CODEWITHSUNDEEP
javagenerics
Random guy
Random guy

Reputation: 923

Confusion in generics in java

I am learning generics in java.At first we made a arraylist of Integer and we added a value in list.

import java.util.ArrayList;
import java.util.List;

public class GenericsRunner {
    public static void main(String[] args) {
        ArrayList<Integer> numbers=new ArrayList<>();
        numbers.add(1);
    }

    static <X extends List> void duplicate(X list) {
        list.addAll(list);
    }
}

The Method duplicate can be only used by those objects which are extending List. But, since List is interface 

public interface List<E> extends Collection<E>

And ArrayList is implemeting List

public class ArrayList<E> extends AbstractList<E>
        implements List<E>, RandomAccess, Cloneable,    
                   java.io.Serializable

I got stuck at this point. Extends is used by those classes which inherits the property from super class. But how Arraylist is extending List? Actually ArrayList is implementing List. How is this extends keyword working for this implements mechanism? I am getting confused that it is using extends and implements are same?

Upvotes: 0

Views: 72

Answers (1)

Dmitrii Abramov
Dmitrii Abramov

Reputation: 106

The reason is <X extends T> means just that X is inherited from T. There is no special syntax for interfaces like when you describe inheritance in class declaration.

ArrayList implements interface List which means that it is inherited from List, thus it is suitable for generic type <X extends List>.

In Java any class can extend only one class. By default it's Object when you do not specify another parent class. And class and interface can implement any amount of interfaces.

Hope this helps.

Upvotes: 1

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