Question

I currently have the following method in a class which I hope I can push to a superclass since I will have a few other classes which will need similar functionality.

public long convertToLong(EnumSet<SomeTypeHere> es) {

  long a = 0;

  for(SomeTypeHere sth : es) {
     a += sth.someLongProperty();

  }
}

It would be great if I can do this, I’ve never really used java generics before other than with collections.

Answer 1

You will need to put a bound on the generic type. If the class which contains convertToLong is parameterized on the same type, you can put the bound there:

import java.util.*;
public class GenericTest<C extends GenericTest.HasLongProperty> {
    static interface HasLongProperty {
        long someLongProperty();
    }
    public long convertToLong(Collection<C> es) {
        long a = 0;
        for(C sth : es)
            a += sth.someLongProperty();
        return a;
    }
}

Or if the class which contains convertToLong is not generic, you can put the bound in the declaration of that one method alone:

import java.util.*;
public class GenericTest {
    static interface HasLongProperty {
        long someLongProperty();
    }
    public <C extends GenericTest.HasLongProperty> long convertToLong(Collection<C> es) {
        long a = 0;
        for(C sth : es)
            a += sth.someLongProperty();
        return a;
    }
}