通过字节码查看int/Integer是如何比较的,为什么nteger(127)==Integer(127)

先看下面三组比较

public static void main(String[] args)  {
		Integer i1 = 100;
		Integer i2 = 100;
		System.out.println(i1 == i2);
		
		Integer i3 = 200;
		Integer i4 = 200;
		System.out.println(i3 == i4);
		
		Integer i5 = 200;
		int i6 = 200;
		System.out.println(i5 == i6);
		
}

结果:

true
false
true

通过把上述源码转成字节码文件分析下问什么是产生上面的结果

先看第一组

		 0: bipush        100
         2: invokestatic  #16                 // Method java/lang/Integer.valueOf:(I)Ljava/lang/Integer;
         5: astore_1
         6: bipush        100
         8: invokestatic  #16                 // Method java/lang/Integer.valueOf:(I)Ljava/lang/Integer;
        11: astore_2
        12: getstatic     #22                 // Field java/lang/System.out:Ljava/io/PrintStream;
        15: aload_1
        16: aload_2
        17: if_acmpne     24
        20: iconst_1
        21: goto          25
        24: iconst_0
        25: invokevirtual #28                 // Method java/io/PrintStream.println:(Z)V

通过第一组生成的字节码可以看出,关键地方还在于java/lang/Integer.valueOf,看看java/lang/Integer.valueOf源码是如何处理的:

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

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 =
                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() {}
    }

通过上面源码可以看出,java/lang/Integer.valueOf在输入的int在﹣128与﹢127之间返回的是IntegerCache中已缓存的对象,所以Integer i1 = 100;Integer i2 = 100;其实是一个对象,结果是true.

再看第二组

明白了第一组其实第二个很好理解,200并没有再IntegerCache中,所有返回的是2个新的对象,2个不同的对象相比返回false.

看最后一组

为什么对象与基本数据类型就相等?

        60: sipush        200
        63: invokestatic  #16                 // Method java/lang/Integer.valueOf:(I)Ljava/lang/Integer;
        66: astore        5
        68: sipush        200
        71: istore        6
        73: getstatic     #22                 // Field java/lang/System.out:Ljava/io/PrintStream;
        76: aload         5
        78: invokevirtual #34                 // Method java/lang/Integer.intValue:()I
        81: iload         6
        83: if_icmpne     90
        86: iconst_1
        87: goto          91
        90: iconst_0
        91: invokevirtual #28                 // Method java/io/PrintStream.println:(Z)V

关键在于ava/lang/Integer.intValue:()I:

private final int value;
public int intValue() {
        return value;
}

Java的自动拆箱,对象最后返回的是int的基本类型,2个基本类型的int对比,200==200,所有返回的是true.