Storing arrays in Set and avoiding duplicates

Question

HashSet<String[]> boog = new HashSet<String[]>(); boog.add(new String[]{"a", "b", "c"}); boog.add(new String[]{"a", "b", "c"}); boog.add(new String[]{"a", "b", "d"}); 

results in

[a, b, c] [a, b, d] [a, b, c] 

where [a,b,c] is repeated, so the hash function is not working as expected. How would I go about overriding the Hash method for String arrays. Or for that matter, a generic array? Is there a better way to accomplish what I'm trying to do?

Answer 1

You can't. arrays use the default identity-based Object.hashCode() implementation and there's no way you can override that. Don't use Arrays as keys in a HashMap / HashSet!

Use a Set of Lists instead.

Answer 2

The "better way" is to use collections. Use a List instead of a String[]:

Set<List<String>> boog = //... boog.add(Arrays.asList("a", "b", "c")); boog.add(Arrays.asList("a", "b", "c")); boog.add(Arrays.asList("a", "b", "d"));  System.out.println(boog.size()); // 2 

Edit

If you absolutely needed to use arrays as keys, you could build a transparent wrapper around each key and put that in the map. Some libraries help you with that. For example, here's how you could do a Set<String[]> using Trove:

Set<String[]> boog = new TCustomHashSet<String[]>(new ArrayHashingStrategy());  boog.add(new String[]{"a", "b", "c"}); boog.add(new String[]{"a", "b", "c"}); boog.add(new String[]{"a", "b", "d"});  System.out.println(boog.size()); // 2  //... public class ArrayHashingStrategy extends HashingStrategy<Object[]> {     public int computeHashCode(Object[] array) {       return Arrays.hashCode(array);    }     public boolean equals(Object[] arr1, Object[] arr2) {       return Arrays.equals(arr1, arr2);    } }