Huffman Encoding Tree, Java

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/**
 *  Huffman Encoding Tree
 *
 *  Author: Jayesh Chandrapal
 *  Version: 1.0
 */

import java.util.*;
import java.lang.*;
import java.io.*;

class Node implements Comparable<Node> {
	Character character;
	Integer frequency;
	Node left;
	Node right;
	
	Node(Character character, Integer frequency) {
		this.character = character;
		this.frequency = frequency;
	}
    
    @Override
    public int compareTo(Node otherNode) {
        return this.frequency.compareTo(otherNode.frequency);
    }
    
    @Override
    public String toString() {
        return "[ " + character + " : " + frequency + " ]";
    }
}
	
class Rextester
{
	Map<Character, Integer> characterMap;
    PriorityQueue<Node> tree;
	
	Rextester() {
		characterMap = new LinkedHashMap<Character, Integer>();
		characterMap.put('a', 5);
		characterMap.put('b', 9);
		characterMap.put('c', 12);
		characterMap.put('d', 13);
		characterMap.put('e', 16);
		characterMap.put('f', 45);
        
        tree = new PriorityQueue<Node>();
	}
	
	public static void main (String[] args) throws java.lang.Exception
	{
		Rextester app = new Rextester();
		
		app.buildTree();
        
        app.displayQueue();
		
		System.out.println("done");
	}
	
	public void buildTree() {
		for(Map.Entry<Character, Integer> entry : characterMap.entrySet()) {
			tree.add(new Node(entry.getKey(), entry.getValue()));
		}
        
        while(tree.size() > 1) {
            Node node1 = tree.poll();
            Node node2 = tree.poll();
            Node newNode = new Node('*', node1.frequency + node2.frequency);
            newNode.left = node1;
            newNode.right = node2;
            tree.add(newNode);
        }
        
        // One remaining node is the root of final tree
        Node finalTree = tree.poll();
        displayTree(finalTree);
	}
    
    public void displayTree(Node root) {
        if(root != null)
            System.out.println(root);
        if(root.left != null)
            displayTree(root.left);
        if(root.right != null)
            displayTree(root.right);
    }
    
    public void displayQueue() {
        for(Node node : tree) {
            System.out.println(node);
        }
    }
}

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Compilation time: 0.84 sec, absolute running time: 0.15 sec, cpu time: 0.09 sec, memory peak: 31 Mb, absolute service time: 1 sec