cannot cast to java.lang.Comparable

Although this question has already been asked but i have an implementation specific doubt.

I am trying to print the top view of the binary tree and following is the complete code for it:

import java.util.*;

class Node{
    int data;
    Node right;
    Node left;
    Node(int data){
        this.data = data;
    }
}

class Pair<F,S>{
    private F first;
    private S second;
    public Pair(F first, S second){
        this.first = first;
        this.second = second;
    }

    public F getFirst(){return first;}
    public S getSecond(){return second;}
}

class BinaryTreeTopView{

    public static void printTopView(Node root){

        if(root == null)
            return;

    Queue <Pair<Node,Integer>> q = new Queue<>();
    Map <Integer,Node> map = new HashMap<>();
    Pair<Node,Integer> p = new Pair<>(root, 0);
    q.add(p);

    /*
    I am storing nodes and the corresponding horizontal distances 
    in the form of a pair which then are being stored in the queue
    to ensure level order traversal
    */

    while(!q.isEmpty()){
        Pair<Node,Integer> temp = q.peek();
        q.remove();

        if(map.containsKey(temp.getSecond())==true){
            map.put(temp.getSecond(),temp.getFirst());
        } else {
            System.out.println(temp.getFirst().data);
            map.put(temp.getSecond(),temp.getFirst());                
        }

        if(temp.getFirst().left!=null){
            Pair<Node,Integer> left = new Pair<>(temp.getFirst().left, temp.getSecond()-1);
            q.add(left);
        }

        if(temp.getFirst().right!=null){
            Pair<Node,Integer> right = new Pair<> (temp.getFirst().right, temp.getSecond()+1);
            q.add(right);
        }

    }
}
public static void main(String[] args) {
    Node root = new Node(1);
    root.left = new Node(2);
    root.right = new Node(3);
    root.left.right = new Node(5);
    root.left.left = new Node(4);
    root.right.left = new Node(6);
    root.right.right = new Node(7);
    root.right.left.right = new Node(8);
    root.right.right.left = new Node(10);
    root.right.right.right = new Node(9);
    root.right.right.left.right = new Node(11);
    root.right.right.left.right.right = new Node(12);

    printTopView(root);
}
}

It compiles fine but an exception is being raised at the runtime.
Now i have been getting the following exception and i am unable to figure out what the problem is:

    Exception in thread "main" java.lang.ClassCastException: 
    Pair cannot be cast to java.lang.Comparable at  java.util.PriorityQueue.siftUpComparable(PriorityQueue.java:652)
    at java.util.PriorityQueue.siftUp(PriorityQueue.java:647)
    at java.util.PriorityQueue.offer(PriorityQueue.java:344)
    at java.util.PriorityQueue.add(PriorityQueue.java:321)

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Jason
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It’s because Pair isn’t implementing Comparable. Either implement it: public class Pair implements Comparable<Pair> { public int compareTo(Pair o) { // ... } } Or use Comparator in Your priority queue Using Comparator ; PriorityQueue<DummyObject> pq = new PriorityQueue<DummyObject>(5, new DummyObjectComparator()); Define your Comparator : class DummyObjectComparator implements Comparator<DummyObject>{ // Overriding compare()method of Comparator public int compare(DummyObject s1, DummyObject s2) { //some code } }

Jason
Guest

You’re trying to add Pair instances to a PriorityQueue, so your Pair class must be Comparable. A reasonable implementation could be to force F and S to be Comparable on their own right, and then compare by the first element, and then the second one: class Pair<F extends Comparable<F>, S extends Comparable<S>> implements Comparable<Pair<F, S>> { // All the code you already have is fine @Override public int compareTo(Pair<F, S> o) { int retVal = getFirst().compareTo(o.getFirst()); if (retVal != 0) { return retVal; } return getSecond().compareTo(o.getSecond()); } }