Really need help!

[Andrew Gray]

I'm trying to override the add(obj) method in the linkedlist class, but I keep getting cannot find symbol errors. What am I doing wrong?

public class SortedList<E extends Comparable<? super E>> extends LinkedList<E>
{


public SortedList()
{

}

public boolean add(Object obj)
{
Object a = null;
int count = 0;
if (size() == 0)
{
super.add(obj);
}
while (count < size())
{
a = super.get(count);
if (a.compareTo(obj) < 0)
{
super.add(count-1, obj);
return true;
}else
{
super.add(count, obj);
}
count++;
}


}
}

[Campbell Ritchie]

Have you imported the classes from java.util?

[Andrew Gray]

Yeah. import java.util.*; is at the top of the code, just forgot to paste it in.

[Campbell Ritchie]

You should always give the exact code (unless it is long) and use the code button and give the full details of the error message. Then we can see what's going on.

[Campbell Ritchie]

By the way: are you returning something from every path through that overridden add method?

[Campbell Ritchie]

Oh, I see you are at ******University. I'm at Teesside. Good thing you're not at S*******d

[Andrew Gray]

Full code:

import java.util.*;

public class SortedList<E extends Comparable<? super E>> extends LinkedList<E>
{


public SortedList()
{

}

public boolean add(Object obj)
{
Object a = null;
int count = 0;
if (size() == 0)
{
super.add(obj);
}
while (count < size())
{
a = super.get(count);
if (a.compareTo(obj) < 0)
{
super.add(count-1, obj);
return true;
}else
{
super.add(count, obj);
}
count++;
}


}
}


error message: cannot find symbol - method add(java.lang.Object). this is the error i'm getting now after adding the first if statement 'if (size() ==0)', before I was getting the same more or less the same error, but replace add(java.lang.Object) with compareTo(java.lang.Object).

[Campbell Ritchie]

Try add(E obj)

[Campbell Ritchie]

Actually try this

@Override public boolean add(E obj)
{
. . .
}

I have a suspicion that your if-else will end up with you adding the element several times. It is difficult to tell because you didn't use the code button and your indentation looks wrong, but I think you will add obj several times in the else.

[Andrew Gray]

Yeah that fixed the add part. Now I'm still getting the cannot find symbol - compareTo(E)

[Andrew Gray]

import java.util.*;

public class SortedLinkedList<E extends Comparable<? super E>> extends LinkedList<E>
{
   

    public SortedLinkedList()
    {

    }
    
    public boolean add(E obj)
    {
        Object a = null;
        int count = 0;
        if (size() == 0)
        {
            super.add(obj);
        }
        while (count < size())
        {
            a = super.get(count);
            if (a.compareTo(obj) < 0)
            {
                super.add(count-1, obj);
                return true;
            }else
            {
                super.add(count, obj);
            }
            count++;
        }
        return false;

              
    }
}       

Now you've pointed it out, yeah I think it will just add the element several times in the else part. I'm going to have to change that, but I can't work out the error I'm getting.

[Steve Luke]

You declare a as an Object, which doesn't have a compareTo method. You should see if you can define it as an E.

[Andrew Gray]

Awesome dude, that's fixed the error! Now I just need to sort the else statement out and I'm done.

Many thanks.

[Campbell Ritchie]

You're welcome

[Campbell Ritchie]

You forgot the Override annotation.

[Andreas Hollmann]

deleted

[Andreas Hollmann]

Andrew Gray wrote:I'm trying to override the add(obj) method in the linkedlist class, but I keep getting cannot find symbol errors. What am I doing wrong?

....

Hi, SortedList encapsulates the sorting-aspect and is implemented as decorator in happy-collections library (Apache License 2.0).
The example below shows how you can convert any list to a SortedList by decorating it:

 
package org.happy.collections.decorators.examples.sorted;

import java.util.ArrayList;
import java.util.Arrays;
import java.util.Comparator;
import java.util.List;

import org.happy.collections.lists.decorators.SortedList_1x0;

/**
 * shows the basic syntax how the list can be sorted-decorated
 */
public class SortedListBasic {
    public static void main(String[] args) {
        // create SortedList
        List<Integer> sortedList = SortedList_1x0.of(new ArrayList<Integer>(),
                new Comparator<Integer>() {
                    @Override
                    public int compare(Integer o1, Integer o2) {
                        return o1.compareTo(o2);
                    }
                });

        // add some elements
        sortedList.add(2);
        sortedList.add(9);
        sortedList.add(7);
        sortedList.add(4);
        sortedList.add(4);
        sortedList.add(8);
        sortedList.add(1);
        sortedList.add(5);
        sortedList.addAll(Arrays.asList(new Integer[] { 5, 0, 6, 5, 3, 4 }));
        // print sorted list
        for (Integer i : sortedList) {
            System.out.print(i + ", ");
        }
    }

}

The SortedList decorator supports two strategies (1)"Array" good for ArrayList or TreeList and (2)"Linked" good for LinkedList (read the javadoc). The strategies are important if you want to improve the performance, here you can find a benchmark where the TreeList combined with SortedList is ten times faster as normal....

:-)
Andrej