A histogram class in Java using Lambdas that I was quite proud of

Histogram.java

import java.util.ArrayList;
import java.util.Collection;
import java.util.Collections;
import java.util.HashMap;
import java.util.Iterator;
import java.util.Map;
import java.util.SortedMap;
import java.util.TreeMap;
import java.util.function.Function;

public class Histogram <U> {
	private SortedMap<Integer, U> result;

 	public <T> Histogram(Collection<T> data, Function<T, U> func) {
		if(data == null){
			 this.result = new TreeMap<Integer, U>();
		} else {
			Map<U, Integer> datamap = new HashMap<U,Integer>();
			result = new TreeMap<Integer,U>(Collections.reverseOrder());
			for(T s : data){
				U key = func.apply(s);
				int value = datamap.containsKey(key)?(int) datamap.get(key)+1:1;
				datamap.put(key, value);
			}
			for(U s : datamap.keySet()){
				this.result.put(datamap.get(s), s);
			}
		}
	}

	public  Histogram(Collection<U> data) {
		this(
				data,
				Function.identity()
		);
	}
	
	public Collection<U> getResult() {
		return this.result.values();
	}
	
	public String toString() {
		return result.toString();
	}
	
	public Collection<U> getTopNResults(int n){
		Iterator<U> it = getResult().iterator();
		Collection<U> outColl = new ArrayList<U>();
		for(int i = 0;it.hasNext() && i < n; i++){
			outColl.add(it.next());
		}
		return outColl;
	}
}