Abstract class needs to be observed

Hi! Thank you for reading my post!
I have been programming seriously in Java for two years now. But sometimes I still feel like I am a rookie.
Here is the situation:
Class A is a parent class. Subclasses B, C, and D extend A. Superclass A extends Observable, and thus it needs to notify its observers when it has changed. Superclass A has an abstract method, which is implemented by subclasses B, C, and D.
Question:
When the abstract method is invoked in subclasses B, C or D, can I somehow set Superclass A as 'changed' and 'notifyObservers' without having to make the 'changed' and 'notifyObservers' call in each of the subclasses B, C and D? In others words, is there a way to implement a non-abstract method (i.e. setChanged() or notifyObservers() ) inside of an abstract method (i.e. method() as shown below )?
Reasoning:
The reason that I want to avoid having to put this code into every subclass is because the subclasses may be created in the future without the programmer knowing that the subclass needs to notify of the change/update. I want to 'dummify' the programming for the future user. In addition, there are a lot of subclasses.
Here is some code to help:
Class A extends Observable
public void abstract method();
Class B extends A
public void method()
    //do something here...
    //I want to avoid having to type these lines in every class
    //that extends Class A:
    setChanged();    // <-- can I avoid putting this here?
    notifyObservers(); // <-- can I put it in Class A's abstract method somehow?
Class C extends A
public void method()
    //do something here...
    //I want to avoid having to type these lines in every class
    //that extends Class A:
    setChanged();    // <-- can I avoid putting this here?
    notifyObservers(); // <-- can I put it in Class A's abstract method somehow?
Class D extends A
public void method()
    //do something here...
    //I want to avoid having to type these lines in every class
    //that extends Class A:
    setChanged();    // <-- can I avoid putting this here?
    notifyObservers(); // <-- can I put it in Class A's abstract method somehow?
}Thanks for any suggestions you might have!
Regards,
Julia

OK. HMMM. Maybe I figured this out. Will this work?
(Sometimes I really feel like a rookie! ;) )
Class A extends Observable
public void method()
    this.method2()
    this.setChanged();
    this.notifyObservers();
public void abstract method2();
Class B extends A
public void method2()
    //do something here ....
Class C extends A
public void method2()
    //do something here ....
Class D extends A
public void method2()
    //do something here ....
}Ok - well, thanks anyways. I think this will work. Now I really feel stupid for even bothering to post. Oh well.
Hope you have a great day!
Thanks!
Julia

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arcadesstuff wrote:
Abstract class: a class that contains at least one abstract method. As has already been mentioned, abstract classes need not contain any methods at all. Have a look at the abstract class you posted; it's still abstract, though it contains no abstract methods. Note that if one declares any method of a class abstract, the class itself must be declared abstract as well.
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~

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Hi Maggie,
1. do I still need to write
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2. this
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".. is an abstract class. It can't be instantiated"

Does anyone have an idea of how I can get rid of the above error message? Here is a bit of my code:
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It is not possible to create instances of an abstract class.
I hope this example helps,
abstract class Vehicle
public static void main(String arg[])
Vehicle a;
a = new Automobile(); // ok, since Automobile is a kind of Vehicle
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class Automobile extends Vehicle

Difference between abstract class and the normal class

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Class vs. interface
Some say you should define all classes in terms of interfaces, but I think recommendation seems a bit extreme. I use interfaces when I see that something in my design will change frequently.
For example, the Strategy pattern lets you swap new algorithms and processes into your program without altering the objects that use them. A media player might know how to play CDs, MP3s, and wav files. Of course, you don't want to hardcode those playback algorithms into the player; that will make it difficult to add a new format like AVI. Furthermore, your code will be littered with useless case statements. And to add insult to injury, you will need to update those case statements each time you add a new algorithm. All in all, this is not a very object-oriented way to program.
With the Strategy pattern, you can simply encapsulate the algorithm behind an object. If you do that, you can provide new media plug-ins at any time. Let's call the plug-in class MediaStrategy. That object would have one method: playStream(Stream s). So to add a new algorithm, we simply extend our algorithm class. Now, when the program encounters the new media type, it simply delegates the playing of the stream to our media strategy. Of course, you'll need some plumbing to properly instantiate the algorithm strategies you will need.
This is an excellent place to use an interface. We've used the Strategy pattern, which clearly indicates a place in the design that will change. Thus, you should define the strategy as an interface. You should generally favor interfaces over inheritance when you want an object to have a certain type; in this case, MediaStrategy. Relying on inheritance for type identity is dangerous; it locks you into a particular inheritance hierarchy. Java doesn't allow multiple inheritance, so you can't extend something that gives you a useful implementation or more type identity.
Interface vs. abstract class
Choosing interfaces and abstract classes is not an either/or proposition. If you need to change your design, make it an interface. However, you may have abstract classes that provide some default behavior. Abstract classes are excellent candidates inside of application frameworks.
Abstract classes let you define some behaviors; they force your subclasses to provide others. For example, if you have an application framework, an abstract class may provide default services such as event and message handling. Those services allow your application to plug in to your application framework. However, there is some application-specific functionality that only your application can perform. Such functionality might include startup and shutdown tasks, which are often application-dependent. So instead of trying to define that behavior itself, the abstract base class can declare abstract shutdown and startup methods. The base class knows that it needs those methods, but an abstract class lets your class admit that it doesn't know how to perform those actions; it only knows that it must initiate the actions. When it is time to start up, the abstract class can call the startup method. When the base class calls this method, Java calls the method defined by the child class.

A Singleton variable in an Abstract Class

Hello there people
I have a quick question. I have made an abstract class in which I want to put an identifier (ID) int to be incremented whenever I make any of the concrete class instances that extend this abstract class.
whould that declairation be as follows?:
public static int IDAdditionally, I have a quick question about abstract constructors. If constructors are defined in the concrete classes, are their declairations needed in the abstract class?
and finally, if I wanted to use the int ID (as mentioned above) then would I have to use constructor from the abstract class?
thanks in advance for the help

I think the OP would want a constructor in the abstract class to update that variable. Otherwise, he'd have to remember to update it in the constructor for all concrete classes. But, you don't declare constructors for the concrete class in the abstract class. You only declare constructors for the abstract class in the abstract class.

"Abstract" method in a non-abstract class

Hi all.
I have a class "SuperClass" from which other class are extended...
I'd like to "force" some methods (method1(), method2, ...) to be implemented in the inherited classes.
I know I can accomplish this just implementing the superclass method body in order to throw an exception when it's directly called:
void method1(){
throw new UnsupportedOperationException();
}...but I was wondering if there's another (better) way...
It's like I would like to declare some abstract methods in a non-abstract class...
Any ideas?

The superclass just models the information held by
the subclasses.
The information is taken from the database, by
accessing the proper table (one for each subclass).??
What do you mean by "models the information"?
You should use inheritance (of implementation) only when the class satisfies the following criteria:
1) "Is a special kind of," not "is a role played by a";
2) Never needs to transmute to be an object in some other class;
3) Extends rather than overrides or nullifies superclass;
4) Does not subclass what is merely a utility class (useful functionality you'd like to reuse); and
5) Within PD: expresses special kinds of roles, transactions, or things.
Why are you trying to force these mystery methodsfrom the superclass?
It's not mandatory for me to do it... I 'd see it
just like a further way to check that the subclasses
implements these methods, as they have to do.That's not a good idea. If the superclass has no relation to the database, it shouldn't contain methods (abstract or otherwise) related to database transactions.
The subclasses are the classes that handle db
transaction.
They are designed as a binding to a db table.And how is the superclass designed to handle db transactions? My guess (based on your description) is that it isn't. That should tell you right away that the subclasses should not extend your superclass.

Abstract class extends other class?

What happens when a abstract class extends other class?
How can we use the abstract class late in other class?
Why do we need an abstract class that extends other class?
for example:-
public abstract class ABC extends EFG {
public class EFG{
private String name="";
private int rollno="";
private void setName(int name)
this.name=name;
private String getName()
return this.name;
}

shafiur wrote:
What happens when a abstract class extends other class?Nothing special. You have defined an abstract class.
How can we use the abstract class late in other class?Define "Late". What "other class"?
Why do we need an abstract class that extends other class?Because it can be useful to define one.

Abstract class needs to be observed

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