About sort Date in TreeMap

Question

import java.util.*;
public class testTreeMap{
public static void main(String[] args){
TreeMap tree=new TreeMap();
Calendar date=Calendar.getInstance();
date.set(2004,9,7);
tree.put(date,"a");
date.set(2003,9,7);
tree.put(date,"b");
date.set(2002,9,7);
tree.put(date,"c");
date.set(2001,9,7);
tree.put(date,"d");
Set s=tree.keySet();
Object[] objs=s.toArray();
for(int i=0;i<objs.length;i++){
Calendar b=(Calendar)objs;
System.out.print(" "+b+" ");
error Report
Exception in thread "main" java.lang.ClassCastException: java.util.GregorianCale
ndar
at java.util.TreeMap.compare(Unknown Source)
at java.util.TreeMap.put(Unknown Source)
what's wrong?who can help me to correct this program?
at testTreeMap.main(testTreeMap.java:10)

Answer 1

what you need is like this:
tree.put(date.getTime(),"xxx");Yes to be able to use a TreeMap the key must be a class which implements the Comparable interface and Date is such a class. Luckily it's possible to convert between Calendar and Date objects using the getTime/setTime pair of methods. So you can store Date objects in the TreeMap but work with Calendar objects outside.
On the other hand if you don't need the ordering on key provided by a TreeMap you can as well store Calendar objects directly in a HashMap because accesses are faster.