HashSet with number of occurences

Question

Hi,
I use an HashSet to store some objects without repetitions,
is there a way to count how many times a specific object has occured?
I better explain:
the main app. finds an object, it tries to store it into the hashset,
so only one istance for each type of object is stored, although I find many copies of the same type of obj.
I wanna be able to count how many times it happens for each type of obj in the most efficient way.
Could you help me?
Thanks!
by

Answer 1

As always, be sure your "objects" have appropriately defined hashCode and equals methods:
http://java.sun.com/developer/Books/effectivejava/Chapter3.pdf
Demo:
import java.util.*;
public class MapExample {
    private Map < String, Integer > wordCounts = new HashMap < String, Integer > ();
    public void increment(String key) {
        Integer count = wordCounts.get(key);
        if (count == null)
            count = 0;
        wordCounts.put(key, 1 + count);
    public String toString() {
        return wordCounts.toString();
    public static void main(String[] args) {
        MapExample app = new MapExample();
        app.increment("one");
        app.increment("two");
        app.increment("three");
        app.increment("four");
        app.increment("two");
        app.increment("three");
        app.increment("four");
        app.increment("three");
        app.increment("four");
        app.increment("four");
        System.out.println(app);
}Message was edited by:
Hippolyte