Check if a collection contains 2 numbers that add up to the input. This uses a hashmap so that the computational complexity is constant.

ArraySumWithMap.java

import java.util.HashMap;
import java.util.Iterator;
import java.util.Map;


public class ArraySumWithMap {

  private Map<Integer, Integer> numbers = new HashMap<Integer, Integer>();

	/**
	 * Do any 2 add up to this number
	 * @param number
	 * @return
	 */
	public boolean canAdd(int number){
		boolean canAdd = false;
		Iterator<Integer> it = numbers.keySet().iterator();
		while(it.hasNext()){
			int numberToFind = number - it.next();
			if(numbers.containsKey(numberToFind)){
				canAdd = true;
				break;
			}
		}
		return canAdd;
	}
	
	public ArraySumWithMap add(int n){
		numbers.put(n,n);
		return this;
	}
	
	public static void main(String[] args) {
		ArraySumWithMap problem = new ArraySumWithMap();
		problem.add(1).add(2).add(3).add(4).add(5);
		System.out.println(problem.canAdd(0));
	}

}