"The type B cannot be a superinterface of C; a superinterface must be an interface" error

Question

Let's assume I got this interface A:

interface A
{
    void doThis();
    String doThat();
}

So, I want some abstracts classes to implement the method doThis() but not the doThat() one:

abstract class B implements A
{
    public void doThis()
    {
        System.out.println("do this and then "+ doThat());
    }

}

abstract class B2 implements A
{
    public void doThis()
    {
        System.out.println(doThat() + "and then do this");
    }
}

There error comes when you finally decide to implement de doThat method in a regular class:

public class C implements B
{
    public String doThat()
    {
        return "do that";
    }
}

This class leads me to the error aforementioned:

"The type B cannot be a superinterface of C; a superinterface must be an interface"

Anyone could now if this hierarchy of classes is valid or should I do other way round?

For more details I recommend looking at the Answers here: http://stackoverflow.com/questions/10839131/implement-vs-extends-when-to-use-whats-the-difference — Angelo Fuchs, Apr 30 '14 at 06:33

score 24 · Accepted Answer · edited May 23 '17 at 11:47

24

You must use extends

public class C extends B

Its important to understand the difference between the implements and extends Keywords. So, I recommend you start reading at this question: Implements vs extends: When to use? What's the difference? and the answers there.

edited May 23 '17 at 11:47

Community

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1

answered May 03 '12 at 16:03

Angelo Fuchs

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1

Thanks! I also forgot to add the public abstract String doThat() to each abstract classes :S – Painy James May 03 '12 at 16:13
1

@PainyJames you don't need to add the `public abstract String doThat()` to each of the abstract classes. – emory May 03 '12 at 17:24

score 5 · Answer 2 · answered May 03 '12 at 16:04

5

Since B is a class, the correct syntax is to use extends:

public class C extends B {

answered May 03 '12 at 16:04

NPE

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score 3 · Answer 3 · answered May 03 '12 at 16:05

3

B is an abstract class and cannot be used with the "implements" keyword. You have to use "extends" instead.

answered May 03 '12 at 16:05

Mister Smith

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"The type B cannot be a superinterface of C; a superinterface must be an interface" error

3 Answers3

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