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Java的8种基本类型(Byte, Short, Integer, Long, Character, Boolean, Float, Double), 除Float和Double以外, 其它六种都实现了常量池, 但是它们只在大于等于-128并且小于等于127时才使用常量池。

由如下例子可以看出：

[java] view plain copy
public static void main(String[] args) {
Integer a = 127;
Integer b = 127;
System.out.println(“等于127:”);
System.out.println(a == b);
System.out.println(“*******”);

a = 128;  b = 128;  System.out.println("等于128:");  System.out.println(a == b);  System.out.println("*****************");  a = -128;  b = -128;  System.out.println("等于-128:");  System.out.println(a == b);  System.out.println("*****************");  a = -129;  b = -129;  System.out.println("等于-129:");  System.out.println(a == b);  System.out.println("*****************");  // 测试Boolean  System.out.println("测试Boolean");  Boolean c = true;  Boolean d = true;  System.out.println(c == d);  d = new Boolean(true);  System.out.println(c == d);  

}
结果如下：

等于127:
true

---

等于128:
false

---

等于-128:
true

---

等于-129:
false

---

测试Boolean
true
false

当我们给Integer赋值时，实际上调用了Integer.valueOf(int)方法，查看源码，其实现如下：

[java] view plain copy
public static Integer valueOf(int i) {
if(i >= -128 && i <= IntegerCache.high)
return IntegerCache.cache[i + 128];
else
return new Integer(i);
}
而IntegerCache实现如下：

[java] view plain copy
private static class IntegerCache {
static final int high;
static final Integer cache[];

static {      final int low = -128;      // high value may be configured by property      int h = 127;      if (integerCacheHighPropValue != null) {          // Use Long.decode here to avoid invoking methods that          // require Integer's autoboxing cache to be initialized          int i = Long.decode(integerCacheHighPropValue).intValue();          i = Math.max(i, 127);          // Maximum array size is Integer.MAX_VALUE          h = Math.min(i, Integer.MAX_VALUE - -low);      }      high = h;      cache = new Integer[(high - low) + 1];      int j = low;      for(int k = 0; k < cache.length; k++)          cache[k] = new Integer(j++);  }  private IntegerCache() {}