How can I use a third party Class Object as Hashmap Key?

Question

OK I understand the working of equals and hashcode and How they are used in hashmap. But This question crossed my mind What if I am having a third party object which does'nt have overridden hashcode and equals and I am not even allowed to modify it.

Consider following Class:

//Unmodifiable class
public final class WannaBeKey{

    private String id;

    private String keyName;

    //Can be many more fields

    public String getId()
    {
          return id;
    }

    public String getKeyName()
    {
          return id;
    }
    //no hashcode or equals :(

  }

Now I want to make this class as my Hashmap key obviously it won't work without equals and hashcode. I want to know is there any way to handle such cases? I am unable to think of any or I am way over my head..

Thanks.

Answer 1

I've encountered this previously, and worked around it by creating a wrapper for the WannaBeKey as such:

public class WannaBeKeyWrapper {

  private final WannaBeKey key;


  public WannaBeKeyWrapper(WannaBeKey key) {
    this.key = key;
  } 

  public boolean equals(Object obj) {

    // Insert equality based on WannaBeKey
  }

  public int hashCode() {
    // Insert custom hashcode in accordance with http://docs.oracle.com/javase/7/docs/api/java/lang/Object.html#hashCode()
  }
}

Obviously this changes your Set from Set<WannaBeKey> to Set<WannaBeKeyWrapper>, but you should be able to account for that.

Answer 2

You can create a wrapper for that object which will have the overridden methods. Then you can use the wrapper class as the key of your hash map.