Curiosity about equality of Integers

Because the Integer class caches values between -128 and 127.

Look at this:

System.out.println(Integer.valueOf("-129") == Integer.valueOf("-129")); // Dá false.
System.out.println(Integer.valueOf("-128") == Integer.valueOf("-128")); // Dá true.
System.out.println(Integer.valueOf("127") == Integer.valueOf("127")); // Dá true.
System.out.println(Integer.valueOf("128") == Integer.valueOf("128")); // Dá false.

Here is some snippets of the Integer source code, as implemented in OpenJDK :

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() {}
}

public static Integer valueOf(int i) {
    assert IntegerCache.high >= 127;
    if (i >= IntegerCache.low && i <= IntegerCache.high)
        return IntegerCache.cache[i + (-IntegerCache.low)];
    return new Integer(i);
}

public static Integer valueOf(String s) throws NumberFormatException {
    return Integer.valueOf(parseInt(s, 10));
}

This is because the Integer Class uses the FlyWeight design pattern.

The Integer class creates an unchanging Singleton object for each of the numbers from -128 to 127 so that these objects can be shared throughout the application, generating memory savings.