Abstract class without absract method

Folks:
I know it's legal to create an abstract class with out any abstract method. I have seen some code like that written by somebody else. But I really don't know the purpose of doing it.
Can anybody give me a hint?
Thanks a lot!

The only reason I can think of is as a flag to
somebody using it that it doesn't do much, andyou're
supposed to provide your own implementation.
I'm not sure, but I think Swings XxxAdapter classes
that implement listeners with empty methods might be
abstract. They do nothing, and you're supposed to
provide "real" implementations for one or more ofthe
methods, but you wouldn't have to. That wouldn't bea
very useful class, so the base class might as wellbe
abstract.its funny that you mention this! i was just having
problems with the listeners because eveyr time i type
"implement actionListener" my IDE tells me that i need
to declare my public class as being abstract. but
once i type the method: public void
actionListener(actionEvent e){} then that error goes
away
just thought i'd give my two cents and support what
jverd is saying (not like he needs support :)Okay, but that's actually not what the OP was asking about or what I was talking about.
He was asking about an abstract class (a class declared as abstract) whose methods are all concrete--i.e. they all have implementations--and why make that class abstrac then.
What you're talking about is a class that's not declared abstract (your "implements ActionListener" class) but that does have abstract (unimplemented) methods.
The OP's case--a class declared abstract but with no abstract methods--is legal, but its value is non-intuitive.
Your case--a class that has abstract methods but is not declared abstract, which is the opposite of the OP's case--is not legal.

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Abstract classes and static methods

I have an abstract report class AbstractReportClass which I am extending in multiple report classes (one for each report, say ReportA, ReportB, ...). Each report class has its own static column definitions, title, etc., which I have to access through a static method getDataMeta() in a web application. Each report has the same exact code in getDataMeta, and no report may exist without these fields. My intuition tells me that AbstractReportClass should contain the code for getDataMeta, but I know that you can't mix abstract and static keywords.
Am I missing a simple solution to unify the getDataMeta code in the abstract base class? or do I really need to have a static function getDataMeta with the same code in each of the base classes?
My apologies if this has been discussed many times before.
Thanks,
-Andrew

I'm not trying to be "right"; rather I just asked a question about whether I can do something that seems intuitive. Perhaps you might write code in a different way than I would or perhaps I wasn't clear about every little detail about my code? Do you regularly belittle people who ask questions here?
I have a loadFromDB() member function in AbstractReport for which all sub classes have an overloaded version. All reports I'm displaying have 4 common fields (a database id and a name and a monetary value, for example), but then each other report has additional fields it loads from the database. Inside ReportX classes' loadFromDB(), I call the superclass loadFromDB() function and augment values to get a completely loaded object. In fact, the loadedData member object resides in AbstractReport.
I can't use a report unless it has these common features. Every report is an AbstractReport. There is common functionality built on top of common objects. Isn't this the point of inheritance? I'm essentially saying that abstract class Shape has a getArea function and then I'm defining multiple types of Shapes (e.g. Rectangle and Circle) to work with...

Abstract class with set and get methods

hi
how to write set and get methods(plain methods) in an abstartc class
ex: setUsername(String)
String getUsername()
and one class is extending this abstract class and same methods existing in that sub class also..... how to write......plz provide some ideas
am new to programming....
asap
thnx in advance

yes... as i told u.... i am new to coding......
and my problem is ..... i have 2 classes one is abstract class without abstract methods.. and another class is extending abstract class.....
in abstract class i have 2 methods...one is setusername(string) and getusername() ..... how to write these two methods.... in abstract class i have private variables username...... when user logins ..... i need to catch the user name and i need to validate with my oracle database and i need to identify the role of that user and based on role of that user i need to direct him to appropriate jsp page.......
for that now i am writing business process classes..... the above mentioned two classes are from business process.....
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Non-abstract methods in a Abstract class

Abstract Class can contain Non-abstract methods.
and Abstract Classes are not instantiable as well
So,
What is the purpose of Non-abstract methods in a Abstract class.
since we can't create objects and use it
so these non-abstract methods are only available to subclasses.
(if the subclass is not marked as abstract)
is that the advantage that has.(availability in subclass)
??

For example, the AbstractCollection class (in
java.util) provides an implementation for many of the
methods defined in the Collection interface.
Subclasses only have to implement a few more methods
to fulfill the Collection contract. Subclasses may
also choose to override the AbstractCollection
functionality if - for example - they know how to
provide an optimized implementation based on
characteristics of the actual subclass.Another example is the abstract class MouseAdapter that implements MouseListener, MouseWheelListener, MouseMotionListener, and that you can use instead of these interfaces when you want to react to one or two types of events only.
Quoting the javadocs: "If you implement the MouseListener, MouseMotionListener interface, you have to define all of the methods in it. This abstract class defines null methods for them all, so you can only have to define methods for events you care about."

Why use abstract classes?

Why should I use abstract classes instead of a regular class with empty method bodies? Just better design? Is there some logical or performance based reason?

Why should I use abstract classes instead of aregular
class with empty method bodies? Just better design?Is
there some logical or performance based reason?Because it describes what you're doing.
If you define a "regular" class with empty message
bodies, everyone who looks at it will say "WTF is he
trying to do" or, if they're charitable, "Look,
Harvey, someone who's trying to make Java look just
like C++!".
sigh Maybe answers like THIS are why people keep asking the same question.
Here's a couple things an abstract class does that a "regular" class with empty methods bodies doesn't:
1) An abstract class cannot be instantiated.
2) An abstract class forces it's abstract methods to be implemented.
If you were to extend a non-abstract class with empty method bodies, you wouldn't have to override the methods... you could just leave them empty. An abstract class forces it.
There's a lot more reasons... those are a couple obvious ones.

Abstract class - subclass

a quik question: if I have a abstract class define a method, and a subclass implementing it, and a third class with an instance casted as the abstract class, would the following method in the subclass then return the instance casted as subclass? :
public Subclass getInstance() {
return this;
Stig.

oops.... cant return SubClass....

Good programming practice - Abstract class

Hi all,
I have been trying to help another soul in this forum, and came to the conclusion that I don't know good
programming practice when it comes to abstract classes.
Is this correct?
You CAN implement methods in an abstract class, but it's not recommended.
I have NEVER done this...when is there possibly a need to?
Regards.
/ k

Yes, absolutely, you can implement methods in an abstract class. Any method that all subclasses will perform in the same way can be implemented in the abstract base class. If subclasses perform similiar functions depending on their type you declare those as abstract in the base class. Here is a contrived example that I have seen on job interviews.
Suppose your developing an application that draws on a panel. We want to provide some canned shapes such as a circle, a square and a triangle. We want to be able to draw the shape set or get its color and calculate its area.
Let's define an abstract base class Shape
public abstract class Shape{
    private Color myColor;
   // since color has nothing to do with what kind of shape we're working with, create concrete implementation
   public Color getColor(){
        return myColor;
public void setColor(Color newColor){
   myColor = newColor;
// however, drawing the shape and calculation its area are depending on the actual shape.
public abstract void draw();
public abstract double getArea();
// so then Square would be something like
public class Square extends Shape{
   public double get Area()
      return sideLength * sideLength // assumes somehow we know sideLength
public void draw(){
              // concrete implementation
}we can do the same things for Circle class and Triangle class.
And, if you think about it you'll notice that we could have made a Rectangle class and then Square would be a subclass of Rectangle where both dimensions are equal.
I hope that somewhat strained example helps answer your question.
DB

Why does this abstract class and method work without implement it?

hi,
I have seen many times that in some examples that there are objects made from abstract classes directly. However, in all books, manual and tutorials that I've read explain that we MUST implement those methods in a subclass.
An example of what I'm saying is the example code here . In a few words that example makes Channels (java.nio.channel) and does operations with them. My problem is in the class to make this channels, because they used the ServerSockeChannel class and socket() method directly despite they are abstracts.
   // Create a new channel: if port == 0, FileChannel on /dev/tty, else
   // a SocketChannel from the first accept on the given port number
private static ByteChannel newChannel (int netPort)
      throws Exception
      if (netPort == 0) {
         FileInputStream fis = new FileInputStream ("/dev/tty");
         return (fis.getChannel());
      } else {
//CONFLICT LINES
         ServerSocketChannel ssc = ServerSocketChannel.open(); //<--I have never thought do that!! Anyway, how it is static method may work.
         ssc.socket().bind (new InetSocketAddress (netPort)); //<--but here, this method (socket) is abstract. WHY RETURN A SOCKET???????? this mehod should be empty by default.
         System.out.print ("Waiting for connection on port "
            + netPort + "...");
         System.out.flush();
         ByteChannel channel = ssc.accept();
         ssc.close();
         System.out.println ("Got it");
         return (channel);
   } I test this code and works fine. So why can it be??
Also, I read that the abstract classes can't have static methods. Is it true???
Please Help!!
PS: i have seen this kind of code many times. So i feel that I don't understand how its really the abstract methods are made.
PS2: I understand that obviously you don't do something like this: *"obj = new AbstractClass(); "*. I dont understand how it could be: ServerSocketChannel ssc = ServerSocketChannel.open(); and the compiler didn't warn.

molavec wrote:
ServerSocketChannel ssc = ServerSocketChannel.open(); //<--I have never thought do that!! Anyway, how it is static method may work.
The static method creates an instance of a class which extends ServerSocketChannel, but is actually another non-abstract class.I thought that, but reading the documentation I saw that about open() method:
Opens a server-socket channel.
The new channel is created by invoking the openServerSocketChannel method of the system-wide default SelectorProvider object.
The new channel's socket is initially unbound; it must be bound to a specific address via one of its socket's bind methods before connections can be accepted.
...and the problem is the same openServerSocketChannel is abstract, so i don't understand how it could return a ServerSocketChannel.There is a concrete implementation class that has implemented that method.
I guess that really the open() method use a SelectorProvider's subclase but it doesn't appear in the doc.It doesn't need to. First, you don't care about those implementation details, and second, you know that if the class is abstract, it must use some concrete subclass.
Ok, I speak Spanish by default (<-- this sounds like "I am a machine", ^_^' ). So, I didn't know how to say that the method would be {}. Is there a way to say that?? I recommendable for me to know, for the future questions o answers.Not sure what you're saying here. But the other respondent was trying to explain to you the difference between an abstract method and an empty method.
// abstract method
public abstract void foo();
// empty method
public void bar() {
Which class does extend ServerSocketChannel? I can not see it.It may be a package-private class or a private nested class. There's no need to document that specific implementation, since you never need to use it directly.

Abstract method called in an abstract class

Hello,
I am writing some code that I'd like to be as generic as possible.
I created an abstract class called Chromozome. This abstract class has a protected abstract method called initialize().
I also created an abstract class called Algorithm which contains a protected ArrayList<Chromozome>.
I would like to create a non abstract method (called initializePopulation()) which would create instances of Chromozome, call their method initialize() and full the ArrayList with them.
In a practical matter, only subclass of Algorithm will be used, using an ArrayList of a subclass of Chromozome implementing their own version of initialize.
I have been thinking of that and concluded it was impossible to do. But I'd like to ask more talented peaple before forgetting it !
Thanks,
Vincent

Ok, let's it is not impossible, juste that I had no idea of how doing it :-)
The difficulty is that Algorithm will never have to deal with Chromozome itself, but always with subclass of Chromozome. This is usually not an issue, but in that case, Algorithm is required to create instances of the desired subclass of Chromozome, but without knowing in advance wich subclass will be used (I hope what I say makes any sense).
Actually I may have found a way in the meantime, but maybe not the best one.
I created in Algorithm an abstract method :
protected abstract Chromozome createChromozome()The method initializePopulation will call createChromozome instead of calling directly the constructor and the initialize() method of Chromozome.
Then subclass of Algorithm will implement the method createChromozome using the desired subclass of Chromozome.

Can i call non -abstract method in abstract class into a derived class?

Hi all,
Is it possible in java to call a non-abstract method in a abstact class from a class derived from it or this is not possible in java.
The following example will explain this Ques. in detail.
abstract class A
void amethod()
System.out.println(" I am in Base Class");
public class B extends A
void amethod()
System.out.println(" I am in Derived Class");
public static void main (String args[])
// How i code this part to call a method amathod() which will print "I am in Base Class
}

Ok, if you want to call a non-static method from a
static method, then you have to provide an object. In
this case it does not matter whether the method is in
an abstract base class or whatever. You simply cannot
(in any object oriented language, including C++ and
JAVA) call a nonstatic method without providing an
object, on which you will call the method.
To my solution with reflection: It also only works,
if you have an object. And: if you use
getDeclaredMethod, then invoke should not call B's
method, but A's. if you would use getMethod, then the
Method object returned would reflect to B's method.
The process of resolving overloaded methods is
performed during the getMethod call, not during the
invoke (at least AFAIK, please tell me, if I'm wrong).You are wrong....
class A {
    public void dummy() {
         System.out.println("Dymmy in A");
class B extends A {
     public void dummy() {
          System.out.println("Dymmy in B");
     public static void main(String[] args) throws Exception {
          A tmp = new B();
          Class clazz = A.class;
          Method method = clazz.getDeclaredMethod("dummy", null);
          method.invoke(tmp, null);
}Prints:
Dymmy in B
/Kaj

I really need abstract static methods in abstract class

Hello all.. I have a problem,
I seem to really need abstract static methods.. but they are not supported.. but I think the JVM should implement them.. i just need them!
Or can someone else explain me how to do this without abstract static methods:
abstract class A {
abstract static Y getY();
static X getX() {
    // this methods uses getY, for example:
    y=getY();
   return new X(y); // or whatever
class B extends A {
static Y getY() { return YofB; }
class C extends A {
static Y getY() { return YofC; }
// code that actually uses the classes above:
// these are static calls
B.getX();
A.getX();I know this wont compile. How should i do it to implement the same?

Damn i posted this in the wrong thread.. anyways.
Yes offcourse i understand abstract and static
But i have a problem where the only solution is to use them both.
I think it is theoretically possible ot implement a JVM with support for abstract static methods.
In fact it is a design decision to not support abstract static methods.. thats why i am asking this question.. how could you implemented this otherwise?
There is an ugly soluition i think: using Aspect Oriented Programming with for example AspectJ.. but that solution is really ugly. So anyone has an OO solution?

Protected methods in abstract classes

Hello All
I have some problem which I cannot find a workaround for.
I have three classes:
package my.one;
public abstract class First {
protected void do();
protected void now();
package my.one;
public class NotWantToHave extends First {
protected First obj;
public NotWantToHave(First O) { obj = O; }
public void do() { obj.do(); }
public void now() { obj.now(); }
package my.two;
public class Second extends my.one.First {
protected void do() { System.out.println("Second does"); }
protected void now() { System.out.println("Second does now"); }
package my.three;
public class Three extends my.one.First {
protected my.one.First obj;
public Three(my.one.First O) { obj = O; }
protected void do() { System.out.println("Doing"); }
protected void now() { obj.now(); } // Not possible, see later text
Problem is, as one can read in http://java.sun.com/docs/books/tutorial/java/javaOO/accesscontrol.html , it says that you cannot access protected members and methods from classes if they are in a different package. However, since my class Three should not concern about the method now() but should use from some other class that implements (i.e. class Second), the question I have is how to do?
One way would be to implement a class that simply make a forward call to any protected method in the same package the abstract class is in like in class NotWantToHave and pass it to the constructor of class Third while this class was created with an instance of class Second. However, such a call would look very clumsy (new my.three.Third(new my.one.NotWantToHave(new my.two.Second()));). Furthermore, everyone could create an instance of class NotWantToHave and can invoke the methods defined as protected in class First, so the access restriction would be quite useless.
Does anyone has a good idea how to do?

Hi
One way I found is to have a nested, protected static final class in my super-class First and provide a protected static final method that returns a class where all methods of the super-class are made public and thus accessible from sub-classes at will. The only requirement is that a sub-class must invoke this method to encapsulate other implementations of the super-class and never publish the wrapper class instance. This will look as follows:
package my.one;
public abstract class First {
protected final static class Wrap extends First { // extend First to make sure not to forget any abstract method
    protected First F;
    public void do() { F.do(); }
    public void now() { F.now(); }
    protected Wrap(First Obj) { F = Obj; }
} // end Wrap
protected final static First.Wrap wrap(First Obj) { return new First.Wrap(Obj); }
protected abstract void do();
protected abstract void now();
} // end First*******
package my.two;
public class Second extends my.one.First {
protected void do() { System.out.println("Second does"); }
protected void now() { System.out.println("Second does now"); }
} // end Second*******
package my.three;
public class Three extends my.one.First {
protected my.one.First.Wrap obj;
public Three(my.one.First O) { obj = my.one.First.wrap(O); }
protected void do() { System.out.println("Doing"); }
protected void now() { obj.now(); } // Not possible, see later text
} // end Third*******
In this way, I can access all methods in the abstract super class since the Wrap class makes them public while the methods are not accessible from outside the package to i.e. a GUI that uses the protocol.
However, it still looks clumsy and I would appreciate very much if someone knows a more clear solution.
And, please, do not tell me that I stand on my rope and wonder why I fall down. I hope I know what I am doing and of course, I know the specification (why else I should mention about the link to the specification and refer to it?). But I am quite sure that I am not the first person facing this problem and I hope someone out there could tell me about their solution.
My requirements are to access protected methods on sub-classes of a super-class that are not known yet (because they are developed in the far, far future ...) in other sub-classes of the same super-class without make those methods public to not inveigle their usage where they should not be used.
Thanks

Java abstract classes and methods

Can anyone please tell me any real time example of abstract classes and methods.
I want to know its real use. If anyone have ever used it for some purpose while programming please do tell me.

Ashu_Web wrote:
No please.. I just want to know if you have used it while programming. Like "an abstract class can be used to put all the common method names in it without having to write actual implementation code."That would describe an Interface better than an abstract class. Abstract classes usually have at least some implementation.
I want to know its usage in programming, not just a definition. I guess you understand what I am looking for.Yes, and I gave you one: java.util.AbstractList. It can be found inside the src.zip in your JDK directory and it is a pretty good example for an abstract class that provides some implementation and defines exactly what is necessary to make a full List implementation.

Using Abstract Class Methods

I just want to know that in how many ways can we use the non abstract methods of the class without extending the class(dont want to implement all abstract methods of the parent class).One way is that if the methods are static then we can use CLASSNAME.METHODNAME().Is their any other way of doing it

none at all. not a single one. by definition, a non-static method has to be invoked on a particular instance of a class, and an abstract class cannot be instantiated. somebody is probably going to say you can do this:
abstract class AbstractClass {
abstract void minceAbout();
void talkRubbish() {
// do stuff
new AbstractClass() {
void minceAbout() {}
}.talkRubbish();but that is extending the AbstractClass, no matter what their argument. why d'you ask, anyway? why not implement the methods? sounds like a design smell

Calling a super.ssuper.method but your super is a abstract class.

Dear guys,
Is that possible to invoke your super's super's method but your super is a abstract class?
like:
class GO { public void draw() { } }
abstract class GORunner extends GO {}
class GOCounter extends GORunner {
public void draw() {
super.super.draw();
I want to do this because I would like to take advantages of the abstract as an layer to achieve some polymorphism programming Therefore, in the later stage of the programming some code may only refer to GORunner but actually it is holding a GOCounter object.
Thank!!

BTW you don't need to write this
public void draw() {
super.draw();
}It works but its basically the same as not having it at all.

Abstract class without absract method

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