Problems with implementing Java Comparator - Generics

Question

So, I have got a problem in my Main class when I would like to call the mergeSort() method caused by the Comparator. I get the following message:

I have no idea how to fix that issue.. please help me!

Notice: Don't wonder that there doesn't happen anything in the code. I got stuck because I cannot prove the functionality of my code because of the above described problem :(

(Sry for my bad english)

class Algorithms
{
    public static <T> void mergeSort(final T[] a, final Comparator<T> c)
    {
        T[] list = a;
        Comparator<T> comp = c;
    }
}


public class Main
{
    public static void main(String[] args)
    {
        int[] unsortedList = {4,5,7,1,98,32}; //Expected = 1,4,5,7,32,98

        Comparator<Integer> sorted = Comparator.naturalOrder();
        int[] sortedList = Algorithms.mergeSort(unsortedList,sorted))
    }
}

Also doesn't help that you're expecting a return value from a mergeSort, which is a void method. — maccaroo
– maccaroo, Commented May 29, 2016 at 14:42
Yeah, you are right! I think that happened while trying many things to get that issue worked... :D — Cedric Michael
– Cedric Michael, Commented May 29, 2016 at 15:00

janos · Accepted Answer · 2016-05-29 14:39:42Z

3

In this code, the types don't match in the Algorithms.mergeSort call:

int[] unsortedList = {4,5,7,1,98,32}; //Expected = 1,4,5,7,32,98

Comparator<Integer> sorted = Comparator.naturalOrder();
Algorithms.mergeSort(unsortedList, sorted))

The type of unsortedList is int[] and the type of sorted is Comparator<Integer>. To make the types match, you need to use Integer[] as the type of unsortedList:

Integer[] unsortedList = {4, 5, 7, 1, 98, 32}; //Expected = 1,4,5,7,32,98

Another problem is that Algorithms.mergeSort returns void, so this still won't compile:

int[] sortedList = Algorithms.mergeSort(unsortedList, sorted);

You need to drop the assignment:

Algorithms.mergeSort(unsortedList, sorted);

Putting it together, this will work (after you implement Algorithms.mergeSort):

public static void main(String[] args) {
    Integer[] unsortedList = {4, 5, 7, 1, 98, 32};

    Comparator<Integer> sorted = Comparator.naturalOrder();
    Algorithms.mergeSort(unsortedList, sorted);
}

answered May 29, 2016 at 14:39

janos

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1 Comment

Cedric Michael Over a year ago

Thank you very much for that detailed answer! :) Now I understood why I have to use Integer instead of int! Have a good day! :)

Jude Niroshan · Accepted Answer · 2016-05-29 14:42:30Z

2

Use an Integer[] instead of int[].

answered May 29, 2016 at 14:42

Jude Niroshan

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1 Comment

Cedric Michael Over a year ago

Thank you very much! Problem is solved.. and that simple!

wake-0 · Accepted Answer · 2016-05-29 14:49:27Z

0

Another possibility would also be to work on a clone:

class Algorithm
{
    public static <T> T[] mergeSort(final T[] a, final Comparator<T> c)
    {
        T[] list = a.clone();
        Comparator<T> comp = c;
        Arrays.sort(list, comp);
        return list;
    }

    public static void main(String[] args)
    {
        Integer[] unsortedList = {4,5,7,1,98,32}; //Expected = 1,4,5,7,32,98

        Comparator<Integer> sorted = Comparator.naturalOrder();
        Integer[] sortedList = Algorithm.mergeSort(unsortedList,sorted);
    }
}

answered May 29, 2016 at 14:49

wake-0

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Comments

Nick Anderson · Accepted Answer · 2016-05-29 14:49:46Z

If you just want things to compile, you can look at this. There were a few problems:

Trying to assign to mergeSort doesn't work because it's a void method.
Currently your mergeSort does nothing, which you probably know.
The above answer is correct that you need to use Integers.

There were multiple syntax problems, like lack of semicolons and too many parens.

import java.util.Comparator;

class Algorithms
{
    public static <T> void mergeSort(final T[] a, final Comparator<T> c)
    {
        T[] list = a;
        Comparator<T> comp = c;
    }
}


public class Main
{

public static void main(String[] args)
{
    Integer[] unsortedList = {4,5,7,1,98,32}; //Expected = 1,4,5,7,32,98

    Comparator<Integer> sorted = Comparator.naturalOrder();
    Algorithms.mergeSort(unsortedList,sorted);
}
}

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