Can a Type Variable Refer to Its Own Subtype in Java?

Can a Type Variable Refer to the Current Type?

In programming languages, type variables are essential for expressing type constraints and polymorphism. However, there are limitations to how these variables can be used. One common question is whether a type variable can refer to the exact subtype it is residing in.

Consider the following code:

class A {
    <T extends A> foo();
}

class B extends A {
    @Override
    T foo();
}

Here, we define a class A with a generic method foo() that takes a type parameter T. We also define a subclass B that overrides method foo(). The question is, can the type parameter T in foo() be used to refer to the exact subtype B in class B?

Answer:

Unfortunately, Java does not allow type variables to refer to the exact subtype they are residing in. The type variable T in the example above remains an unknown type within the scope of foo() and cannot be resolved to the subtype B.

Curiously Recurring Template Pattern (Self Type):

To achieve the desired behavior of having a "self" type, one can use a design pattern known as the Curiously Recurring Template (CRT) pattern, also called "self type." This pattern involves defining abstract base classes that enforce a contract for returning the runtime type of an instance. Derived classes implement the self type by resolving the type parameter to their own type.

Here's an example of the CRT pattern in Java:

abstract class SelfTyped<SELF extends SelfTyped<SELF>> {
    abstract SELF self();
}

public class MyLeafClass extends SelfTyped<MyLeafClass> {
    @Override
    MyLeafClass self() {
        return this;
    }
}

MyLeafClass mlc = new MyLeafClass();
mlc.self(); // returns mlc

While this pattern provides a way to refer to the current type using self(), it should be used with caution as it introduces potential for misuse and can compromise type safety.

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