学习： 

public static void main(String [] args) {        Integer i = 120 ;     //装箱            int a  = i;                   //拆箱    }

通过 javap -c Test.class 查看。class文件可以看到

会执行 

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

public int intValue() {        return value;    }

　　

　

该方法表示 当 传入的 int 的值 在 -128 ~127 之间时 会从缓冲区中取出一个Integer对象

private static class IntegerCache {        static final int low = -128;        static final int high;        static final Integer cache[];        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;        }        private IntegerCache() {}    }

　　当值大于127 或着 小于 -128 时时会新建一个 Integer 

面试考察点：

Integer a = 1; // 等价于 Integer a = valueOf(1)    Integer b = 1;    System.out.println(a == b); // true   Integer c = 128;    Integer d = 128;    System.out.println(c == d); // false    System.out.println(c.equals(d)); // true