Combinations in Java

combinations.java

package nl.falcon108;

import java.util.ArrayList;
import java.util.List;

public class Collections {
	
	/**
	 * Creates 2-dimensional ArrayList from a List with all the combinations of length k
	 * @param collection
	 * @param k
	 * @return 2-dimensional ArrayList with combinations
	 */
	public static <T> ArrayList<ArrayList<T>> combinations(List<T> collection, int k) {
		ArrayList<ArrayList<T>> comb    = new ArrayList<ArrayList<T>>();
		ArrayList<ArrayList<T>> chCombs;
		ArrayList<T> temp;
		for(int i = 0; i < collection.size() - k + 1; i++) {
			if(k > 1) {
				chCombs = combinations(collection.subList(i + 1, collection.size()), k - 1);
				for(ArrayList<T> c : chCombs) {
					temp = new ArrayList<T>(c);
					temp.add(0, collection.get(i));
					comb.add(temp);
				}
			} else {
				temp = new ArrayList<T>();
				temp.add(collection.get(i));
				comb.add(temp);
			}
		}
		return comb;
	}
	
	/**
	 * Creates an ArrayList with all the unique values from a List
	 * @param collection
	 * @return ArrayList with unique values
	 */
	public static <T> ArrayList<T> unique(List<T> collection) {
		ArrayList<T> unique = new ArrayList<T>();
		for(T t : collection) {
			if(!unique.contains(t)) {
				unique.add(t);
			}
		}
		return unique;
	}
}