Java 基础巩固：装箱拆箱你真的熟悉吗

先考两道题：

Integer a1 = 300;

Integer a2 =300;

System.out.print(a1 == a2);

Integer b1 = 1;

Integer b2 = 1;

System.out.print(a1 == a2);

答案是：

false

true

Integer a1=1的时候，java编译器会默认编译成Integer a1=Integer.valueOf(1),valueOf方法里边会查看a1的值是否在-128到127，如果在这个范围，会从IntegerCache这个缓存中去取有没有值等于1的对象，如果有的话会返回缓存里的对象，没有的话会new一个。

马丹，那人说错了 integer 在-128 到127之间是静态初始化的直接从缓存里面取得

JDK 1.7:

JDK 1.8:

IntegerCache:

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() {}

}