Collection.sort does not work for my Array

Question

I created my own MyArrayList but it can not be sorted. A normal Arraylist can be sorted by Collections.sort but mine can not. Could some one tell me where is wrong?

import java.util.*;

public class TestMyArrayList
{
    public static void main(String[] args){
        String[] subjects = {"EIE320", "EIE558", "EIE375", "EIE424"};
        Integer[] marks = {90,85,70,80};
        MyArrayList<String> sList = new MyArrayList<String>(subjects);
        Collections.sort(sList);
        sList.print();
        MyArrayList<Integer> mList = new MyArrayList<Integer>(marks);
        Collections.sort(mList);
        mList.print();
    }
}

public class MyArrayList<E> extends ArrayList<E>
    implements List<E>
{    
    private List<E> data;

    public MyArrayList()
    {
    }

    public MyArrayList(E[] inputdata){
        super();
        data = new ArrayList<E>();
        for(E e:inputdata){
            data.add(e);
        }
    }

    public void print(){
        for(Iterator iter = data.iterator();iter.hasNext();){
            System.out.print(iter.next() + " ");
        }

        System.out.println(" ");
    }
}

When you create a List inside a List you have two lists. Some methods use one List and some use the other. You would need use the debugger to see this. I suggest you either use delegations (i.e. don't extend ArrayList) or you use inheritance, but don't do a bit of both. — Peter Lawrey
– Peter Lawrey, Commented Mar 14, 2016 at 7:47

Eran · Accepted Answer · 2016-03-14 07:42:58Z

5

The constructor of MyArrayList is wrong. Since you are extending ArrayList, you shouldn't create another ArrayList instance in the MyArrayList constructor. Just add the elements to this ArrayList.

When you call Collections.sort() on your MyArrayList instance, it doesn't sort the data member List.

public MyArrayList(E[] inputdata){
    super();
    for(E e:inputdata){
        super.add(e);
    }
}

Your data member is not needed. You would only need such a member if you use composition instead of inheritance, in which case your class wouldn't extend ArrayList.

You should also modify your print method :

public void print(){
    for(Iterator iter = iterator();iter.hasNext();){
        System.out.print(iter.next() + " ");

    System.out.println(" ");
}

answered Mar 14, 2016 at 7:42

Eran

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nits.kk · Accepted Answer · 2016-03-14 07:59:43Z

If you see the source code of Collections.java you will see that Collections.sort calls the sort method defined in the concrete list which in this case is 'MyArrayList.java'. You have not provided this method so your class inherits this from the super class ArrayList.java Now if you see the source code of ArrayList.java you will find that sort method defined in it does the process of sorting on the Object[] elementData;. You have not set your data in elementData (defined in ArrayList.java).

You can use public ArrayList(Collection<? extends E> c) constructor from your public MyArrayList(E[] inputdata). You will have to transform the inputdata and then instead of calling super(), you should call the constructor public ArrayList(Collection<? extends E> c).

As a suggestion, also pointed by answer given by Eran, you can use composition, in you class you can have a reference to ArrayList and then can use it for addition, deletion and do sorting on it (define a method sort and within it call Collections.sort on the object of ArrayList you have).

vikingsteve · Accepted Answer · 2016-03-14 09:12:01Z

0

Just a heads up that you can use Arrays.asList() instead of doing what you are doing here.

    String[] subjects = {"EIE320", "EIE558", "EIE375", "EIE424"};
    ArrayList<String> sList = new ArrayList<String>(Arrays.asList(subjects));

If you are doing this as an excercise that's fine, but I would usually avoid re-writing Collections classes if at all possible.

answered Mar 14, 2016 at 9:12

vikingsteve

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