
" +
                "c == d ? {}", c == d);   //false

        Integer e = 127; //Integer.valueOf(127)
        Integer f = new Integer(127); //new instance
        log.info("
Integer e = 127;
" +
                "Integer f = new Integer(127);
" +
                "e == f ? {}", e == f);   //false

        Integer g = new Integer(127); //new instance
        Integer h = new Integer(127); //new instance
        log.info("
Integer g = new Integer(127);
" +
                "Integer h = new Integer(127);
" +
                "g == h ? {}", g == h);  //false

        Integer i = 128; //unbox
        int j = 128;
        log.info("
Integer i = 128;
" +
                "int j = 128;
" +
                "i == j ? {}", i == j); //true
    }
}

结果：

Integer a = 127;
Integer b = 127;
a == b ? true
2020-04-13 22:30:47.162  INFO 4492 --- [  restartedMain] com.example.newdemo.IntegerCompare       : 
Integer c = 128;
Integer d = 128;
c == d ? false
2020-04-13 22:30:47.162  INFO 4492 --- [  restartedMain] com.example.newdemo.IntegerCompare       : 
Integer e = 127;
Integer f = new Integer(127);
e == f ? false
2020-04-13 22:30:47.162  INFO 4492 --- [  restartedMain] com.example.newdemo.IntegerCompare       : 
Integer g = new Integer(127);
Integer h = new Integer(127);
g == h ? false
2020-04-13 22:30:47.162  INFO 4492 --- [  restartedMain] com.example.newdemo.IntegerCompare       : 
Integer i = 128;
int j = 128;
i == j ? true

结论：

• =比较的是引用

• equal比较的是integer实际的值

• 

第一个相等，第二个不相等，是因为Interger.valueOf内部做了缓存（-127，127）。

• a, b相同的引用。
• c，d超过了缓存的范围直接new，产生不同的引用，所以不相等。

 Integer e = 127;

Integer f = new Integer(127);

e从缓存中读取。f则是新的对象，用=判断时候是两个不同的引用，所以不相等。

Integer g = new Integer(127);
Integer h = new Integer(127);
g和h都是新的引用。

Integer i = 128;
int j = 128;
把装箱的 Integer 和基本类型 int 比较，前者会先拆箱再比较，比较的肯定是数值而不是引用，因此返回 true