Question

I’m having some trouble writing a hashCode() method for a class I created. This class is meant to be used inside a TreeSet, and as such, it implements Comparable. The class has the following variables:

public class Node implements Comparable<Node> {
   Matrix matrix;
   int[] coordinates= new int[2];
   Node father;
   int depth;
   int cost;

Here’s the implementation of the compareTo() method. I want the TreeSet to organize these Node structures by their cost, therefore, compareTo() returns the result of a simple subtraction.

public int compareTo(Node nodeToCompare) {
    return this.cost - nodeToCompare.cost;
}

I also implemented an equals() method.

public boolean equals(Object objectToCompare) {
    if(objectToCompare== this) {return true;}
    if(objectToCompare== null || objectToCompare.getClass()!= this.getClass()) {return false;}

    Node objectNode= (Node) objectToCompare;
    return this.father.equals(objectNode.father) &&
            this.depth== objectNode.depth &&
            this.cost== objectNode.cost &&
            this.matrix.equals(objectNode.matrix) &&
            Arrays.equals(this.coordinates, objectNode.coordinates);
}

Having said all of that, I have a few questions:

1. Since I implemented a new equals() method, should I implement a new hashCode() method?
2. How can I go about implementing a new hashCode method() with those variables? (Note that the variable matrix of the type Matrix has a hashCode() method implemented)

That’s all!

Answer 1

Your compareTo method is not consistent with your equals method: your compareTo method says that two instances are equivalent if they have the same cost — such that a TreeSet can only ever contain at most one instance with a given cost — but your equals method says that they’re only equivalent if they have the same cost and are the same in various other ways.

So, assuming that your equals method is correct:

• you need to fix your compareTo method to be consistent with it.
• you need to create a hashCode method that is consistent with it. I recommend using the same sort of logic as is used by java.util.List.hashCode(), which is a straightforward and effective way to assemble the hash-codes of component objects in a specific order; basically you would write something like:

  int hashCode = 1;
  hashCode = 31 * hashCode + (father == null ? 0 : father.hashCode());
  hashCode = 31 * hashCode + depth;
  hashCode = 31 * hashCode + cost;
  hashCode = 31 * hashCode + matrix.hashCode();
  hashCode = 31 * hashCode + java.util.Arrays.hashCode(coordinates);
  return hashCode;