Sorting ArrayLists

[Steve Dyke]

The following sort class works fine when I use regular arrays however:

I get the following message when using ArrayList on the line that contains 'Comparable[] array1 = (Comparable[])o1;':

java.lang.ClassCastException: java.util.ArrayList incompatible with [Ljava.lang.Comparable;

I build my ArrayList of ArrayList then I call the following sort class:

Collections.sort(this.indentedBillArray, new ColumnSorter(0,1));

My sort class:

public class ColumnSorter implements Comparator  {
	int column;
	int column2;
	
	 
	   public ColumnSorter(int columnToSortBy, int columnToSortBy2) {
	      this.column = columnToSortBy;
	      this.column2 = columnToSortBy2;
	      
	   }
	 
	   public void setColumnToSortBy(int columnToSortBy, int columnToSortBy2) {
	      this.column = columnToSortBy;
	      this.column2 = columnToSortBy2;
	      
	   }
	 
	   public int compare(Object o1, Object o2) {
		   
		   int result = 0;
		   
	      Comparable[] array1 = (Comparable[])o1;
	      Comparable[] array2 = (Comparable[])o2;
	 
	      //return array1[this.column].compareTo(array2[this.column]);
	      System.out.println(array1[this.column]+","+array2[this.column]);
	      if((result = array1[this.column].compareTo(array2[this.column])) == 0)
	    		    {
	          /* If same last name, sort on second element (first name) */
	    	  result = array1[this.column2].compareTo(array2[this.column2]);
	        }
	      return result;
	   }

}

[Paul Clapham]

Well, yeah. The comparator is written to assume it's going to be given arrays to compare, or what you call "regular arrays". But an ArrayList isn't an array of any kind, despite its name, hence the error message.

[Steve Dyke]

Paul Clapham wrote:Well, yeah. The comparator is written to assume it's going to be given arrays to compare, or what you call "regular arrays". But an ArrayList isn't an array of any kind, despite its name, hence the error message.

Can you guide me in a sort soultion for ArrayList

[Paul Clapham]

Replace

          Comparable[] array1 = (Comparable[])o1;  
          Comparable[] array2 = (Comparable[])o2; 

by

          List array1 = (List)o1;  
          List array2 = (List)o2;  

and make the corresponding changes in the following lines.

[Steve Dyke]

Paul Clapham wrote:Replace

          Comparable[] array1 = (Comparable[])o1;  
          Comparable[] array2 = (Comparable[])o2; 

by

          List array1 = (List)o1;  
          List array2 = (List)o2;  

and make the corresponding changes in the following lines.

I am really struggling. What if I just want to sort my ArrayList<ArrayList<String>> on the first column assending?

[Paul Clapham]

I'm sorry, I don't understand that question. The first column of what?

[Campbell Ritchie]

There is something wrong when you are trying to sort columns in an object‑oriented language. Those columns should be incorporated into a class, and objects created from that class, which you can apply Comparators to. Then you can sort an ordinary array.

[Steve Dyke]

Paul Clapham wrote:I'm sorry, I don't understand that question. The first column of what?

I picture the ArrayList<ArrayList<String>> as having elements separated by commas with each iteration of the first ArrayList as being the row. So I would like to re-arange the rows based on the first string of each element.

The strings should be as:

862356-401, Seat Assy
854698-41, Bracket
859365-1, Angle

So I want to sort these strings in asending order by the part number.

[Winston Gutkowski]

Steve Dyke wrote:I picture the ArrayList<ArrayList<String>> as having elements separated by commas with each iteration of the first ArrayList as being the row. So I would like to re-arange the rows based on the first string of each element.

The strings should be as:
862356-401, Seat Assy
854698-41, Bracket
859365-1, Angle

So I want to sort these strings in asending order by the part number.

OK, there's a few things going on here:
1. Collections.sort() has no way of knowing that your first String is, in fact, a Part Number, so it will sort it as a String, which means that
862356-401 will sort before
862356-41
If you're OK with that, I don't see any particular problem with creating a Comparator to compare two Lists based on a particular element, viz:

public class ColumnComparator<T extends Comparable<? super T>>
   implements Comparator<List<T>>
{
   private int element;
   public ColumnComparator(int column) {
      this.element = column;      
   }
   public compare(List<T> row1, List<T> row2) {
      return row1.get(element).compareTo(row2.get(element));
   }
}

then your code might look something like:

List<List<String>> matrix = new ArrayList<ArrayList<String>>();
// populate 'matrix'
for (int i = 0, columns = matrix.get(0).size(); i < columns; i++) {
   Collections.sort(matrix, new ColumnComparator(i));
   // do whatever you need to do with your sorted 'matrix'
}

However, there are several possible 'gotchas', particularly if rows don't always have the same number of columns, and the whole thing feels a bit 'clunky' to me.

HIH

Winston

[Jeff Verdegan]

Steve Dyke wrote:

Paul Clapham wrote:I'm sorry, I don't understand that question. The first column of what?

I picture the ArrayList<ArrayList<String>>

Except as already stated you shouldn't have a list of lists. Instead you should have something like List<Product>, where the Product class defines productId and productName member variables, or whatever those columns represent.

[Henry Wong]

Steve Dyke wrote:

Paul Clapham wrote:I'm sorry, I don't understand that question. The first column of what?

I picture the ArrayList<ArrayList<String>> as having elements separated by commas with each iteration of the first ArrayList as being the row. So I would like to re-arange the rows based on the first string of each element.

The strings should be as:

862356-401, Seat Assy
854698-41, Bracket
859365-1, Angle

So I want to sort these strings in asending order by the part number.

While this discussion into the design, and the requirements, is a really a good idea -- I am wondering if this is a detour of the original question.

In the original question, you have a working comparator that compares using Comparable arrays, and the elements in the array are the "columns". So, to convert it to using an arraylist, isn't it just a 5 minute refactoring exercise? Just use a List<Comparable> and have the elements (based on index) of the list to be the "columns"?

Henry

[Steve Dyke]

Thanks for all the help.

I have it working by using the following:

public class ColumnSorterArrayList implements Comparator<List<String>> {   
  public int compare(List<String> list1, List<String> list2) {   
    return list1.get(0).compareTo(list2.get(0));   
  }   
}