Creating a 'sort' method for array objects in a linked list

Question

I have a project that requires we create a an object array and then we place that in a linked list, I am kind of stuck because I have trouble writing/implementing my sort method that is supposed to sort the the linked list. Here is my code of how far I've gone. By the way, the object name is 'temperature'; Thank you.

public class SelectionSort
{
   private static void SelectionSort (Temperature[] array, int size)
   { 
      for ( int i = 0; i < size - 1; i++ )
      { 
         int indexLowest = i; 
         for ( int j = i + 1; j < size; j++ )
         {
            if ( array[j] < array[indexLowest] ) 
               indexLowest = j;

            if ( array[indexLowest] != array[i] )
            { 
               Temperature temp = array[indexLowest];
               array[indexLowest] = array[i]; 
               array[i] = temp; 
            }// if
         }//for j
      }// for i 
   }// method 
}

I believe Temperature is your own class. Can you post that and on what ground you wanna sort. Also check this - mkyong.com/java/… — Atul O Holic
– Atul O Holic, Commented Mar 29, 2014 at 5:23
"Have trouble" in which way? Explain what results you're getting and how they differ from the desired results. — Adi Inbar
– Adi Inbar, Commented Mar 29, 2014 at 5:29
This isn't C++. If it were, then have a look at std::sort. — juanchopanza
– juanchopanza, Commented Mar 29, 2014 at 8:02

Seelenvirtuose · Accepted Answer · 2014-03-29 10:07:10Z

I think, your problem is the line

if ( array[j] < array[indexLowest] )

Both - array[j] and array[indexLowest] - are of type Temperature, according to your method's signature. So they are not primitive types and thus cannot be compared with <. This obviously results in a compiler error, that you really should have told us.

To compare objects like this, you have two possibilities:

1) Let the class Temperature implement Comparable<Temperature>. This interface will force you to add a method public int compareTo(Temperatue other) to your class Temperature. Implement this in the following way:

@Override
public int compareTo(Temperatue other) {
     if (/* this is smaller than other */) {
        return -1;
    } else if (/* this is greater than other */) {
        return 1;
    } else {
        return 0;
    }
}

You could return any other positive or negative integer if you want to. Implement the comparisonyourself based on the fields in Temperature.

Use this in your problematic line as:

if ( array[j].compareTo(array[indexLowest]) < 0 )

2) Write a Comparator for your class Temperature.

public class TemperatureComparator implements Comparator<Temperature> {
    public int compare(Temperature t1, Temperature t2) {
        if (/* t1 is smaller than t2 */) {
            return -1;
        } else if (/* t1 is greater than t2 */) {
            return 1;
        } else {
            return 0;
        }
    }
}

The logic is similar. Now you can use this comparator in your sort method

private static void SelectionSort (Temperature[] array, int size) {
    Comparator<Temperature> comparator = new TemperatureComparator();
    ...
        if ( comparator.compare(array[j], array[indexLowest]) < 0 )
    ...
}

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