Inconvenience due to abstract class does not have a constructor

[www]

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.

 

[Lew]

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

public abstract MyClass {
protected List _list;

You should not expose implementation details to subclasses. Make this
variable private.

Don't use underscores in names for non-constant (i.e., non-final or mutable)
variablea.

Instance variables should be initialized in the declaring class. You cannot
count on a subclass implementor to do this correctly.

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

You do not show a getList() method. Where does this come from?

}
..
}

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.

Not only is it not a requirement, it's terrible coding to set things up where
an instance variable is instantiated only by a subclass.

The point of an abstract class is to fix certain aspects of the
implementation. There is no such thing as an "abstract instance variable".

Should I do like this:

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

They should *never* need to implement a superclass variable.

public abstract void doIt();
}

Yes.

 

[Lew]

> [complete lack of SSCCE]

You've been around the newsgroup long enough to know about
<http://www.physci.org/codes/sscce.html>

Why did you refuse to do that? Your example leaves out significant details.