how to sort an arraylist by column?

I am not sure I understand fully. What exactly do you mean "contains column A, B, C"? Can you elaborate?


Regards

You probably mean a List of ArrayList. You can use a custom Comparator:

import java.util.*;

public class ColumnSorter implements Comparator {
  int columnIndex;

  public ColumnSorter(int columnIndex) {
    this.columnIndex = columnIndex;
  }

  public int compare(Object o1, Object o2) {
    //return ((int[])o1)[columnIndex] - ((int[])o2)[columnIndex];
    java.util.List list1 = (java.util.List)o1;
    java.util.List list2 = (java.util.List)o2;
    return ((Comparable)list1.get(columnIndex)).compareTo((Comparable)list2.get(columnIndex));
  }

}

thanks, what do you mean by Comparable?

Things like String, Integer, Date are Comparable

What will your ArrayList contain?

class test {

int a
string b
}

(That would sort on field 'a')

If the above comments have answered your question then please accept an answer, if not then post your further questions and we shall attempt to assist you further.

thanks, but I am trying to put everything together and getting ClassCastException:

   
     public int compareTo(Object other) {
          Test otherTest = (Test)other;
          return this.a - otherTest.a;
          // (Or however you want to sort them)
     }

do you try to minus these two value? so, this will return a number? then, combined with above codes you wrote:

public class ColumnSorter implements Comparator {
  int columnIndex;

  public ColumnSorter(int columnIndex) {
    this.columnIndex = columnIndex;
  }

  public int compare(Object o1, Object o2) {
    //return ((int[])o1)[columnIndex] - ((int[])o2)[columnIndex];
    java.util.List list1 = (java.util.List)o1;
    java.util.List list2 = (java.util.List)o2;
    return ((Comparable)list1.get(columnIndex)).compareTo((Comparable)list2.get(columnIndex));
  }

}

Collections.sort(yourListOfLists, new ColumnSorter(2));
*********************************************
don't know why is getting ClassCastException??? thanks.l

Have you made your Test class Comparable?

>>do you try to minus these two value? so, this will return a number?

Yes and yes

i did. it looks like the exception comes from

public int compare(Object o1, Object o2) {
    //return ((int[])o1)[columnIndex] - ((int[])o2)[columnIndex];
    List list1 = (List)o1;
    List list2 = (List)o2;
    return ((Comparable)list1.get(columnIndex)).compareTo((Comparable)list2.get(columnIndex));
  }

OK. As the first two lines of that method, do

System.out.println(o1.getClass());
System.out.println(o2.getClass());

Oh and get rid of that commented-out line i stupidly left in ;-)

it returns the "test" class, not the ArrayList of test....

:-)

Thanks for closing the question :)