Working with an ArrayList of Functions in Java-8

Question

Problem Description: I want to be able to use an ArrayList of Functions passed in from another class (where the Functions have been defined in that other c

Answer 1

I'm not convinced I understand correctly, but I think the problem you're facing is how to call the functions in your List? The JavaDoc for the Function interface states that it has a single non-abstract method, apply() that you call to use the Function, as follows:

public double getResult(){
    double result = 0.0;

    for(int i = 0; i < operations.size(); i++){
        if(i == 0)
            result = operations.get(i).apply(initialValue);
        else
            result += operations.get(i).apply(result);
    }
    return result;
}

As an aside, that method could be tidied up a bit to make it simpler:

public double getResult() {
    double result = initialValue;

    //Not sure if this if-statement is a requirement, but it is to replicate
    //the functionality in the question
    if (operations.isEmpty()) {
        return 0.0;
    }

    for (Operation operation : operations) {
        result = operation.apply(result);
    }

    return result;
}

And as others have said in the comments, you should use generics when passing around your List objects (I guess you'll have to use something like List<Function<? extends Number, ? extends Number>)

Answer 2

The method of a java.util.function.Function object is apply. You need to call it like this:

operations.get(i).apply(initialValue)

However you use raw Function and therefore the result could be Object and you'd need to convert it to the appropriate type. Also you can't use the + (or the +=) operator with it. I'd suggest restricting the parameter types with Number:

List<Function<Number, ? extends Number>> operations = Arrays.asList(
        num ->  num.intValue() + 1,
        num -> num.intValue() - 1,
        num -> num.doubleValue() * 3.14
        ); // just an example list here 

public double getResult() {
    double result = 0.0;

    for (int i = 0; i < operations.size(); i++) {
        if (i == 0) {
            result = operations.get(i).apply(initialValue).doubleValue();
        } else {
            result += operations.get(i).apply(result).doubleValue();
        }
    }
    return result;
}