Hi,

I have an Abstract class and other classes which are subclasses of it.

public abstract MyClass {
    protected List _list;

    public abstract void doIt();
}

public class ClassA extends MyClass {

    public ClassA() {
        _list = new ArrayList(); //this is mandatory since _list is not
implemented YET!
        ...//other things
    }
   
    public void doIt() {
        double d = getList().get(1);  //using super_list
    }
    ..
}

public class ClassB extends MyClass {

    public ClassB() {
        _list = new ArrayList(); //this is mandatory since _list is not
implemented YET!
        ...//other things
    }

    ..
}

It is kind of hard or tricky for anybody to remember, if you extend your
class from MyClass, be sure to add "_list = new ArrayList()". Actually,
I didn't know such a "requirement" and got null point exception when I
ran my program.

Should I do like this:

public abstract MyClass {
    protected List _list = new ArralyList(); //now, the subclass no need to
implement it in their constructors

    public abstract void doIt();
}

Thank you for your help.

Thank you very much.