Question

Let’s say I have a class A and this class has a List l. There is also a class B that extends A that will access the list l, as shown below. In my program, there must only be an instance of the class B (I’m using Singleton pattern), and there must be only one instance of the List l also, so I’m doing it like this:

public abstract class A {

    protected List<String> l;

    public A() {}

    protected synchronized List<String> getList() {
        if (l == null)
            l = new LinkedList<String>();
        return l;
    }
}

//---

public class B extends A {

    private static B instance;

    private B() {
        super();
    }

    public static synchronized B getInstance() {
        if (instance == null)
        instance = new B();
        return instance;
    }
}

What I want to do is:

In one instance of B:

System.out.println(super.getList().size()); //must print 0
super.getList().add("a");
System.out.println(super.getList().size()); //will print 1

In another instance of B:

System.out.println(super.getList().size()); //should print 1, cause i've already
                                            //added "a", but prints 0
super.getList().add("b");
System.out.println(super.getList().size()); //should print 2, but prints 1

That is not working as expected though. What am I doing wrong? Can anyone help me?

---

EDITED:

Hi, Bill the Lizard, here are the two classes:

public abstract class A {
    protected static List<String> l;

    public A() {}

    protected static synchronized List<String> getList() {
        if (l == null)
            l = new LinkedList<String>();
        return l;
    }
}

//---

public class B extends A {
    private static B instance;

    private B() {
        super();
    }

    public static synchronized B getInstance() {
        if (instance == null)
            instance = new B();
        return instance;
    }

    public void metodo() {
        System.out.println(super.getList().size());
        super.getList().add("a");
        System.out.println(super.getList().size());
    }
}

And this is how i create an instance of B:

public class ClassTeste {
    public static void main(String[] args) {
        B b = B.getInstance();
        b.metodo();
    }
}

Answer 1

I’ve run your main revised code, with the following main method,

public static void main(String[] args) {
 B b1 = B.getInstance();
 b1.metodo();

 B b2 = B.getInstance();
 b2.metodo();
}

and got the following output, which matches what you’re looking for

0
1
1
2