Common generic method to handle operations on subclasses of java.lang.Number

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I have a class MyGen which takes two generic arguments and has a method getValue()

public interface MyGen<E,T> {

    T getValue();
}

Usually the second generic type would be either Long or Integer.

Then, I wrote a method to combine the objects when the second generic type is Long as below:

public static <E extends MyGen<?, Long>> long combineValue(Set<E> set) {
    return set.stream()
              .map(MyGen::getValue)
              .reduce(0L, (a,b) -> a | b);
}

Now I want to have a similar method when the second type is an Integer. So I tried updating the above same method to :

public static <E extends MyGen<?, ? extends Number>> long combineValue(Set<E> set) {
    return (long) set.stream()
                     .map(MyGen::getValue)
                     .reduce((a,b) -> a | b) // error1
                     .orElse(0); // error2

}

But the following errors are displayed:

  1. error1

The operator | is undefined for the argument type(s) capture#4-of ? extends java.lang.Number, capture#4-of ? extends java.lang.Number

  1. error2

The method orElse(capture#4-of ? extends Number) in the type Optional<capture#4-of ? extends Number> is not applicable for the arguments (int)

Is there any way to handle both Long and Integer in the same method or could it be done only using two separate methods?

java-stream generics java
1 Answers

You seem to be fine about the return value of this common method being long. In that case, it is possible, because you can get a long from a Number using longValue:

public static long combineValue(Set<? extends MyGen<?, ? extends Number>> set) {
    return set.stream()
              .map(MyGen::getValue)
              .mapToLong(Number::longValue)
              .reduce(0L, (a,b) -> a | b);
}

You basically turn every number into longs first, and then do the reduction.

user_51335850
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