CODEWITHSUNDEEP
javasortingcollectionslinked-list
csharpener
csharpener

Reputation: 33

Collection Sort method not working properly on linked list

Here is my code: 

import java.util.Collections;
import java.util.*;
public class ListMerge<String> extends LinkedList<String>
{
    public LinkedList<String> mergeLists(LinkedList<String> one, LinkedList<String> two)
    {
        LinkedList<String> newList = new LinkedList<String>(); 
        newList.addAll(one); 
        newList.addAll(two); 
        newList = Collections.sort(newList); 

        return newList; 
    }  
}

I get the error "cannot find symbol - method sort(java.util.LinkedList), and I am completely unaware as to why it is not working. I've imported the collections class and in the in-line coding I've called Collections, so I'm not sure why it can't seem to find the method.

I'm a beginner in coding so my vocabulary in coding isn't too strong, so please accommodate!

Thank you!

Upvotes: 0

Views: 1020

Answers (3)

Sotirios Delimanolis
Sotirios Delimanolis

Reputation: 280179

Your issue after changing to java.util.Collections is the following

                                                  v this is a type argument
public class ListMerge<String> extends LinkedList<String>
                       ^ this is a type parameter

The type parameter you have in ListMerge<String>, ie. <String> is the simple name of the parameter. It has nothing to do with java.lang.String. As such you are obscuring the type java.lang.String.

This might actually be what you wanted in the first place, ie. you planned to have ListMerge work for all types, equivalent to

public class ListMerge<T> extends LinkedList<T>

If it isn't, then what you want is 

public class ListMerge extends LinkedList<String>

Go through the official Java generics tutorial to understand more about the syntax of generic class declarations.

Upvotes: 0

sdanzig
sdanzig

Reputation: 4500

If you're intending on extending LinkedList, you have to extend the generic class "LinkedList... you can't extend a version of a generic class with the type specified: "LinkedList". You only specify a generic class's template arguments "with String, for instance" when you instantiate an instance of it

LinkedList<String> list = new LinkedList<String>()

You might find it simpler, however, to have a ListMerger class, that has a method "mergeLists" which returns a LinkedList object.

public class ListMerger<E extends Comparable>
{
    public LinkedList<E> mergeLists(LinkedList<E> one, LinkedList<E> two)
    {
        LinkedList<E> newList = new LinkedList<E>(); 
        newList.addAll(one); 
        newList.addAll(two); 
        Collections.sort(newList);

        return newList; 
    }  
}

And as others have mentioned, yes, the Collections.sort method modifies the list you input. It doesn't return a sorted copy of it. So see how I did it in my code. Then, all you have to do to get your merged LinkedList is this:

ListMerger<String> merger = new ListMerger<String>();
LinkedList<String> mergedList = merger.mergeLists(one, two);

Notice how I specified "String" when I was USING the generic class. Not defining it. Oh, and notice I changed the name from ListMerge to ListMerger. Just personal preference I guess.

EDIT: Updated the example to require E to be "Comparable". That way, Collections.sort() will accept it.

Upvotes: 0

Admit
Admit

Reputation: 4987

Watch out for return type: void sort(List<T> list), it sorts given list.

Compiler expects method with signature LinkedList<String> sort(List<T> list), which is not exist in Collections class.

This line is illegal:

newList = Collections.sort(newList);

You could easily fix that:

 Collections.sort(newList);

Upvotes: 3

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