How do I create a leaf node?

This is homework. I've been stuck on this a long time...

I have to create leaf nodes. For this I have the class LeafTreeNode.

LeafTreeNode.java :

import java.util.Map;

public class LeafTreeNode implements TreeNode {

    private char symbol;
    private int symbolCount;

    public LeafTreeNode(char symbol, int symbolCount) {
        this.symbol = symbol;
        this.symbolCount = symbolCount;
    }

    public int symbolCount() {
        return symbolCount;
    }

    public void getSymbolCodes(String prefix, Map<Character, String> symbolCodes) {
        symbolCodes.put(symbol, prefix);
    }

}

Now I have to use the method characterHistogram(String s) and create a leaf node for each character occurring in s. This method is in a new class Huffman.

I have to do this in the method huffmanCode(String s).

Huffman

import java.util.Map;

public class Huffman{

    private LeafTreeNode Blattknoten;

    public int[] characterHistogram(String s) {
        // Only ASCII
        int[] histogram = new int[256];

        for (int i = 0; i < s.length(); i++) {
            char c = s.charAt(i);
            if (c < 256) {
                histogram[c]++;
            }
            System.out.println(s.charAt(i) + " <-> " + histogram[c]);
        }
        return histogram;
    }

    public void printHuffmanCoding(Map<Character, String> map) {
        // for (Character c : map.keySet()) {
        // System.out.println("'" + c + "': " + map.get(c));
        // }
    }

    /**
     * Compute the Huffman coding map for s
     * 
     * @param s
     *          String to encode
     * @return Huffman coding map
     */
    public Map<Character, String> huffmanCode(String s) {

        int[] histogram = characterHistogram(s);
        System.out.println(1);

        // Blattknoten.symbolCount();
        // Blattknoten
        // Blattknoten.getSymbolCodes(s, null);

        // for( int i = 0; i < histogram.length;i++){
        // histogram[i];
        // }
        // LeafTreeNode.getSymbolCodes(s, null)
        // TODO 3.
        return null;
    }

    /**
     * @param s
     *            String to encode
     * @param map
     *            Huffman coding map
     * @return the length of the Huffman coding of s
     */
    public long lengthHuffmanCoding(String s, Map<Character, String> map) {
        // TODO 4.
        return 42L;
    }

    /**
     * @param s
     *          String to encode
     * @return the length of the 8-bit coding of s
     */
    public long length8BitCoding(String s) {
        // TODO 4.
        return 42L;
    }

    /**
     * Compute the compression ratio we can achieve for s given the Huffman
     * encoding map as compared to 8-bit coding of the symbols.
     * 
     * @param s
     *            String to encode
     * @param map
     *            Huffman coding map
     * @return compression ratio
     */
    public float computeCompressionRatio(String s, Map<Character, String> map) {
        // TODO 4.
        return 42.f;
    }

    public static void main(String[] args) {

        Huffman h = new Huffman();
        String s = "Insert String here";
        Map<Character, String> map = h.huffmanCode(s);
        h.printHuffmanCoding(map);
        float cr = h.computeCompressionRatio(s, map);

        System.out.println("Compression ratio: " + (cr * 100) + "%");
    }

}

I tried to use the methods from LeafTreeNode but this didn't work. How I can create a leaf node?

Go through all 256 histogram entries, and for each one whose count is not zero, you use new LeafTreeNode(i, histogram[i]).

The next question will be: what do you do with each leaf once it's created?