Help please: static instance of a class

Question

Hello guys,
I'm quite new to Java (I had abandonned it for years, and now i'm recovering the good habbits) and i am finding trouble with an static instance of a class.
The thing is that I created a JSP page that calls a Java class. My JSP page will be reloaded several times for one same user, therefore, every teme the JSP page is reloaded, a new instance of my Java class is created.
Since I NEEDED to always have the same object of my Java class, I decided to create an static instance of this class. The snippet of my class is:
public class MyClass{
     private static MyClass instance = null;
     private Vector v=null;
     private myClass(){
     v = new Vector();     
public Vector getItems() {
     return this.v;
public static MyClass getInstance() {
          if(instance == null) {
          instance = new MyClass();
          return instance;
For the first time I call getInstance() I get a new instance of MyClass, and if i'm not wrong, the following times I call getInstance(), I should get the same instance, right???
The thing is that I implemented another method that changes the value of Vector v. But when calling this method from the static instance, the change is never reflected on Vector v (every time I try to recover the value of v I get a Null Pointer Exception)
Could anyone tell me what i am doing wrong, please?Is there any other way to do this??
Thanx in advance
Sergio

Answer 1

thanks for the answers, but now I have two questions
instead of one:
import java.util.Vector;
public class TestSingleton {
    public static void main(String[] args) throws Exception {
        MyClass clazz = MyClass.getInstance();
        System.out.println("#" +clazz);
        clazz.addItem("Test");
        clazz.addItem("Hello!");
        Vector vec = clazz.getItems();
        System.out.println(vec.size());
class MyClass{
    private static MyClass instance = null;
    private Vector v=null;
    private MyClass() {
        v = new Vector();
    public Vector getItems() {
        return this.v;
    public void addItem(Object obj) {
        this.v.add(obj);
    public static synchronized MyClass getInstance() {
        if(instance == null) {
            instance = new MyClass();
        return instance;
}Take a look at this code and see if this answers both your questions.
I love it when the questions multiply! :)