Method with generics don't accept class as argument

Question

I'm studying java in my class and I need to sort a doubly linked list, and I got the code, it works, but when I use the code in my environment, I receive an error:

The method sort(DoubleLinkedList<T>) in the type DoubleLinkedListSort is not applicable for the arguments (DoubleLinkedList<Item>)

This is the class that I got here: https://stackoverflow.com/a/2938529/4611395

Just changed the List class to my DoubleLinkedList class. So, I'm using this way.

DoubleLinkedList<Item> it = new DoubleLinkedList<Item>();

it = DoubleLinkedListSort.sort( it );

If I change the generic T to my class Item, it works with no problem. Anyone know why this problem? I learned that it should be working.

Thank you.

Edit:

This is my DoubleLinkedListSort that I edited.

package list;

import list.DoubleLinkedList;

public class DoubleLinkedListSort
{
    private static int depth = 0;

    public static <T extends Comparable<T>> DoubleLinkedList<T> sort( DoubleLinkedList<T> Ts )
    {
        depth++;
        String tabs = getTabs();

        System.out.println( tabs + "Sorting: " + Ts );

        if( Ts.listSize() <= 1 ) 
        {
            depth--;
            return Ts;
        }

        DoubleLinkedList<T> left = new DoubleLinkedList<T>();
        DoubleLinkedList<T> right = new DoubleLinkedList<T>();
        DoubleLinkedList<T> result = new DoubleLinkedList<T>();

        int middle = Ts.listSize() / 2;
        int added = 0;

        for( int i = 0; i < Ts.listSize(); i++ )
        {
            try
            {
                if( added++ < middle )
                    left.insertFirst( Ts.getElementAtPosition( i ) );
                else
                    right.insertFirst( Ts.getElementAtPosition( i ) );
            }
            catch( DoubleLinkedListException d )
            {

            }
        }

        left = sort( left );
        right = sort( right );
        result = merge( left, right );

        System.out.println( tabs + "Sorted to: " + result );

        depth--;
        return result;
    }

    private static <T extends Comparable<T>> DoubleLinkedList<T> merge( DoubleLinkedList<T> left, DoubleLinkedList<T> right ) 
    {
        String tabs = getTabs();
        System.out.println( tabs + "Merging: " + left + " & " + right );

        DoubleLinkedList<T> result = new DoubleLinkedList<T>();

        while( left.listSize() > 0 && right.listSize() > 0 )
        {
            try
            {
                if( left.getElementAtPosition( 0 ).compareTo( right.getElementAtPosition( 0 ) ) < 0 )
                {
                    result.insertLast( left.getElementAtPosition( 0 ) );

                    left.deleteElementAtPosition( 0 );
                }
                else
                {
                    result.insertLast( right.getElementAtPosition( 0 ) );

                    right.deleteElementAtPosition( 0 );
                }
            }
            catch( DoubleLinkedListException d )
            {

            }
        }

        try
        {
            if( left.listSize() > 0 )
            {
                for( int i = 0; i < left.listSize(); i++ )
                {
                    result.insertLast( left.getElementAtPosition( i ) );
                }
            }
            else
            {
                for( int i = 0; i < right.listSize(); i++ )
                {
                    result.insertLast( right.getElementAtPosition( i ) );
                }
            }
        }
        catch( DoubleLinkedListException d )
        {

        }

        return result;
    }

    private static String getTabs()
    {
        StringBuffer sb = new StringBuffer( "" );

        for( int i = 0; i < depth; i++ )
            sb.append( '\t' );

        return sb.toString();
    }
}

Edit 2:

As Method with generics don't accept class as argument and https://stackoverflow.com/a/30061121/4611395 commented, it was just implement Comparable and it worked.

public class Item implements Comparable<Item>
.
.
.
@Override
public int compareTo( Item it )
{
    return this.name.compareTo( it.name );
}

Thank you guys for the help.

GriffeyDog stole my thunder. That's the answer. Take a look at your linked code: public static > List sort(List list) { ... } — jgitter, May 05 '15 at 18:58
Put the relevant code in your question. In this case, the relevant code is the headers of the class declaration and the method declaration. — John Bollinger, May 05 '15 at 18:58
Please copy the _current_ version of the code you're using into this question. — Louis Wasserman, May 05 '15 at 18:58
You just copied your code out of some Stack Overflow answer? That's not going to teach you much. — user2357112, May 05 '15 at 18:59
"If I change the generic T to my class Item, it works with no problem. Anyone know why this problem?" - Doesn't make much sense. — Bubletan, May 05 '15 at 19:03
@Bubletan: If `Item` has a `compareTo` method but doesn't implement `Comparable`, you'd see that result when you change `sort` to refer to `Item`. — user2357112, May 05 '15 at 19:07

score 1 · Accepted Answer · edited May 23 '17 at 11:58

1

It looks like you will have to implement Comparable on your DoubleLinkedList class. You can follow this example to see how to implement Comparable

edited May 23 '17 at 11:58

Community

1
1

answered May 05 '15 at 19:00

nullPointer

133
7

score 0 · Answer 2 · answered May 05 '15 at 19:01

0

Are you using this class? http://www.java2s.com/Code/Java/Collections-Data-Structure/DoubleLinkedList.htm

If so, it doesn't implements List< T > so it is not compatible with the signature of the method

List<T> sort(List<T> list)

answered May 05 '15 at 19:01

B3rn475

1,057
5
14

Method with generics don't accept class as argument

2 Answers2