CODEWITHSUNDEEP
javagenerics
Denis Steinman
Denis Steinman

Reputation: 7799

How to return `this` in the generic class?

For example, I have a some class:

public class Test<T> {
    public T setField() {
        return this;
    }
}

Of course, it's an incorrect class. But can I write it some else? (to it hasn't errors). P.S. I know that I can write an abstract method to override it in the child class. But I ask about the generic class only (in the its class body only).

Upvotes: 3

Views: 1482

Answers (3)

newacct
newacct

Reputation: 122489

It is not possible for something like this to work safely in general in Java. this is only known to be an instance of Test<T>, which is not known to be a subtype of T, the type you want to return. There is no bound on T that can guarantee that this is an instance of T. No matter what bounds you give T, for any type that you can use for T that satisfies those bounds (call it X), I can just define a new class (unrelated to X) that extends Test<X>, and you cannot return an instance of this class as X, because it is not an X.

Upvotes: 1

Andrew
Andrew

Reputation: 49646

If you want to return an instance of your class (this), it does simply by using the class declaration (Test<T>):

public Test<T> setField() {
    return this;
}

If you want to return a type of a generic parameter, look at @wero's answer.

Upvotes: 8

wero
wero

Reputation: 33000

Casting this to T makes only sense if you use a recursive type bound. Still you need to add a cast and suppress the warning: 

abstract class Test<T extends Test<T>> {
     @SuppressWarnings("unchecked")
     public T setField()
     {
         return (T)this;
     }
}

Define derived classes like:

public class DerivedTest extends Test<DerivedTest> { ... }

and now this works without any casts needed in the client code:

DerivedTest d = new DerivedTest().setField();

Upvotes: 6

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