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javaclasstemplatesgenericstypes
dammkewl
dammkewl

Reputation: 651

Java type mismatch, cannot convert to return itself as the generic type

It error's on the "return this;" line with this: Type mismatch: cannot convert from A to T.

public class A<T extends A<T>>{
    public T method() {
        return this;
    }
}

I don't understand why the compiler can't convert from A to T, when A is a suitable candidate for T.

Upvotes: 2

Views: 1081

Answers (1)

Tanmay Patil
Tanmay Patil

Reputation: 7057

Let us assume your code was valid...

You might have a subclass defined like this

public class B extends A<B> {

}

Here type parameter T is same as type of this.
Which is why I guess you said

A is a suitable candidate for T

Now consider this case

public class C extends A<B> {

}

which is perfectly valid since B passes all criteria required for T.

In this case your method declaration in class A becomes invalid.
(return type is B but you are returning instance of class C.)

So your assumption that

A is a suitable candidate for T

is invalid. Hence the error.

Hope this helps.
Good luck.

Upvotes: 2

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