Integer i = 10; //装箱,反编译后发现调用Integer.valueOf(int i) int t = i; //拆箱,反编译后发现调用i.intValue()
public class Solution { public static void main(String args[]) { Integer i1 = 1,i2 = 1,i3 = 200,i4 = 200; System.out.println(i1 == i2); System.out.println(i3 == i4); } }
输出结果:
true
false
原因在于Integer.valueOf(int i)为了优化运行的效率,减少对象的new操作,做了一个IntegerCache缓存,-128-127之间的Integer对象直接映射到缓冲中。
源码如下:
public static Integer valueOf(int i) { if (i >= -128 && i <= IntegerCache.high) return IntegerCache.cache[i + 128]; else return new Integer(i); } 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() {} }
同样类型的还有Short、Byte、Character、Long;
Double和Float由于本身自带偏差,所以不带有Cache。
对于Boolean类型的变量,直接采用类似于单例模式的方法进行valueOf(boolean b)方法
public static final Boolean TRUE = new Boolean(true); public static final Boolean FALSE = new Boolean(false); public static Boolean valueOf(boolean b) { return (b ? TRUE : FALSE); }
因此
public class Solution {
public static void main(String args[]) {
Boolean b1 = true, b2 = true;
System.out.println(b1 == b2);
b2 = false;
System.out.println(b1 + " " + b2);
}
}
输出
true
true false(类似String,是引用操作,不是赋值操作)