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Is there possible to add a new custom method to the well-known interface List in a way in Java? I tried to implement List interface to "public abstract class MyList", however, I would like to achieve it remaining the same name of the interface "List". For example, later implememnt in the following way:

List mylist = new ArrayList();
mylist.newmethod();
Darius Miliauskas
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  • extend it `public interface YourList extends java.util.List` – jmj Jan 15 '15 at 03:00
  • Imagine if this was possible. You could write this method: `public static void doStuff(List l) {l.newmethod();}`. Then someone else could write `doStuff(new ArrayList())`. What happens? – user253751 Jan 15 '15 at 03:05
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    @immibis other languages (e.g. Objective C) support these types of extensions and avoid these problems. It can be very convenient. – sprinter Jan 15 '15 at 03:09

3 Answers3

6

No, Java does not support such a feature. Use composition.

Sotirios Delimanolis
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6

You could extend it to another interface 

interface MyList<E> extends List<E> {
    //add new methods here
}
jmj
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    The nice thing about this solution is that you can extend `List` implementations, and have them implement this. So you can write something like `public class MyArrayList extends ArrayList implements MyList` and just write those methods in it that you've listed in `MyList`. – Dawood ibn Kareem Jan 15 '15 at 03:07
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    The disadvantage of this approach (as opposed to languages that allow extension) is that it doesn't then apply to all the classes that have already implemented List. – sprinter Jan 15 '15 at 03:10
  • Haha, bring on Java 8's "default implementations". @sprinter – Dawood ibn Kareem Jan 15 '15 at 03:13
1

Here's what you want to do. I wouldn't recommend it though, it can be confusing to have two List classes.

public abstract class List implements java.util.List {
    public void myMethod() {
        ...
    }
}
eduardohl
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