Java常量池

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：

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：

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：

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() {}
}

注意cache数组是静态的。