A DESCRIPTION OF THE PROBLEM :
In the javadoc of java.lang.Object.hashCode(), it says "the hashCode method defined by class Object returns distinct integers for distinct objects". However, the method might return the same hashCode for distinct objects. See the test program below. Moreover, it's also impossible since 64-bit JVMs can obtain more than 4294967296 objects.

ACTUAL -
The test program prints the following message:
duplicate hashCode 2134400190
o != old

---------- BEGIN SOURCE ----------
// Run this program on a 64-bit JVM
// You might need -Xmx16G option to increase maximum memory limit
public class Main {
    private static final int ARRAY_LEN = Integer.MAX_VALUE - 32;
    private static final long MAX_ITERATION_COUNT = (1L << 32) + 2L;

    public static void main(String[] args) {
        Object[] array = new Object[ARRAY_LEN];
        for (long i = 0; i < MAX_ITERATION_COUNT; ++i) {
            Object o = new Object();
            int h = o.hashCode();
            if (h >= 0 && h < ARRAY_LEN) {
                Object old = array[h];
                if (old != null) {
                    System.out.println("duplicate hashCode " + h);
                    if (o == old) {
                        System.out.println("o == old");
                    } else {
                        System.out.println("o != old");
                    }
                    break;
                }
                array[h] = o;
            }
        }
    }
}
---------- END SOURCE ----------