Huffman - Coder Algorithm

Implementing huffman algo might tough for newbies but it is one of the most prominent algorithm out there and one must have it's proper knowledge. So, I thought I should share a simple algo to implement it
Learning it will help you gain knowledge about about heaps, trees and data compression algorithms.
Below is a simple method to implement it form scratch.

import java.nio.file.Files;
import java.nio.file.Path;
import java.nio.file.Paths;
import java.util.ArrayList;
import java.util.BitSet;
import java.util.Comparator;
import java.util.HashMap;


public class HEncoder {
    private HashMap<Character, String> encoder = new HashMap<>();
    private HashMap<String, Character> decoder = new HashMap<>();

    private static class Node{
        Character data;
        int freq;
        Node left;
        Node right;
        private static final NodeComparator Ctor = new NodeComparator();

        private static class NodeComparator implements Comparator<Node>{
            @Override
            public int compare(Node o1, Node o2) {
                // TODO Auto-generated method stub
                return o2.freq - o1.freq;
            }
        }
    }

    // 1. freq map
    // 2. prepare the heap from keyset
    // 3. prepare tree - remove two, merge, add it back
    // 4. traverse
    public HEncoder(String feeder){
        // 1. freq map
        HashMap<Character, Integer> fm = new HashMap<>();
        for(int i = 0; i < feeder.length(); i++){
            char ch = feeder.charAt(i);

            if(fm.containsKey(ch)){
                fm.put(ch, fm.get(ch) + 1);
            } else {
                fm.put(ch, 1);
            }
        }

        // 2. create the heap
        PriorityQueue<Node> heap = new PriorityQueue<>(Node.Ctor);
        ArrayList<Character> keys = new ArrayList<>(fm.keySet());
        for(Character key: keys){
            Node node = new Node();
            node.data = key;
            node.freq = fm.get(key);

            heap.add(node);
        }

        // 3. create the binary tree - remove two, merge, put it back till size is 1
        while(heap.size() != 1){
            Node one = heap.removeHP();
            Node two = heap.removeHP();

            Node merged = new Node();
            merged.freq = one.freq + two.freq;
            merged.left = one;
            merged.right = two;

            heap.add(merged);
        }

        // 4. traverse the tree
        Node finalNode = heap.removeHP();
        traverse(finalNode, "");
    }

    private void traverse(Node node, String osf) {
        // work
        if(node.left == null && node.right == null){
            encoder.put(node.data, osf);
            decoder.put(osf, node.data);
            return;
        }

        traverse(node.left, osf + "0");
        traverse(node.right, osf + "1");
    }

    public String compress(String str) throws Exception {
        String rv = "";

        for(int i = 0; i < str.length(); i++){
            rv += encoder.get(str.charAt(i));
        }

        byte[] barr = null;

        if(rv.length() % 8 == 0){
            barr = new byte[rv.length() / 8];
        } else {
            barr = new byte[rv.length() / 8 + 1];
        }

        int counter = 0;
        for(int i = 0; i < rv.length(); ){
            barr[counter] = (byte)Integer.parseInt(rv.substring(i, Math.min(i + 8, rv.length())), 2);
            counter++;
            i = i + 8;
        }

        Path path = Paths.get("E:\\1.obj");
        Files.write(path, barr);

        return rv;
    }

    public String decompress(String str) throws Exception{
        Path path = Paths.get("E:\\1.obj");
        byte[] arr = Files.readAllBytes(path);

        for(int i = 0; i < arr.length; i++){
            str += String.format("%8s", Integer.toBinaryString(arr[i] & 0xFF)).replace(' ', '0');
        }

        String rv = "";

        String code = "";
        for(int i = 0; i < str.length(); i++){
            code += str.charAt(i);

            if(decoder.containsKey(code)){
                rv += decoder.get(code);
                code = "";
            }
        }

        return rv;
    }
}