Java8Sample2.java

//Vendor's code

interface ConditionCheck {
	public boolean check(Integer i);
}

class MathUtility {
	public static int sum(List<Integer> list, ConditionCheck condition){
		//ability to add is only here
		int sum=0;
		for(Integer i: list){
			if(condition.check(i)){
				sum=sum+i;
			}
		}
		return sum;
}

//client creates list of Ints
List<Integer> listOfInts = new ArrayList<Integer>();
listOfInts.add(2);
listOfInts.add(3);
listOfInts.add(4);

//client creates reusuable objects that will do the filtering 
ConditionCheck isEven = new ConditionCheck<Integer>(){
			@Override
			public boolean check(Integer i) {
				return i%2==0;
			}	
};

//client calls Vendor provided function
int result = MathUtility.sum(listOfInts, isEven);

//client feels like creating another filter  
final List<Integer> special = Arrays.asList(new Integer[]{2,3});
ConditionCheck isSpecial = new ConditionCheck(){
	@Override
	public boolean check(Integer i) {
		return special.contains(i);
	}
};

int result = MathUtility.sum(listOfInts, isSpecial);