Ordering items in collection

Question

Using this code I am able to order each item in collection:
import java.util.Iterator;
import java.util.TreeSet;
public class Item implements Comparable<Item>{
     public Integer param1 = 0;
     public Integer param2 = 0;
     public int compareTo(Item i) {
          return param1.compareTo(i.param1);
     public static void main(String[] args) {
          // collection
          TreeSet<Item> mySet = new TreeSet<Item>();
          // create items
          Item i1 = new Item();
          i1.param1 = 4;
          i1.param2 = 4;
          Item i2 = new Item();
          i2.param1 = 3;
          i2.param2 = 7;
          Item i3 = new Item();
          i3.param1 = 5;
          i3.param2 = 7;
          // add items
          mySet.add(i1);
          mySet.add(i2);
          mySet.add(i3);
          // output
          for (Iterator<Item> iter = mySet.iterator(); iter.hasNext();) {
               Item element = iter.next();
               System.out.println("ITEM: param1="+element.param1+"  param2="+element.param2);
}Please, could anyone give me some hint - I need to have for example this classes:
class TreeSetOrderByParam1 { ... }and
class TreeSetOrderByParam2 { ... }and I need inserted items will be ordered differently (according to name of its class).
Thanks in advance for any help.
Message was edited by:
radone

Answer 1

Hello,
there appears to be some confusion here. You can't order the items in a set. There are basically three collection types in java.util: lists, sets and maps.
Lists are an ordered collection of objects. You have ArrayList, which is fast for access but slow for inserting and deleting objects. You have LinkedList, which is a bit slower for access, but much faster for inserting and deleting objects.
Sets are an unordered collection of objects. You have HashSet, which is fastest for access. You have TreeSet, which is a bit slower for access, but stores the objects in their natural order. However, you can't change the order!
Maps are for storing (key, value) pairs. You have HashMap, which is fastest for access and TreeMap, which stores the pairs according to the natural order of the keys.
If you want to order and reorder objects you have to use lists!
There are two ways for ordering objects. The first is that the objects implement the Comparable interface. The method
public int compareTo(Object o)determines the natural order of the objects. The method returns a negative integer if this object is less than the object in the argument. The method returns 0 if this object is equal to the method in the argument. The method returns a positive integer if this object is greater than the object in the argument.
You can order the objects in a list according to their natural order with the method Collections.sort(List<T> list) in the package java.util.
If you want to define a custom order on your objects, you have to create an object that implements the Comparator<T> interface in the package java.util. This interface has two methods:
public int compare(T o1, T o2)
public boolean equals(Object obj)The compare method returns a negative integer if o1 < o2, returns 0 if o1 == o2 and returns a positive integer if o1 > o2. The equals method returns true if and only if this comparator object is equal to the object in the argument. (The latter method makes only sense if you also include the comparator object in your list.)
You can order the objects in a list according to a custom order defined by a comparator with the method Collections.sort(List<T> list, Comparator<? super T> c) in the package java.util.