How to make HashMap work with Arrays as key?

Question

I am using boolean arrays as keys for a HashMap. But the problem is HashMap fails to get the keys when a different array is passed as key, although the elements are same. (As they are different objects).

How can I make it work with arrays as keys ? Here is the code :

public class main { public static HashMap<boolean[], Integer> h;   public static void main(String[] args){     boolean[] a = {false, false};      h = new HashMap<boolean[], Integer>();     h.put(a, 1);       if(h.containsKey(a)) System.out.println("Found a");      boolean[] t = {false, false};      if(h.containsKey(t)) System.out.println("Found t");     else System.out.println("Couldn't find t");  }  } 

Both the arrays a and t contain the same elements, but HashMap doesn't return anything for t.

How do I make it work ?

Answer 1

You cannot do it this way. Both t and a will have different hashCode() values because the the java.lang.Array.hashCode() method is inherited from Object, which uses the reference to compute the hash-code (default implementation). Hence the hash code for arrays is reference-dependent, which means that you will get a different hash-code value for t and a. Furthermore, equals will not work for the two arrays because that is also based on the reference.

The only way you can do this is to create a custom class that keeps the boolean array as an internal member. Then you need to override equals and hashCode in such a way that ensures that instances that contain arrays with identical values are equal and also have the same hash-code.

An easier option might be to use List<Boolean> as the key. Per the documentation the hashCode() implementation for List is defined as:

int hashCode = 1; Iterator<E> i = list.iterator(); while (i.hasNext()) {     E obj = i.next();     hashCode = 31*hashCode + (obj==null ? 0 : obj.hashCode()); } 

As you can see, it depends on the values inside your list and not the reference, and so this should work for you.