Integeter127与128

Posted 2020-08-19 jackkke

 1 package com.hangao.basic;
 2 
 3 /**
 4  * @author hangao [email protected]
 5  *
 6  */
 7 public class TestInt {
 8     public static void main(String[] args) {
 9         Integer ie1 = 127;
10         Integer ie2 = 127;
11         System.out.println("Integer 127==127:" + (ie1 == ie2));//Integer 127==127:true
12         
13         Integer ie3 = new Integer(127);
14         Integer ie4 = new Integer(127);
15         System.out.println("new Integer 127==127:" + (ie3 == ie4));//new Integer 127==127:false
16         
17         Integer ie5 = 128;
18         Integer ie6 = 128;
19         System.out.println("Integer 128==128:" + (ie5 == ie6));//Integer 128==128:false
20         
21         Integer ie7 = new Integer(128);
22         Integer ie8 = new Integer(128);
23         System.out.println("new Integer 128==128:" + (ie7 == ie8));//new Integer 128==128:false
24         
25         analysis();
26     }
27 
28     private static void analysis() {
29         Integer ie0 = 127;
30         System.out.println(ie0);
31         /* L0
32         LINENUMBER 5 L0
33         BIPUSH 127
34         INVOKESTATIC java/lang/Integer.valueOf (I)Ljava/lang/Integer;
35         ASTORE 1*/
36         /*从字节码文件可以看出此处有一个自动装箱的动作*/
37         
38         /*public static Integer valueOf(int i) {
39             if (i >= IntegerCache.low && i <= IntegerCache.high)
40                 return IntegerCache.cache[i + (-IntegerCache.low)];
41             return new Integer(i);
42         }*/
43         /*而Integeter这个类的装箱过程从Integeter源代码可以看出在(-128 - 127)之间的int值
44         其实返回的是IntegerCache里面的cache[]数组里面的对象*/
45         
46         /*private static class IntegerCache {
47             static final int low = -128;
48             static final int high;
49             static final Integer cache[];
50 
51             static {
52                 // high value may be configured by property
53                 int h = 127;
54                 String integerCacheHighPropValue =
55                     sun.misc.VM.getSavedProperty("java.lang.Integer.IntegerCache.high");
56                 if (integerCacheHighPropValue != null) {
57                     try {
58                         int i = Integer.parseInt(integerCacheHighPropValue);
59                         i = Math.max(i, 127);
60                         // Maximum array size is Integer.MAX_VALUE
61                         h = Math.min(i, Integer.MAX_VALUE - (-low) -1);
62                     } catch( NumberFormatException nfe) {
63                         // If the property cannot be parsed into an int, ignore it.
64                     }
65                 }
66                 high = h;
67 
68                 cache = new Integer[(high - low) + 1];
69                 int j = low;
70                 for(int k = 0; k < cache.length; k++)
71                     cache[k] = new Integer(j++);
72 
73                 // range [-128, 127] must be interned (JLS7 5.1.7)
74                 assert IntegerCache.high >= 127;
75             }
76 
77             private IntegerCache() {}
78         }*/
79         /*IntegerCache是Integer的内部类，在被jdk加载的时候，会去初始化创建（-128 - 127）之间所有int值对应的Integer类数组
80         使用的时候可以从里面取*/
81     }
82 }