Sorting in Java

[Sam Karny]

1. I have a Collection of 'JT' objects. JT has date, description, amount as its properties.
2. I want to sort this collection based on a key specified by the user (date, desc, amt). What is the best way to do this in Java?
3. Additionally, the sorting could either be ascending or descending, based on what the previous sorting was. e.g. if the previous sort was date ascending, the current sort has to be date descending.
I want to use the Comparator, but don't know how (or if) to use it.
Thanks in advance!

[Ernest Friedman-Hill]

Hi,
Welcome to JavaRanch!
Yes, you should use Comparator. In a nutshell, use Collections.sort() method, and pass as arguments two things: one, your Collection (it has to be a List) and two, an instance of a class that implements Comparator. It will look something like

class MyAmountComparator implements Comparator {
  public int compare(Object o1, Object o2) {
    JT jt1 = (JT) o1;
    JT jt2 = (JT) o2;
    return jt2.amount() - jt1.amount();
  }
}

To reverse the order of the sort, reverse the sign of the return value. To sort on other fields, implement other classes that compare those other fields. Easy!

[Joe Ess]

Check out the Java Tutorial: Collections

[steve souza]

Although it works for Arrays (Object[][]) the following class in the FormattedDataSet API may be of interest

   Object[][] array=...;
   ArrayComparator ac=new ArrayComparator();
   ac.addSortCol(0,"asc"); // sort by column 0 in ascending order first
   ac.addSortCol(1,"desc"); // then by column 1 in descending order
   ac.addSortCol(2,"asc");  // then by column 2 in ascending order
   ac.sort(array);  // note ac can be used to sort as many arrays as needed after it has been created.

For the javadocs: http://www.fdsapi.com/javadocs/com/fdsapi/ResultSetUtils.html
Go to http://www.fdsapi.com to download and read more about the API. It's primary use is to generate dynamic text such as xml and html.