Generic method to perform a map-reduce operation. (Java-8)

Question

How to overload a Function with generic parameter in Java 8?

public class Test {

    List list = new ArrayList<>();

    public int          


        

Answer 1

Benji Weber once wrote of a way to circumvent this. What you need to do is to define custom functional interfaces that extend the types for your parameters:

public class Test {

    List list = new ArrayList<>();

    @FunctionalInterface
    public interface ToIntFunction extends Function{}
    public int sum(ToIntegerFunction function) {
        return list.stream().map(function).reduce(Integer::sum).get();
    }


    @FunctionalInterface
    public interface ToDoubleFunction extends Function{}
    public double sum(ToDoubleFunction function) {
        return list.stream().map(function).reduce(Double::sum).get();
    }
}

Another way is to use java.util.function.ToIntFunction and java.util.function.ToDoubleFunction instead:

public class Test {

    List list = new ArrayList<>();

    @FunctionalInterface
    public int sum(ToIntFunction function) {
        return list.stream().mapToInt(function).sum();
    }

    public double sum(ToDoubleFunction function) {
        return list.stream().mapToDouble(function).sum();
    }
}