代碼

 1 @Slf4j
 2 public class IntegerCompare {
 3     public static void doCompare() {
 4 
 5         Integer a = 127; //Integer.valueOf(127)
 6         Integer b = 127; //Integer.valueOf(127)
 7         log.info("\nInteger a = 127;\n" +
 8                 "Integer b = 127;\n" +
 9                 "a == b ? {}", a == b);    // true
10 
11         Integer c = 128; //Integer.valueOf(128)
12         Integer d = 128; //Integer.valueOf(128)
13         log.info("\nInteger c = 128;\n" +
14                 "Integer d = 128;\n" +
15                 "c == d ? {}", c == d);   //false
16 
17         Integer e = 127; //Integer.valueOf(127)
18         Integer f = new Integer(127); //new instance
19         log.info("\nInteger e = 127;\n" +
20                 "Integer f = new Integer(127);\n" +
21                 "e == f ? {}", e == f);   //false
22 
23         Integer g = new Integer(127); //new instance
24         Integer h = new Integer(127); //new instance
25         log.info("\nInteger g = new Integer(127);\n" +
26                 "Integer h = new Integer(127);\n" +
27                 "g == h ? {}", g == h);  //false
28 
29         Integer i = 128; //unbox
30         int j = 128;
31         log.info("\nInteger i = 128;\n" +
32                 "int j = 128;\n" +
33                 "i == j ? {}", i == j); //true
34     }
35 }

結果：

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

結論：

• =比較的是引用

• equal比較的是integer實際的值

• 

第一個相等，第二個不相等，是因為Interger.valueOf內部做了緩存（-127，127）。

• a, b相同的引用。
• c，d超過了緩存的範圍直接new，產生不同的引用，所以不相等。

 Integer e = 127;

Integer f = new Integer(127);

e從緩存中讀取。f則是新的對象，用=判斷時候是兩個不同的引用，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;
把裝箱的 Integer 和基本類型 int 比較，前者會先拆箱再比較，比較的肯定是數值而不是引用，因此返回 true