CODEWITHSUNDEEP
javagenerics
G Bisconcini
G Bisconcini

Reputation: 764

Java Generic Class Method Overriding

I'm doing an experience, trying to override a Method of a Generic Class, in this way:

public class MyGenFun<T extends CharSequence> extends Do<String>{
    public void doX(T s){
        System.out.println("in CHILD");
    }

    public static void main (String ...a){
        MyGenFun<String> x = new MyGenFun<>();
        x.doX("Test");  /// LINE 1
    }
}

class Do<A>{

    public void doX(A a){
        System.out.println("in Super");
    }
}

Why I can't? The compile error is

yGenFun.java:15: error: reference to doX is ambiguous, both method doX(A) in Do and method doX(T) in MyGenFun match x.doX("Test"); ^ where A,T are type-variables: A extends Object declared in class Do T extends CharSequence declared in class MyGenFun 1 error

If I comment the "Line 1", I can compile the code. So I think I am overriding correctly.

Upvotes: 1

Views: 135

Answers (3)

Konstantin Yovkov
Konstantin Yovkov

Reputation: 62864

Having this class hierarchy, you have two overloaded methods with signatures:

public void doX(String s){

}

public void doX(CharSequence s){

}

When you pass a String to doX, the compiler cannot infer which of the two methods to invoke, because String is compatible with String and CharSequence at the same time.

So, you should rather override like this:

public class MyGenFun<T extends CharSequence> extends Do<T>{

    @Override
    public void doX(T s){
        System.out.println("in CHILD");
    }
}

Upvotes: 3

beosign
beosign

Reputation: 433

This is because you are not actually overriding the method but just overloading it. To override, all argument types must match. So a correct override method would be

public void doX(String s) {       
}

T is something that extends CharSequence, which may be String but does not need to be String, it could for example be CharBuffer as well.

You instantiate the following:

MyGenFun<String> x = new MyGenFun<>();

So because you are not overriding this method but overloading it, there are now two methods with the same signature in the current context, one comes from the child class, one from the super class:

public void doX(String a) {
...
}

Upvotes: 4

swapyonubuntu
swapyonubuntu

Reputation: 2110

your method signature says that you need a parameter to pass , public void doX(T s);

so call like : x.doX("abc");

Upvotes: 0

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