Challenging Issue Encountered with Java Generics - Type Mismatch Error

I've encountered a puzzling issue while working on a project using Java Generics. I have two classes, ClassA<T> and ClassB<T extends Number>. ClassB extends ClassA and overrides a method in ClassA. Here are the bare-bone versions of the classes:

public class ClassA<T> {
    public T doSomething(T input) {
        // some operations here
        return input;
    }
}

public class ClassB<T extends Number> extends ClassA<T> {
    @Override
    public T doSomething(T input) {
        // some operations here
        return input;
    }
}

Now, when I'm trying to create a new object of ClassB with type String like this:

ClassB<String> objB = new ClassB<String>();

I'm getting a type mismatch error: Type parameter 'java.lang.String' is not within its bound; should extend 'java.lang.Number'.

I'm struggling to understand why the compiler is giving me this error when ClassA should accept any type, including String. Can anyone explain to me what's going wrong and how I might resolve it?

Thank you for your help.

avatar pri-shav
@pri-shav

2 Kontribusi 0 Poin

Diperbarui 8 bulan yang lalu

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The error you are encountering is due to how you have set up your classes with generics.

When you define ClassB<T extends Number> as an extension of ClassA<T>, the compiler uses the bounds from ClassB for its generic type. In this case, ClassB has a stricter definition of T than ClassA does. According to your definition, T in ClassB must extend Number, which is not the case for String.

Even though ClassA can accept any type, ClassB, which extends ClassA, cannot. Therefore, when you try to create an instance of ClassB<String>, the compiler checks the bounds of T in ClassB and finds that String doesn't extend Number, thus it throws an error.

To solve the issue, you would have to make sure the types you're using to instantiate ClassB comply with its generic bounds. That is, only types which extend Number can be used to instantiate ClassB. Here is an example:

ClassB<Integer> objB = new ClassB<>();

In this example, Integer extends Number, so it's a valid type for ClassB.

If you want ClassB to accept any type as ClassA does, you should not restrict T to extend Number. You could declare ClassB as follows:

public class ClassB<T> extends ClassA<T> { @Override public T doSomething(T input) { // some operations here return input; } }

Now, you can instantiate ClassB with String or any other type:

ClassB<String> objB = new ClassB<>();

This will definitely help you.

avatar jamesmillere
@jamesmillere

5 Kontribusi 4 Poin

Dipost 8 bulan yang lalu

Tanggapan

Wait let me try this.

Hi, It is working perfectly, thank you so much for the solution.

avatar pri-shav
@pri-shav

2 Kontribusi 0 Poin

Dipost 8 bulan yang lalu

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