How do I use multiple classes to access the same object?

Question

This should be staightforward. I have and object and I have multiple GUIs which operate on the same object. Do all the classes creating the GUIs need to be inner classes of the data class? I hope this question makes sense.
Thanks,
Mike

Answer 1

public final class SingletonClass {
private static SingletonClass instance;
private int lastIndex = 10;
public final static SingletonClass getInstance()
if (instance == null) instance = new SingletoClass();
return instance;
public int getLastIndex() {
return lastIndex;
}1) This won't work since one could create a new SingletonClass. You need to add a private constructor. Also, because the constructor is private the class doesn't have to be final.
2) Your design isn't thread-safe. You need to synchronize the access to the shared variable instance.
public class SingletonClass {
    private static SingletonClass instance;
    private static Object mutex = new Object( );
    private int lastIndex = 10;
    private SingletonClass() { }
    public static SingletonClass getInstance() {
        synchronized (mutex) {
            if (instance == null) {
                instance = new SingletonClass();
        return instance;
    public int getLastIndex() {
        return lastIndex;
}if you are going to create an instance of SingletonClass anyway then I suggest you do it when the class is loaded. This way you don't need synchronization.
public class SingletonClass {
    private static SingletonClass instance=new SingletonClass();
    private int lastIndex = 10;
    private SingletonClass() { }
    public static SingletonClass getInstance() {
        return instance;
    public int getLastIndex() {
        return lastIndex;
}If you don't really need an object, then you could just make getLastIndex() and lastIndex static and not use the singleton pattern at all.
public class SingletonClass {
    private static int lastIndex = 10;
    private SingletonClass() { }
    public static int getLastIndex() {
        return lastIndex;
}- Marcus