Question

What I’m simply trying to do is extending a class from LinkedList. Here is my code:

  import java.util.*;

  class Test {

    public static void main( String [] args ) {

        OrderedLinkedList ol = new OrderedLinkedList();
    }

    public class OrderedLinkedList extends LinkedList<Integer> {

        public boolean add( Integer item ) {
            for (int i=0; i < size(); i++) {
                Integer itemOfList = get( i );
                if ( itemOfList.compareTo( item ) > 0 ) {
                    add( i, item );
                    break;
                }
            }
            return true;
        }
    }

}

However, I got compile error with this message:

Test.java:7: non-static variable this cannot be referenced from a static context
        OrderedLinkedList ol = new OrderedLinkedList();
                               ^
1 error

I believe I’m not referencing anything but instantiating.

Answer 1

An inner class has an implicit reference to its outer class, unless marked static.

You need to mark the inner class static to avoid this:

public static class OrderedLinkedList extends LinkedList<Integer> {

So the compiler is complaining that there isn’t an instance of Test for the inner class as main is a static method.