HashMap , int [][] - key , compareTo override problem , help

Question

i've a hashmap Map<int [][] , Integer> and i want to get the corresponding value for int a[][] from the map.
if i use , map.containsKey(a)
it doesn't compare as required( it doesn't consider order )
so i thought of using class key implementing comparable like this,
class key implements Comparable
          int a[][];
          public key(int p[][])
          {     a=p;     }
          public int compareTo(Object o)
                        key k = (key)o;
               for(int i = 0 ; i < 3 ; i++)
                    for(int j = 0 ;j < 3 ; j++)
                         if(a[i][j]!=k.a[i][j])     return 1;
               return 0;
}now when i create a key ,
key KK = new key(a) // a is int[][]
map.containsKey(KK) doesn't invoke compareTo method itself ..
don't what the problem is.
but when i try . TreeMap it invokes compareTo method
but don't know why , it acts as before when Map<int [][] , Integer> was used.. any idea
Thanx

Answer 1

manishmulani@nitk wrote:
wil the following way of overriding hashcode work??
class key
          int a[][];
          public key(int p[][])
          {     a=p;     }
          public boolean equals(Object o)
               key k = (key)o;
               for(int i = 0 ; i < 3 ; i++)
                    for(int j = 0 ; j < 3 ; j++)
                         if(a[i][j]!=k.a[i][j])     return false;
               return true;
          public int hashCode()
               return a.hashCode();
NO! When doing that, you will get undesirable results. Execute the following lines of code and examine the output:
System.out.println(new int[][]{{1,2},{3,4}}.hashCode());
System.out.println(new int[][]{{1,2},{3,4}}.hashCode());This is better:
public int hashCode() {
  int hash = 1, mutliplier = 1;
  for(int[] row: array) {
    for(int val: row) {
      hash ^= (val * mutliplier);
      mutliplier *= 10;
  return hash;
}(Untested!!!)
ok this will be same if i use TreeMap and implement Comparable<key> and override compareTo() right??Err, yes, more or less. Of course, a HashMap and TreeMap are not the same.