Issues with using HashTable in Java

Question

I am trying to use hashtable and when trying to search for an object,I do not see the object, but I can see it if I print it.

Node Class:

public class Node {

    int x;
    int y;


    public Node() {

        this.x=0;
        this.y=0;

    }

    public Node(int x,int y) {
        this.x=x;
        this.y=y;
    }



    public String toString(){

        return "(Node: x,y="+Integer.toString(x)+","+Integer.toString(y)+")";

    }

}

Main Class:

public class GridWalk {


    static Hashtable <Node, Integer> myMap;
    static Stack<Node> nodes;

    public static void main(String[] args) {

        myMap = new Hashtable<Node,Integer>();
        nodes=new Stack<Node>();

        Node start=new Node(0,0);

        Node new1= new Node(100,100);
        myMap.put(new1,new Integer(1));
        Node new2=new Node (100,100);
        System.out.println("Already there ? huh: "+new2.toString()+" at "+myMap.get(new2)); 

    }
}

I am getting NULL when I do the print line. Any idea why ?

Cameron Skinner · Accepted Answer · 2012-03-11 01:44:40Z

3

You need to override and implement the equals method in your Node class. The default implementation from java.lang.Object only compares equality of references, which is not suitable in your case.

Node new1 = new Node(100, 100);
Node new2 = new Node(100, 100);

System.out.println(new1.equals(new2)); // Your current code will print false

HashMap's rely on proper implementation of both equals and hashCode method in order to operate correctly. You should implement an equals method that reflects the objects logic. Something like:

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

    final Node other = (Node) o;
    return ((getX() == o.getX()) && (getY() == o.getY());
}

You might also want to implement a hashCode() method on your Node object.

edited Mar 11, 2012 at 1:44

Cameron Skinner

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answered Mar 11, 2012 at 1:27

Perception

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6 Comments

Ahsan Over a year ago

so, the method should be something like: Boolean equals(Node node1,Node node2), right ?

Ahsan Over a year ago

got it: public boolean equals(Object obj) { return (this == obj); }

Louis Wasserman Over a year ago

No. It needs to be in the Node class and have the exact signature boolean equals(Object).

Anthony Accioly Over a year ago

No public boolean equals(Object o). You can convert o to a Node object and compare it against this. this == obj will not help you, since it is the default Object.equals implementation that is causing your trouble. Shamelessly advertise on my blog post. You can translate the article using the right menu "Traduzir" combo box.

Cameron Skinner Over a year ago

@Perception: It's not "you might also want to implement a hashCode() method"; it's "you must also implement a hashCode() method". If you override equals but not hashCode then they will be inconsistent and you will wind up with undefined behaviour from the HashMap.

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