Java判断Integer类型的值是否相等

public class Demo {

	public static void main(String[] args) {
		Integer c = -128;
		Integer d = -128;
		System.out.println("c == d: " + (c == d));
		System.out.println("c.equals(d): " + c.equals(d));
		System.out.println("c.intValue() == d.intValue(): " + (c.intValue() == d.intValue()));
		System.out.println("Objects.equals(c, d): " + Objects.equals(c, d));
		
		Integer e = 127;
		Integer f = 127;
		System.out.println("e == f: " + (e == f));
		System.out.println("e.equals(f): " + e.equals(f));
		System.out.println("e.intValue() == f.intValue(): " + (e.intValue() == f.intValue()));
		System.out.println("Objects.equals(e, f): " + Objects.equals(e, f));
		
		Integer g = 128;
		Integer h = 128;
		System.out.println("g == h: " + (g == h));
		System.out.println("g.equals(h): " + g.equals(h));
		System.out.println("g.intValue() == h.intValue():" + (g.intValue() == h.intValue()));
		System.out.println("Objects.equals(g, h): " + Objects.equals(g, h));
	}
}

结果如下

c == d: true
c.equals(d): true
c.intValue() == d.intValue(): true
Objects.equals(c, d): true
e == f: true
e.equals(f): true
e.intValue() == f.intValue(): true
Objects.equals(e, f): true
g == h: false
g.equals(h): true
g.intValue() == h.intValue():true
Objects.equals(g, h): true

（1）当用“==”进行比较时，jvm默认是比较数据在java堆的地址。int是一种基本数据类型，jvm会自动将Integer转成int数值进行比较。在Integer类中，有一个内部静态类IntegerCache ，用来支持自动拆箱和装箱，如下，数值范围[-128,127]

 /**
     * Cache to support the object identity semantics of autoboxing for values between
     * -128 and 127 (inclusive) as required by JLS.
     *
     * The cache is initialized on first usage.  The size of the cache
     * may be controlled by the -XX:AutoBoxCacheMax=<size> option.
     * During VM initialization, java.lang.Integer.IntegerCache.high property
     * may be set and saved in the private system properties in the
     * sun.misc.VM class.
     */

    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) {
                int i = parseInt(integerCacheHighPropValue);
                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() {}
    }

如果Integer对象的数值在上述范围之外，会比较Integer对象，此时如果再用“==”会比较出两个对象地址不同。

比较两个Integer的值是否相同，方法比较多，推荐用Objects.equals(a, b)，这个还可以避免一些空指针问题的出现。