Overloading / generics in Java

Question

I want to run certain tests in Lists. The Lists can contain entirely different classes.

I have one method to check the consistency of the list - not null, not empty, no more than x elements. This is common to all the lists. Then I want to test each of the objects, using overloading.

The idea would be something like:

public static <T> void check(List<T> list) {

    //do general checks

    for (T element : list) {
        check(element);
    }
}

and then

public static void check(SomeType element) {...}

public static void check(SomeOtherType element) {...}

But I also had to add a method like this:

public static void check(T element) {...}

And this was called at runtime - not my other methods with the specific classes. Although the class was exactly the same. I'm evidently missing some generics understanding.

Now if I don't use the general method at all and try to solve it this way:

    public static void check(List<SomeType> list) {...}

    public static void check(List<SomeOtherType> list) {...}

Compiler error - "Method check(List) has the same erasure check(List) as another method..."

So is there any elegant solution for this? I could just use different method names but would like to know how it's possible without that.

Thanks!

Answer 1

This isn't something about generics that you're missing. Java does not have double dispatch. The call to check must be resolved at compile-time, and check(T) is the only match since the compiler can't tell if T is SomeType or SomeOtherType in a given scenario. It needs to choose one method to call that will work for all possible Ts.

This is sometimes solved using the visitor pattern.