Selection Sort with Boxes

Here is selection sort modified to sort an array of references to Box objects:

// Sort an array of Box references
  public static void selectionSort( Box[] array )
  { 
    // Find the Box reference that should go in each cell of
    // the array, from cell 0 to the end
    for ( int j=0; j < array.length-1; j++ )
    {
      // Find min: the index of the reference that should go into cell j.
      // Look through the unsorted references (those at j or higher)
      int min = j;
      for ( int k=j+1; k < array.length; k++ )
        if ( array[k].compareTo( array[min] ) < 0 ) min = k;  

      // Swap the reference at j with the reference at min 
      Box temp   = array[j];
      array[j]   = array[min];
      array[min] = temp;
    }
  }

The method is the same as the method to sort Strings, except that references have been changed from String to Box.

Examine how compareTo() is used:

        if ( array[k].compareTo( array[min] ) < 0 ) min = k;  

This statement is deciding if the the Box pointed to by array[k] is less than the Box pointed to by array[min]. If so, min is updated. The statement is using the compareTo() method that the programmer wrote for Box. Changing that method will change the order.