Questions on SortedMap & regards on implementing compareTo.

Question

the problem: im trying to run search algorithms on TreeMaps which contain string keys and values. these algorithms generally require the known first and last key. the interface Sorted Map, implemented by TreeMap, gives the methods firstKey() and lastKey(), however im having trouble dealing with them.
what gets me is that all keys inserted into the map must implement the Comparable interface. this requires me to define compareTo in my code but im having trouble with this. how do i compare the value of two strings together without using equals() which uses compareTo? im in a loop =)
all i want to do is run a binary search on the contents of a TreeMap!
can anyone help?

Answer 1

When you put String keys and values into a TreeMap, you don't need to implement anything. String implements the Comparable interface.
// file Test.java
import java.util.*;
public class Test {
  public static void main(String args[]) {
    TreeMap tree = new TreeMap();
    tree.put("a", "object a");
    tree.put("c", "object c");
    tree.put("b", "object b");
    System.out.println(tree.firstKey());
    System.out.println(tree.lastKey());
}This will show the following output.
a
cWhen you use a different class for your keys, that class should implement the Comparable interface, as in:
// file MyKey.java
import java.lang.*;
public class MyKey implements Comparable {
  private String s;
  public String getS() { return s; }
  public MyKey(String s) {
    this.s = s;
  public int compareTo(Object o) {
    if( o instanceof MyKey ) return s.compareTo(((MyKey)o).getS());
    else throw new ClassCastException("object is not an instance of MyKey");
  public String toString() { return s; }
}When you change the testprogram a little, you can add keys of class MyKey.
// file test2.java
import java.util.*;
public class Test2 {
  public static void main(String args[]) {
    TreeMap tree = new TreeMap();
    tree.put(new MyKey("a"), "object a");
    tree.put(new MyKey("c"), "object c");
    tree.put(new MyKey("b"), "object b");
    System.out.println(tree.firstKey());
    System.out.println(tree.lastKey());
}This will show the same output as the first program. Hope this will help.