Question

can somebody please explain what is wrong with my functions, thanks in advance:
I have those classes:

main

 public class Main {
        public static void main(String[] args) {
            String[] inArr = {"Hi", "Great", "Man", "Yeap", "Why"};
            int[] outArr = new int[5];

            Summer summing = new Summer();
            outArr = summing.sum(inArr, 2, new sumStringToInt(), outArr); //here problem

            for(int i = 0; i < outArr.length; i++){
                System.out.println(outArr[i] + " ");
            }

        }
    }

summer

public class Summer{
    public <X,Y> Y[] sum(X[] inArr, Y first, SumFunction<Y,X> f, Y[] outArr){
        for(int i = 0; i < inArr.length; i++){
            outArr[i] = f.op(first, inArr[i]);
            first = outArr[i];
        }
        return outArr;
    }
}

sum function

public abstract class SumFunction<Y,X> {
public abstract Y op (Y y, X x);
}

sumStrinToInt

public class sumStringToInt  extends SumFunction<Integer, String>{
    public Integer op(Integer num, String s){
        return s.length() + num;
    }
}

I have an error:

The method sum(X[], Y, SumFunction<Y,X>, Y[]) in the type Summer is not applicable for the arguments (String[], int, sumStringToInt, int[])

Answer 1

Primitives and generics don’t mix.

Make your array of the wrapper type Integer and it will compile:

Integer[] outArr = new Integer[5];

But note that you will have to initialize the array elements, because otherwise they’ll be null.