Java获取表示类实例的类的泛型类型

Question

I am able to declare class like this:

class TestClass<K extends TestClass<K>> {

}

I need a method returning K type:

class TestClass<K extends TestClass<K>> {
    K getK() {

    }
}

If I return class instance:

class TestClass<K extends TestClass<K>> {
    K getK() {
        return this;
    }
}

I have an error about incompatible types.

Sure, I can do cast:

class TestClass<K extends TestClass<K>> {
    K getK() {
        return (K) this;
    }
}

But in such a case I have warning about unchecked cast.

Is there a way to do that without warning supression?

EDIT. What I really want to do - to have these 2 classes:

class BaseViewHolder<T extends BaseViewHolder<T, K>, K extends BaseViewModel<T, K>> {
    K model;

    void setModel(K model) {
        this.model = model;
        model.setHolder((T) this);
    }
}

and

class BaseViewModel<T extends BaseViewHolder<T, K>, K extends BaseViewModel<T, K>> {
    T holder;

    void setHolder(T holder) {
        this.holder = holder;
    }
}

I want each concrete ViewModel know about a corresponding concrete ViewHolder, and each concrete ViewHolder know about corresponding concrete ViewModel.

That's why I want to have such structure.
Actually, I didn't find another way to tell BaseModel about holder rather than above.

Answer 1

Declaring a class with generics like that doesn't make that class an instance of K. You are not extending K. Your cast is not only unchecked but it is wrong.

Consider the example:

public static void main(String[] args) throws InterruptedException {
        new TestClass<AnotherClass>().getK().get0();        
    }

class AnotherClass extends TestClass<AnotherClass>{
    int get0() {
        return 0;
    }   
}

class TestClass<K extends TestClass<K>> {
    K getK() {
        return (K)this;
    }
}

This results into a runtime exception:

Exception in thread "main" java.lang.ClassCastException: TestClass cannot be cast to AnotherClass

Basically when you cast (K) this you are casting an object of type TestClass to an object which extending TestClass. The other way around it is true - K is a TestClass but TestClass is not a K.

EDIT:

I saw your problem but still that cast is not alright. You don't know what implementation/subclass of BaseViewHolder you will get and generics when used like that point to a specific type. K is a child of BaseViewHolder and casting any BaseViewHolder to K which is a specific subclass is not alright.

Why don't you just make it like:

class BaseViewModel {
    BaseViewHolder  holder;


   //You can call this setter with any subclass of BaseViewHolder  without casting!
    void setHolder(BaseViewHolder  holder) {
        this.holder = holder;
    }
}


class BaseViewHolder {
    BaseViewModel model;

   //You can call this setter with any subclass of BaseViewModel without casting!
    void setModel(BaseViewModel  model) {  
        this.model = model;
        model.setHolder( this);
    }
}

I cannot see any reason to use generics in your case. Making it like that you can extend BaseViewHolder and BaseViewModel and use the subclasses as instances of the base classes without any issues.