TreeSet and ClassCastException

Question

Hi,
I know that I shouldn't add abjects to TreeSet which can't be compared (it's called
mutually comparable, isn't it ?) - so basically thay all have to be the same type.
But could anybody explain me why if I break this tule I get this error:
Exception in thread "main" java.lang.ClassCastException: java.lang.Integer
     at java.lang.String.compareTo(String.java:90)
     at java.util.TreeMap.compare(TreeMap.java:1093)
     at java.util.TreeMap.put(TreeMap.java:465)
     at java.util.TreeSet.add(TreeSet.java:210)
     at com.adrian.Book.main(Book.java:26)
What interests me here is what a TreeMap is doing here ?
And how am I supposed to read this information (step by step) ?
I don't understand this output ...
Here is the code:
public class Book {
     public static void main(String[] args) {          
          Set s=new TreeSet();
          s.add(new Integer(1));
          s.add("a");
          s.add(new Integer(5));
          s.add(new Integer(3));
          for(Object o:s)
               System.out.print(o+" ");
}Thanks,
Adrian

Answer 1

Ok - I found that TreeSet uses TreeMap in its
implementation - that's why it appears
there.That is correct.
TreeSet makes a call to TreeMap's put(...) method on line 210 from the add(...) method.
Exception in thread "main" java.lang.ClassCastException: java.lang.Integer
at java.lang.String.compareTo(String.java:90)
at java.util.TreeMap.compare(TreeMap.java:1093)
at java.util.TreeMap.put(TreeMap.java:465)
at java.util.TreeSet.add(TreeSet.java:210)
at com.adrian.Book.main(Book.java:26)
If you're curious what is happening exactly, you can always (download and) take a look at the source code:
http://download.java.net/jdk6/