Java usage of static declarations in generics

Question

I am confused on the following:

I want to do:

public class MyGenericClass<SomeType> extends SomeMoreAbstractClass<SomeType>{
    private Node<SomeType> beginNode;
    private static final Node<SomeType> NOT_FOUND = null;

    private static class Node<SomeType>{  
     //class definition here
    }
}

Ie I want to use the NOT_FOUND in the code in checks for null

I get the error Cannot make a static reference to the non-static type AnyType
in the line that I declare private static final Node<SomeType> NOT_FOUND = null;

How can I get arround this? I prefer to use it as a class variable and not an instance variable

Answer 1

You need to remove the static modifiers. Generics are, by definition, coupled to the concrete object. It is not possible to have static (= identical for all objects) members, that have a generic parameter.

Don't forget that SomeType is just a placeholder for what ever class you use, when you instanciate the object. (like this: MyGenericClass<Integer> x = new MyGenericClass<Integer>(); )

Answer 2

Simply declare the field as

private static final Node<?> NOT_FOUND = null;

As it's static, it will be shared between all instances of MyGenericClass , so it can't refer to SomeType which is a type parameter specified per class instance. Node<?> makes it compatible with all possible type parameters. It is then not useful for anything that depends on a specific type parameter, but fortunately, comparison with null still works.