Question

Let’s say you have a class and you create a HashSet which can store this instances of this class. If you try to add instances which are equal, only one instance is kept in the collection, and that is fine.

However if you have two different instances in the HashSet, and you take one and make it an exact copy of the other (by copying the fields), the HashSet will then contain two duplicate instances.

Here is the code which demonstrates this:

 public static void main(String[] args)
    {
         HashSet<GraphEdge> set = new HashSet<>();
        GraphEdge edge1 = new GraphEdge(1, "a");
        GraphEdge edge2 = new GraphEdge(2, "b");
        GraphEdge edge3 = new GraphEdge(3, "c");

        set.add(edge1);
        set.add(edge2);
        set.add(edge3);

        edge2.setId(1);
        edge2.setName("a");

        for(GraphEdge edge: set)
        {
            System.out.println(edge.toString());
        }

        if(edge2.equals(edge1))
        {
            System.out.println("Equals");
        }
        else
        {
            System.out.println("Not Equals");
        }
    }

    public class GraphEdge
    {
        private int id;
        private String name;

        //Constructor ...

        //Getters & Setters...

        public int hashCode()
        {
        int hash = 7;
        hash = 47 * hash + this.id;
        hash = 47 * hash + Objects.hashCode(this.name);
        return hash;    
        }

        public boolean equals(Object o)
        {
            if(o == this)
            {
                return true;
            }

            if(o instanceof GraphEdge)
            {
                GraphEdge anotherGraphEdge = (GraphEdge) o;
                if(anotherGraphEdge.getId() == this.id && anotherGraphEdge.getName().equals(this.name))
                {
                    return true;
                }
            }

                return false;
        }
    }

The output from the above code:

1 a
1 a
3 c
Equals

Is there a way to force the HashSet to validate its contents so that possible duplicate entries created as in the above scenario get removed?

A possible solution could be to create a new HashSet and copy the contents from one hashset to another so that the new hashset won’t contain duplicates however I don’t like this solution.

Answer 1

The situation you describe is invalid. See the Javadoc: “The behavior of a set is not specified if the value of an object is changed in a manner that affects equals comparisons while the object is an element in the set.”