Java class generic class references

Question

I have this abstract class

public abstract class AbstractA<T extends Comparable<? super T>> {
    protected T value;
}

and my class

public class A<T> extends AbstractA {
}

I want to use the compareTo function in class a

But java gives me the error:

The method compareTo(capture#5-of ? super capture#4-of ?) in the type Comparable is not applicable for the arguments (Comparable)

the function looks like:

protected find(Comparable value){
    if( this.value.compareto(value) == 0){
    //
    }
}

But the function should be Called with (T value)

How do I have to extend the abstract class?

As a point of style, in Java, class names (and interface names) should begin with a capital letter. Variables start with a lowercase letter. — ᴇʟᴇvᴀтᴇ, Jan 17 '18 at 17:44
Do you want class `A` still be bound to generic type `T` or a concrete type? — tsolakp, Jan 17 '18 at 17:50
Please share the code you're invoking `compareTo` with. It's hard to answer a question about an error if you don't show the actual lines of code causing the error. — dimo414, Jan 17 '18 at 17:51

ᴇʟᴇvᴀтᴇ · Answer 1 · 2018-01-17T17:54:11.123

To extend it you need to specify a generic interface that implements Comparable. For example:

public class A<T extends Comparable<? super T>> extends AbstractA<T> {
    // ...
}

Or a concrete class:

public class A extends AbstractA<String> {
    // ...
}

AbstractA does not implement Comparable. If you want A to implement Comparable you need to add it as an implements clause. E.g.

public class A extends AbstractA<String> implements Comparable<String> {

    @Override
    public int compareTo(String arg0) {
        return 0;
    }
}

To understand generics, there is a good Java tutorial.

Java class generic class references

1 Answers1