Java8Samples5.java

//Vendor makes this change to utilize java 8 features but he faces this issue
public static <T extends Number> Optional<T> sum(List<T> list, Predicate<T> condition){
	//ability to add is only here
		return list.parallelStream()
		.filter(condition)
		.map(i -> i)
        .reduce((a,b)->a+b); //Compile error: Operator + is undefined for type T, T
}

//client
int result = MathUtility.sum(listOfInts, i->i<4).get();


//one solution is to move the addition logic back to client 

int result = MathUtility.sum(listOfInts, i->i<4, (a,b)->a+b).get();

//vendor code
public static <T extends Number> Optional<T> sum(List<T> list, Predicate<T> condition, BinaryOperator<T> operation){
	//ability to add is only here
		return list.parallelStream()
		.filter(condition)
		.map(i -> i)
        .reduce(operation);
}

//But we don't want to do this!!! As our inital assumption is that client doesn't or don't want to do the arithmetic operation