Question

Possible Duplicate:
How can I sort the keys of a Map in Java?

In class TreeMap the Java API says:

A Red-Black tree based NavigableMap implementation. The map is sorted according to the natural ordering of its keys, or by a Comparator provided at map creation time, depending on which constructor is used.

What is meant by natural ordering ? A class used as key does not have to implement the Comparable interface, but what ordering will be used instead ?

Answer 1

If you were to try this yourself you’d find that you cannot use a TreeMap that has a K that does not implement Comparable (Unless you explicitly provide a Comparator via the TreeMap constructor).

public class App 
{
    public static void main( String[] args )
    {
        TreeMap<App,String> tm = new TreeMap<App,String>();
        tm.put(new App(), "value");
    }
}

Exception in thread “main” java.lang.ClassCastException: App cannot be cast to java.lang.Comparable

The javadoc for put() states this explicitly:

Throws:
ClassCastException – if the specified key cannot be compared with the keys currently in the map

The link in javadocs for TreeMap for “Natural Ordering” takes you to the Comparable interface