为什么此示例中的Integer.valueOf（...）比较返回不同的值？

Question

From the answer to a question about primitive types and autoboxing in java :

for biziclop:

class biziclop {

 public static void main(String[] args) { System.out.println(new Integer(5) == new Integer(5)); System.out.println(new Integer(500) == new Integer(500)); System.out.println(Integer.valueOf(5) == Integer.valueOf(5)); System.out.println(Integer.valueOf(500) == Integer.valueOf(500)); } 

}

Results in:

 C:\\Documents and Settings\\glow\\My Documents>java biziclop false false true false C:\\Documents and Settings\\glow\\My Documents> 

Why is that?

Answer 1

Integer.valueof caches objects for values around zero as required by the Java Language Specification.

Inspired by ilya's answer see the latest, actual source for Integer.valueOf() in the upcoming JDK7, lines 638-643.

Answer 2

请参阅Integer.valueOf实现： http ：//docjar.com/html/api/java/lang/Integer.java.html（850s行）

Answer 3

You should use the equal method not the == operator. == test if two objects are equal but you create different objects with same value and need the equal() method to compare the object's values.

Update:
Reason for different behavior of Integer.valouOf(5) and Integer.valouOf(500) is indeed that Integer implementation uses a static valueOfCache of size -128..127.
As of Java 7 this is configurable with the command-line argument -XX:AutoBoxCacheMax=<size>

Answer 4

Integer.valueOf缓存值，特别是-128到127。