Integer类小细节随笔记录

　　先看一段简单的代码：

Integer v1 = Integer.valueOf(12);
Integer v2 = Integer.valueOf(12);
 
Integer v3 = Integer.valueOf(129);
Integer v4 = Integer.valueOf(129);
 
System.out.println(v1 == v2);
System.out.println(v3 == v4); 

　　输出结果是啥呢？第一个是 true，第二个是false。

　　为啥呢？

public static Integer valueOf(int i) {
        if (i >= IntegerCache.low && i <= IntegerCache.high)
            return IntegerCache.cache[i + (-IntegerCache.low)];
        return new Integer(i);
    }

　　看源代码得知，当 i 在默认条件下（-127到128）之间，从缓存中取值，否则重新 new 一个 Integer 对象。详细代码如下：

static {
           // high value may be configured by property
           int h = 127;
           String integerCacheHighPropValue =
               sun.misc.VM.getSavedProperty("java.lang.Integer.IntegerCache.high");
           if (integerCacheHighPropValue != null) {
               try {
                   int i = parseInt(integerCacheHighPropValue);
                   i = Math.max(i, 127);
                   // Maximum array size is Integer.MAX_VALUE
                   h = Math.min(i, Integer.MAX_VALUE - (-low) -1);
               } catch( NumberFormatException nfe) {
                   // If the property cannot be parsed into an int, ignore it.
               }
           }
           high = h;
 
           cache = new Integer[(high - low) + 1];
           int j = low;
           for(int k = 0; k < cache.length; k++)
               cache[k] = new Integer(j++);
 
           // range [-128, 127] must be interned (JLS7 5.1.7)
           assert IntegerCache.high >= 127;
       }