Test.java

/**
 * $java Test
 * 1 == 1:             true
 * 100 == 100:         true
 * 127 == 127:         true
 * new 127 == new 127: false
 * 128 == 128:         false
 * 1000 == 1000:       false
 */


public class Test {

    public static void main(String[] args) {

        Integer i1 = 1;
        Integer i2 = 1;
        System.out.println("1 == 1:             " + (i1 == i2));

        Integer i3 = 100;
        Integer i4 = 100;
        System.out.println("100 == 100:         " + (i3 == i4));

        Integer i5 = 127;
        Integer i6 = 127;
        System.out.println("127 == 127:         " + (i5 == i6));

        Integer i5n = new Integer(127);
        Integer i6n = new Integer(127);
        System.out.println("new 127 == new 127: " + (i5n == i6n));

        Integer i7 = 128;
        Integer i8 = 128;
        System.out.println("128 == 128:         " + (i7 == i8));

        Integer i9 = 1000;
        Integer i10 = 1000;
        System.out.println("1000 == 1000:       " + (i9 == i10));


    }

}

Test.md

//
// Source code recreated from a .class file by IntelliJ IDEA
// (powered by Fernflower decompiler)
//

public class Test {
    public Test() {
    }

    public static void main(String[] var0) {
        Integer var1 = Integer.valueOf(1);
        Integer var2 = Integer.valueOf(1);
        System.out.println("1 == 1:             " + (var1 == var2));
        Integer var3 = Integer.valueOf(100);
        Integer var4 = Integer.valueOf(100);
        System.out.println("100 == 100:         " + (var3 == var4));
        Integer var5 = Integer.valueOf(127);
        Integer var6 = Integer.valueOf(127);
        System.out.println("127 == 127:         " + (var5 == var6));
        Integer var7 = new Integer(127);
        Integer var8 = new Integer(127);
        System.out.println("new 127 == new 127: " + (var7 == var8));
        Integer var9 = Integer.valueOf(128);
        Integer var10 = Integer.valueOf(128);
        System.out.println("128 == 128:         " + (var9 == var10));
        Integer var11 = Integer.valueOf(1000);
        Integer var12 = Integer.valueOf(1000);
        System.out.println("1000 == 1000:       " + (var11 == var12));
    }
}

    /**
     * Returns an {@code Integer} instance representing the specified
     * {@code int} value.  If a new {@code Integer} instance is not
     * required, this method should generally be used in preference to
     * the constructor {@link #Integer(int)}, as this method is likely
     * to yield significantly better space and time performance by
     * caching frequently requested values.
     *
     * This method will always cache values in the range -128 to 127,
     * inclusive, and may cache other values outside of this range.
     *
     * @param  i an {@code int} value.
     * @return an {@code Integer} instance representing {@code i}.
     * @since  1.5
     */
    public static Integer valueOf(int i) {
        if (i >= IntegerCache.low && i <= IntegerCache.high)
            return IntegerCache.cache[i + (-IntegerCache.low)];
        return new Integer(i);
    }

    /**
     * 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 {@code -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) {
                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() {}
    }