Question

Hi guys i’ve never written a comparator b4 and im having a real problem. I’ve created a hashtable.

Hashtable <String, Objects> ht;

Could someone show how you’d write a comparator for a Hashtable? the examples i’ve seen overide equals and everything but i simply dont have a clue. The code below is not mine but an example i found, the key thing in hashtables means i cant do it like this i guess.

 public class Comparator implements Comparable<Name> {
        private final String firstName, lastName;

        public void Name(String firstName, String lastName) {
            if (firstName == null || lastName == null)
                throw new NullPointerException();
        this.firstName = firstName;
            this.lastName = lastName;
        }

        public String firstName() { return firstName; }
        public String lastName()  { return lastName;  }

        public boolean equals(Object o) {
            if (!(o instanceof Name))
                return false;
            Name n = (Name)o;
            return n.firstName.equals(firstName) &&
                   n.lastName.equals(lastName);
        }

        public int hashCode() {
            return 31*firstName.hashCode() + lastName.hashCode();
        }

        public String toString() {
        return firstName + " " + lastName;
        }

        public int compareTo(Name n) {
            int lastCmp = lastName.compareTo(n.lastName);
            return (lastCmp != 0 ? lastCmp :
                    firstName.compareTo(n.firstName));
        }
    }

Answer 1

Comparators are used to sort a list. A Hashtable (note the case) is not ordered by its elements. You can order a table by iterating over its keys (in the case you’d want to order on its keys, I presume) and put them in a List. The next thing to do is to sort the List and iterate over the List, and use a get out of the Hashtable to get its associated value.

Here is an example (using HashMap, since it’s more integrated with the rest of the Java Collections. A HashMap is essentially the same as Hashtable.):

public static void main(String... arg) {
    HashMap<String, Object> x = new HashMap<String, Object>();
    x.put("second", " ordered!");
    x.put("first", "Correctly");

    LinkedList<String> keys = new LinkedList<String>();
    for(final String f : x.keySet()) {
        keys.add(f);
    }
    Collections.sort(keys, new Comparator<String>() {
        public int compare(String first, String second) {
            // return -1 is "first <  second"
            // return 1  is "first >  second"
            // return 0  is "first == second"
            return first.compareTo(second);
        }
    });

    for(final String f : keys) {
        System.out.print(x.get(f));
    }
    System.out.println();
}

The order of the list keys is sorted by the anonymous Comparator class. It will sort alphabetically, as is the default for Strings. You can use your own key object, like you mentioned. If you don’t implement Comparator in this key object, then you can supply, as in the above example. Else you can use the default Comparator by calling:

Collections.sort(keys);

Which will use the classes implementation of Comparator. If it does not implement Comparator, then it will throw an exception (since it will cast to a Comparator)