[Solved] Java – If I specify multiple generics, do I have to use them all?

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If you’re using generics*, you always need to include both types. Depending on your specific use, there are a couple of ways you could do this:

  1. Use a wildcard for one of them:

    Foo<?, Integer> foo = new Foo<>(69);

  2. Use the Void type:

    Foo<Void, Integer> foo = new Foo<>(69);

Both of these will prevent you from passing any value for A except for null. Return types for the first approach will be of type Object. For the second solution, they will be Void, which can only be null**, because it has no public constructor.


*You can use a raw type and specify no type parameters, but this is strongly discouraged.
**Unless you use reflection to instantiate it.

solved Java – If I specify multiple generics, do I have to use them all?