equals and hashCode: Is Objects.hash method broken?

Question

I am using Java 7, and I have the following class below. I implemented equals and hashCode correctly, but the problem is that equals r

Answer 1

HashCode being 32 bit int value, there is always a possibility of collisions(same hash code for two objects), but its rare/coincidental. Your example is one of the such a highly coincidental one. Here is the explanation.

When you call Objects.hash, it internally calls Arrays.hashCode() with logic as below:

public static int hashCode(Object a[]) {
    if (a == null)
        return 0;
    int result = 1;
    for (Object element : a)
        result = 31 * result + (element == null ? 0 : element.hashCode());
    return result;
}

For your 3 param hashCode, it results into below:

   31 * (31 * (31 *1 +hashOfString1)+hashOfString2) + hashOfString3

For your first object. Hash value of individual Strings are:

chamorro --> 1140493257 english --> 1698758127 notes --> 0

And for second object:

chamorro --> 1140494218 english --> 1698728336 notes -->0

If you notice, first two values of the hash code in both objects are different.

But when it computes the final hash code as:

  int hashCode1 = 31*(31*(31+1140493257) + 1698758127)+0;
  int hashCode2 = 31*(31*(31+1140494218) + 1698728336)+0;

Coincidentally it results into same hash code 1919283673 because int is stored in 32 bits.

Verify the theory your self be using the code segment below:

  public static void main(String... args) {
    ChamorroEntry entry1 = new ChamorroEntry("Åguigan", 
                         "Second island south of Saipan. Åguihan.", "");
    ChamorroEntry entry2 = new ChamorroEntry("Åguihan", 
                         "Second island south of Saipan. Åguigan.", "");
    System.out.println(entry1.equals(entry2)); // returns false
    System.out.println("Åguigan".hashCode());
    System.out.println("Åguihan".hashCode());
    System.out.println("Second island south of Saipan. Åguihan.".hashCode());
    System.out.println("Second island south of Saipan. Åguigan.".hashCode());
    System.out.println("".hashCode());
    System.out.println("".hashCode());
    int hashCode1 = 31*(31*(31+1140493257) + 1698758127)+0;
    int hashCode2 = 31*(31*(31+1140494218) + 1698728336)+0;
    System.out.println(entry1.hashCode() + "\n" + entry2.hashCode()); 
    System.out.println(getHashCode(
                    new String[]{entry1.chamorro, entry1.english, entry1.notes}) 
                    + "\n" + getHashCode(
                    new String[]{entry2.chamorro, entry2.english, entry2.notes})); 
    System.out.println(hashCode1 + "\n" + hashCode2); // returns same hash code!
  }

    public static int getHashCode(Object a[]) {
        if (a == null)
            return 0;
        int result = 1;
        for (Object element : a)
            result = 31 * result + (element == null ? 0 : element.hashCode());
        return result;
    }

If you use some different string parameters, hope it will result into different hashCode.