Purpose/Advantage of Abstract Class

Hi,
I'm aware of the functionality of the Abstract class.
But what is the purpose or advantage of using it. Where ever we are using an Abstract class, we can replace it with a normal class & achieve the same functionality. Is there any scenario like only Abstract class can be used?
Thanks & Regards,
Adithya M.

By using the Abstract class, we achieve the Polymorphism. Polymorphism simplifies the binding of the objects. By using the abstract class as the reference as object definition time, we can easily replace it with the concrete object at the runtime. This is called as the Dynamic Binding.
When we use the dynamic binding, the caller or the user of the object doesn't need to worry of what type of object is being passed to it at design time. At runtime the object would be assigned to it.
You can achieve the same Polymorphism and Dynamic binding using the interfaces, too. But the advantage using the Abstract class is, you could have the implementation of the non-abstract methods of the class where as Interface would only define the object. This non-abstract methods could contain the common logic which is valid for both Super class and Subclass.
Regards,
Naimesh Patel

Similar Messages

What is the advantage of abstract class and method???

hi,
* Why a class is declared as abstract???
* What is the use of declaring a class as abstract???
* At what situation abstract class will be used???Thanks
JavaImran

To save you from the wrath of the Java experts on this forum, allow me as a relatively new Java user to advise you: do NOT post homework problems here; you're just going to get told to go google the answer. Which would be a good move on your part. Especially since I found the answer to your questions by googling them myself.

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."

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.

Inheriting from an abstract class

Hi all,
I'm using a certain graphics package and I need to modify a functionality of one of its classes called, say, PlainAxis. For this purpose I define a class MyAxis that derives from the class Axis which is the base class for PlainAxis as well. The Axis class is abstract; it has two abstract methods - draw() and copy(). I provide these two methods in MyAxis, so, it can be instantiated. Still I get the following error message during the compilation:
MyAxis should be declared abstract; it does not define draw() in Axis
Here is the command I'm executing (sgt.jar contains classes of the graphics package I'm using):
javac -g -classpath .:./sgt.jar *.java -d .
What am I doing wrong? Why does this error occure for the draw() method only?
Thank you for the help!

You probably didn't define draw() with the exactsame
method signatureBelieve me I did :) This was the first thing I
checked when I saw this error for the first time...
Any other suggestions? :)The possibilities that I can think of are (most of these have already been suggested):
1. you didn't define draw with the correct signature
2. you're not extending the class you think you're extending
3. you're not compiling the class you think you're compiling
4. there's a bug in your compiler
I'd say 3 is extremely unlikely. So without seeing your code, we can only guess, but I would guess #1.

Abstract class Vs interface

Hi,
I have to buid a report in ECM with complete details of the engineering as well as production. This include workflow as well as various fucntionality depends upon the criterion and user's event.
I am implementating in OOPS and I Want to know that when I should use the Abstract class and when interface ?
Because as per me both serve the same purpose. Kindly send me the exact difference so that i can efficiently use the same.
Thanks
Prince

When inheriting A Interface We have to inherit all the methods of the Interface there's no other option whereas with abstract classes we can inherit the members that we are in need of.
Just the interface has to have body of the method and the method is to be used by the classes inheriting it. Whereas in the case of Abstract Class it can have declarations (Other than the abstract method) and it can be further extended in the classes inheriting the Abstract Class.
Interface contains all abstract methods,all methods compulsory implemented by particular class, interface does not contain Constructor
abstract classes are designed with implemantion gaps for sub-class to fill in.
interfaces are sintacticlly similar to classes but they lack insance variables & methods.
abstract classes can also have both abstract methods & non-abstract methods. where as in interface methods are abstract only, & variables are implicitly static&final
regards
Preetesh

Instanciate generic subclass of an abstract class

Hello,
I'm writing a litle file manager API for my application, organised like a Database: a table filled by records.
I want my table to be able to be able to manage different types of records (but one record type by table!), so I have this hierarchy:
the abstract record class:
public abstract class Record {
     public Record() {
     public abstract void readFrom(DataInput in, int length) throws IOException;
     public abstract void writeTo(DataOutput out) throws IOException;
     public abstract int prepareToWrite();
}the abstract atomic record class, extending the abstract record class:
public abstract class AtomicRecord<T> extends Record {
     public AtomicRecord() {
     public AtomicRecord(T value) {
          super();
     public abstract T getValue();
     public abstract void setValue(T value);
}and, for example, the StringRecord class:
public class StringRecord extends AtomicRecord<String> {
     code doesn't matter.
}now, I want to make a table of generic records:
public class RandomRecordAccessAppendableFileTable<RecordType extends Record>{
     public final RecordType getRecord(long id) throws IOException {
          RecordType record = new RecordType();
}problem: the RecordType is obviously not instanciable, as it extends Record, and not StringRecord. But I still want to instanciate a new StringRecord from a RandomRecordAccessAppendableFileTable<StringRecord> and a FooRecord from a RandomRecordAccessAppendableFileTable<FooRecord>.
Is there a way I can do this?
Thank you in advance.

This is just my opinion, so you might ignore it: The design you are targeting is a result of misunderstanding what Generics are for and what Generics may provide in their current state. Their only purpose is to provide compile-time type safety. Creating instances the way you want to do seems more runtime focused, i.e., when generic information is gone.
Maybe, you should consider passing a factory at construction time that enables to create a record of a specific type. Using the class object requires reflection, which will introduce implicit contracts to record classes and potential runtime failures.

Abstract class extending a nonabstract

i want to know what are the effects , advantages and cases were we want to use this cinda achitecture.
I have a class(NA) which is NonAbstract, There is a Abstract class(A) which extends a NA. Doing so is legally right, but what are the advantages of doing that are there any situations were we will be using this technique.
TIA
Zoha.

Every abstract class extends a non-abstract class already: Object is non-abstract, and since all abstract classes directly or indirectly extend Object... see?
So if you can see the reason for abstract classes at all, then you don't need to ask this question.

Access overriden method of an abstract class

class Abstract
abstract void abstractMethod(); //Abstract Method
void get()
System.out.print("Hello");
class Subclass extends Abstract
void abstractMethod()
System.out.print("Abstract Method implementation");
void get()
System.out.print("Hiiii");
In the above code, i have an abstract class called "Abstract", which has an abstract method named "abstractMethod()" and another method called "get()".
Now, this class is extended by "Subclass", it provides implementation for "abstractMethod()", and also overrides the "get()" method.
Now my problem is that i want to access the "get()" method of "Abstract" class. Since it is an abstract class, i cant create an object of it directly, and if i create an object like this:
Abstract obj = new Subclass();
then, obj.get() will call the get() method of Subclass, but how do i call the get() method of Abstract class.
Thanks in advance

hey thanks a lot,, i have another doubt regarding Abstract classes.
i was just trying something, in the process, i noticed that i created an abstract class which does not have any abstract method, it gave no compilation errors. was wondering how come this is possible, and what purpose does it solve?

Abstract classes & interfaces

In the API we have an interface namely:Set
which is implemented by an abstract class AbstractSet.
Class HashSet extends AbstractSet implements Set.
Now What is need for HashSet class to implement a Set interface when it is already extending from AbstractSet class that inturn implements Set interface.

This is probably an case of the folks at Sun refactoring their code and just missing one cleanup detail. The initial implementation of HashSet probably did not have a subclass. Then when they started to implement TreeSet, they realized that there was common functionality that they could reuse.
As the previous writer mentioned and you observed, there is no advantage or reason for HashSet to implement Set, it's probably just a harmless oversight.

Abstract Class & Interface

Hi ,
I have a fundamental doubt regarding Abstract Class & Interface!!!
What is their real benefit...whether we implement an interface or extend an Abstract class we have to write the code for the abstract method in the concrete class.Then where the benefit remained....
And it is said that Abstract class provide default behaviour...what is the actual meaning of that?
Thanks & Regards
Santosh

In this section we will redesign our OneRowNim game to fit within a hierarchy of classes of two-player games. There are many games that characteristically involve two players: checkers, chess, tic-tac-toe, guessing games, and so forth. However, there are also many games that involve just one player: blackjack, solitaire, and others. There are also games that involve two or more players, such as many card games. Thus, our redesign of OneRowNim as part of a two-player game hierarchy will not be our last effort to design a hierarchy of game-playing classes. We will certainly redesign things as we learn new Java language constructs and as we try to extend our game library to other kinds of games.
This case study will illustrate how we can apply inheritance and polymorphism, as well as other object-oriented design principles. The justification for revising OneRowNim at this point is to make it easier to design and develop other two-player games. As we have seen, one characteristic of class hierarchies is that more general attributes and methods are defined in top-level classes. As one proceeds down the hierarchy, the methods and attributes become more specialized. Creating a subclass is a matter of specializing a given class.
8.6.1. Design Goals
One of our design goals is to revise the OneRowNim game so that it fits into a hierarchy of two-player games. One way to do this is to generalize the OneRowNim game by creating a superclass that contains those attributes and methods that are common to all two-player games. The superclass will define the most general and generic elements of two-player games. All two-player games, including OneRowNim, will be defined as subclasses of this top-level superclass and will inherit and possibly override its public and protected variables and methods. Also, our top-level class will contain certain abstract methods, whose implementations will be given in OneRowNim and other subclasses.
Generic superclass
A second goal is to design a class hierarchy that makes it possible for computers to play the game, as well as human users. Thus, for a given two-player game, it should be possible for two humans to play each other, or for two computers to play each other, or for a human to play against a computer. This design goal will require that our design exhibit a certain amount of flexibility. As we shall see, this is a situation in which Java interfaces will come in handy.
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Another important goal is to design a two-player game hierarchy that can easily be used with a variety of different user interfaces, including command-line interfaces and GUIs. To handle this feature, we will develop Java interfaces to serve as interfaces between our two-player games and various user interfaces.
8.6.2. Designing the TwoPlayerGame Class
To begin revising the design of the OneRowNim game, we first need to design a top-level class, which we will call the TwoPlayerGame class. What variables and methods belong in this class? One way to answer this question is to generalize our current version of OneRowNim by moving any variables and methods that apply to all two-player games up to the TwoPlayerGame class. All subclasses of TwoPlayerGamewhich includes the OneRowNim classwould inherit these elements. Figure 8.18 shows the current design of OneRowNim.
Figure 8.18. The current OneRowNim class.
What variables and methods should we move up to the TwoPlayerGame class? Clearly, the class constants, PLAYER_ONE and PLAYER_TWO, apply to all two-player games. These should be moved up. On the other hand, the MAX_PICKUP and MAX_STICKS constants apply just to the OneRowNim game. They should remain in the OneRowNim class.
The nSticks instance variable is a variable that only applies to the OneRowNim game but not to other two-player games. It should stay in the OneRowNim class. On the other hand, the onePlaysNext variable applies to all two-player games, so we will move it up to the TwoPlayerGame class.
Because constructors are not inherited, all of the constructor methods will remain in the OneRowNim class. The instance methods, takeSticks() and getSticks(), are specific to OneRowNim, so they should remain there. However, the other methods, getPlayer(), gameOver(), getWinner(), and reportGameState(), are methods that would be useful to all two-player games. Therefore these methods should be moved up to the superclass. Of course, while these methods can be defined in the superclass, some of them can only be implemented in subclasses. For example, the reportGameState() method reports the current state of the game, so it has to be implemented in OneRowNim. Similarly, the getWinner() method defines how the winner of the game is determined, a definition that can only occur in the subclass. Every two-player game needs methods such as these. Therefore, we will define these methods as abstract methods in the superclass. The intention is that TwoPlayerGame subclasses will provide game-specific implementations for these methods.
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Constructors are not inherited
Given these considerations, we come up with the design shown in Figure 8.19. The design shown in this figure is much more complex than the designs used in earlier chapters. However, the complexity comes from combining ideas already discussed in previous sections of this chapter, so don't be put off by it.
Figure 8.19. TwoPlayerGame is the superclass for OneRowNim and other two-player games.
To begin with, note that we have introduced two Java interfaces into our design in addition to the TwoPlayerGame superclass. As we will show, these interfaces lead to a more flexible design and one that can easily be extended to incorporate new two-player games. Let's take each element of this design separately.
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8.6.3. The TwoPlayerGame Superclass
As we have stated, the purpose of the TwoPlayerGame class is to serve as the superclass for all two-player games. Therefore, it should define the variables and methods shared by two-player games.
The PLAYER_ONE, PLAYER_TWO, and onePlaysNext variables and the getPlayer(), setPlayer(), and changePlayer() methods have been moved up from the OneRowNim class. Clearly, these variables and methods apply to all two-player games. Note that we have also added three new variables, nComputers, computer1, computer2, and their corresponding methods, getNComputers() and addComputerPlayer(). We will use these elements to give our games the capability to be played by computer programs. Because we want all of our two-player games to have this capability, we define these variables and methods in the superclass rather than in OneRowNim and subclasses of TwoPlayerGame.
Note that the computer1 and computer2 variables are declared to be of type IPlayer. IPlayer is an interface containing a single method declaration, the makeAMove() method:
public interface IPlayer {
public String makeAMove(String prompt);
Why do we use an interface here rather than some type of game-playing object? This is a good design question. Using an interface here makes our design more flexible and extensible because it frees us from having to know the names of the classes that implement the makeAMove() method. The variables computer1 and computer2 will be assigned objects that implement IPlayer via the addComputerPlayer() method.
Game-dependent algorithms
The algorithms used in the various implementations of makeAMove() are game-dependentthey depend on the particular game being played. It would be impossible to define a game playing object that would suffice for all two-player games. Instead, if we want an object that plays OneRowNim, we would define a OneRowNimPlayer and have it implement the IPlayer interface. Similarly, if we want an object that plays checkers, we would define a CheckersPlayer and have it implement the IPlayer interface. By using an interface here, our TwoPlayerGame hierarchy can deal with a wide range of differently named objects that play games, as long as they implement the IPlayer interface. Using the IPlayer interface adds flexibility to our game hierarchy and makes it easier to extend it to new, yet undefined, classes. We will discuss the details of how to design a game player in Section 8.6.7.
The IPlayer interface
Turning now to the methods defined in TwoPlayerGame, we have already seen implementations of getPlayer(), setPlayer(), and changePlayer() in the OneRowNim class. We will just move those implementations up to the superclass. The getNComputers() method is the assessor method for the nComputers variable, and its implementation is routine. The addComputerPlayer() method adds a computer player to the game. Its implementation is as follows:
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public void addComputerPlayer(IPlayer player) {
if (nComputers == 0)
computer2 = player;
else if (nComputers == 1)
computer1 = player;
else
return; // No more than 2 players
++nComputers;
As we noted earlier, the classes that play the various TwoPlayerGames must implement the IPlayer interface. The parameter for this method is of type IPlayer. The algorithm we use checks the current value of nComputers. If it is 0, which means that this is the first IPlayer added to the game, the player is assigned to computer2. This allows the human user to be associated with PLAYERONE if this is a game between a computer and a human user.
If nComputers equals 1, which means that we are adding a second IPlayer to the game, we assign that player to computer1. In either of these cases, we increment nComputers. Note what happens if nComputers is neither 1 nor 2. In that case, we simply return without adding the IPlayer to the game and without incrementing nComputers. This, in effect, limits the number of IPlayers to two. (A more sophisticated design would throw an exception to report an error. but we will leave that for a subsequent chapter.)
The addComputerPlayer() method is used to initialize a game after it is first created. If this method is not called, the default assumption is that nComputers equals zero and that computer1 and computer2 are both null. Here's an example of how it could be used:
OneRowNim nim = new OneRowNim(11); // 11 sticks
nim.add(new NimPlayer(nim)); // 2 computer players
nim.add(new NimPlayerBad(nim));
Note that the NimPlayer() constructor takes a reference to the game as its argument. Clearly, our design should not assume that the names of the IPlayer objects would be known to the TwoPlayerGame superclass. This method allows the objects to be passed in at runtime. We will discuss the details of NimPlayerBad in Section 8.6.7.
The getrules() method is a new method whose purpose is to return a string that describes the rules of the particular game. This method is implemented in the TwoPlayerGame class with the intention that it will be overridden in the various subclasses. For example, its implementation in TwoPlayerGame is:
public String getRules() {
return "The rules of this game are: ";
Overriding a method
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and its redefinition in OneRowNim is:
public String getRules() {
return "\n*** The Rules of One Row Nim ***\n" +
"(1) A number of sticks between 7 and " + MAX_STICKS +
" is chosen.\n" +
"(2) Two players alternate making moves.\n" +
"(3) A move consists of subtracting between 1 and\n\t" +
MAX_PICKUP +
" sticks from the current number of sticks.\n" +
"(4) A player who cannot leave a positive\n\t" +
" number of sticks for the other player loses.\n";
The idea is that each TwoPlayerGame subclass will take responsibility for specifying its own set of rules in a form that can be displayed to the user.
You might recognize that defining geTRules() in the superclass and allowing it to be overridden in the subclasses is a form of polymorphism. It follows the design of the toString() method, which we discussed earlier. This design will allow us to use code that takes the following form:
TwoPlayerGame game = new OneRowNim();
System.out.println(game.getRules());
Polymorphism
In this example the call to getrules() is polymorphic. The dynamic-binding mechanism is used to invoke the getrules() method defined in the OneRowNim class.
The remaining methods in TwoPlayerGame are defined abstractly. The gameOver() and getWinner() methods are both game-dependent methods. That is, the details of their implementations depend on the particular TwoPlayerGame subclass in which they are implemented.
This is good example of how abstract methods should be used in designing a class hierarchy. We give abstract definitions in the superclass and leave the detailed implementations up to the individual subclasses. This allows the different subclasses to tailor the implementations to their particular needs, while allowing all subclasses to share a common signature for these tasks. This enables us to use polymorphism to create flexible, extensible class hierarchies.
Figure 8.20 shows the complete implementation of the abstract TwoPlayerGame class. We have already discussed the most important details of its implementation.
Figure 8.20. The TwoPlayerGame class
(This item is displayed on page 381 in the print version)
public abstract class TwoPlayerGame {
public static final int PLAYER_ONE = 1;
public static final int PLAYER_TWO = 2;
protected boolean onePlaysNext = true;
protected int nComputers = 0; // How many computers
// Computers are IPlayers
protected IPlayer computer1, computer2;
public void setPlayer(int starter) {
if (starter == PLAYER_TWO)
onePlaysNext = false;
else onePlaysNext = true;
} // setPlayer()
public int getPlayer() {
if (onePlaysNext)
return PLAYER_ONE;
else return PLAYER_TWO;
} // getPlayer()
public void changePlayer() {
onePlaysNext = !onePlaysNext;
} // changePlayer()
public int getNComputers() {
return nComputers;
} // getNComputers()
public String getRules() {
return "The rules of this game are: ";
} // getRules()
public void addComputerPlayer(IPlayer player) {
if (nComputers == 0)
computer2 = player;
else if (nComputers == 1)
computer1 = player;
else
return; // No more than 2 players
++nComputers;
} // addComputerPlayer()
public abstract boolean gameOver(); // Abstract Methods
public abstract String getWinner();
} // TwoPlayerGame class
Effective Design: Abstract Methods
Abstract methods allow you to give general definitions in the superclass and leave the implementation details to the different subclasses.
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8.6.4. The CLUIPlayableGame Interface
We turn now to the two interfaces shown in Figure 8.19. Taken together, the purpose of these interfaces is to create a connection between any two-player game and a command-line user interface (CLUI). The interfaces provide method signatures for the methods that will implement the details of the interaction between a TwoPlayerGame and a UserInterface. Because the details of this interaction vary from game to game, it is best to leave the implementation of these methods to the games themselves.
Note that CLUIPlayableGame extends the IGame interface. The IGame interface contains two methods that are used to define a standard form of communication between the CLUI and the game. The getGamePrompt() method defines the prompt used to signal the user for a move of some kindfor example, "How many sticks do you take (1, 2, or 3)?" And the reportGameState() method defines how the game will report its current statefor example, "There are 11 sticks remaining." CLUIPlayableGame adds the play() method to these two methods. As we will see shortly, the play() method contains the code that will control the playing of the game.
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Extending an interface
The source code for these interfaces is very simple:
public interface CLUIPlayableGame extends IGame {
public abstract void play(UserInterface ui);
public interface IGame {
public String getGamePrompt();
public String reportGameState();
} // IGame
Note that the CLUIPlayableGame interface extends the IGame interface. A CLUIPlayableGame is a game that can be played through a CLUI. The purpose of its play() method is to contain the game-dependent control loop that determines how the game is played via a user interface (UI). In pseudocode, a typical control loop for a game would look something like the following:
Initialize the game.
While the game is not over
Report the current state of the game via the UI.
Prompt the user (or the computer) to make a move via the UI.
Get the user's move via the UI.
Make the move.
Change to the other player.
The play loop sets up an interaction between the game and the UI. The UserInterface parameter allows the game to connect directly to a particular UI. To allow us to play our games through a variety of UIs, we define UserInterface as the following Java interface:
public interface UserInterface {
public String getUserInput();
public void report(String s);
public void prompt(String s);
Any object that implements these three methods can serve as a UI for one of our TwoPlayerGames. This is another example of the flexibility of using interfaces in object-oriented design.
To illustrate how we use UserInterface, let's attach it to our KeyboardReader class, thereby letting a KeyboardReader serve as a CLUI for TwoPlayerGames. We do this simply by implementing this interface in the KeyboardReader class, as follows:
public class KeyboardReader implements UserInterface
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As it turns out, the three methods listed in UserInterface match three of the methods in the current version of KeyboardReader. This is no accident. The design of UserInterface was arrived at by identifying the minimal number of methods in KeyboardReader that were needed to interact with a TwoPlayerGame.
Effective Design: Flexibility of Java Interfaces
A Java interface provides a means of associating useful methods with a variety of different types of objects, leading to a more flexible object-oriented design.
The benefit of defining the parameter more generally as a UserInterface instead of as a KeyboardReader is that we will eventually want to allow our games to be played via other kinds of command-line interfaces. For example, we might later define an Internet-based CLUI that could be used to play OneRowNim among users on the Internet. This kind of extensibilitythe ability to create new kinds of UIs and use them with TwoPlayerGamesis another important design feature of Java interfaces.
Generality principle
Effective Design: Extensibility and Java Interfaces
Using interfaces to define useful method signatures increases the extensibility of a class hierarchy.
As Figure 8.19 shows, OneRowNim implements the CLUIPlayableGame interface, which means it must supply implementations of all three abstract methods: play(), getGamePrompt(), and reportGameState().
8.6.5. Object-Oriented Design: Interfaces or Abstract Classes?
Why are these methods defined in interfaces? Couldn't we just as easily define them in the TwoPlayerGame class and use inheritance to extend them to the various game subclasses? After all, isn't the net result the same, namely, that OneRowNim must implement all three methods.
These are very good design questions, exactly the kinds of questions one should ask when designing a class hierarchy of any sort. As we pointed out in the Animal example earlier in the chapter, you can get the same functionality from an abstract interface and an abstract superclass method. When should we put the abstract method in the superclass, and when does it belong in an interface? A very good discussion of these and related object-oriented design issues is available in Java Design, 2nd Edition, by Peter Coad and Mark Mayfield (Yourdan Press, 1999). Our discussion of these issues follows many of the guidelines suggested by Coad and Mayfield.
Interfaces vs. abstract methods
We have already seen that using Java interfaces increases the flexibility and extensibility of a design. Methods defined in an interface exist independently of a particular class hierarchy. By their very nature, interfaces can be attached to any class, and this makes them very flexible to use.
Flexibility of interfaces
Another useful guideline for answering this question is that the superclass should contain the basic common attributes and methods that define a certain type of object. It should not necessarily contain methods that define certain roles that the object plays. For example, the gameOver() and getWinner() methods are fundamental parts of the definition of a TwoPlayerGame. One cannot define a game without defining these methods. By contrast, methods such as play(), getGamePrompt(), and reportGameState() are important for playing the game but they do not contribute in the same way to the game's definition. Thus these methods are best put into an interface. Therefore, one important design guideline is:
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Effective Design: Abstract Methods
Methods defined abstractly in a superclass should contribute in a fundamental way to the basic definition of that type of object, not merely to one of its roles or its functionality.
8.6.6. The Revised OneRowNim Class
Figure 8.21 provides a listing of the revised OneRowNim class, one that fits into the TwoPlayerGame class hierarchy. Our discussion in this section will focus on the features of the game that are new or revised.
Figure 8.21. The revised OneRowNim class, Part I.
(This item is displayed on page 385 in the print version)
public class OneRowNim extends TwoPlayerGame implements CLUIPlayableGame {
public static final int MAX_PICKUP = 3;
public static final int MAX_STICKS = 11;
private int nSticks = MAX_STICKS;
public OneRowNim() { } // Constructors
public OneRowNim(int sticks) {
nSticks = sticks;
} // OneRowNim()
public OneRowNim(int sticks, int starter) {
nSticks = sticks;
setPlayer(starter);
} // OneRowNim()
public boolean takeSticks(int num) {
if (num < 1 || num > MAX_PICKUP || num > nSticks)
return false; // Error
else // Valid move
{ nSticks = nSticks - num;
return true;
} // else
} // takeSticks()
public int getSticks() {
return nSticks;
} // getSticks()
public String getRules() {
return "\n*** The Rules of One Row Nim ***\n" +
"(1) A number of sticks between 7 and " + MAX_STICKS +
" is chosen.\n" +
"(2) Two players alternate making moves.\n" +
"(3) A move consists of subtracting between 1 and\n\t" +
MAX_PICKUP + " sticks from the current number of sticks.\n" +
"(4) A player who cannot leave a positive\n\t" +
" number of sticks for the other player loses.\n";
} // getRules()
public boolean gameOver() {   /*** From TwoPlayerGame */
return (nSticks <= 0);
} // gameOver()
public String getWinner() {        /*** From TwoPlayerGame */
if (gameOver()) //{
return "" + getPlayer() + " Nice game.";
return "The game is not over yet."; // Game is not over
} // getWinner()
The gameOver() and getWinner() methods, which are nowinherited from the TwoPlayerGame superclass, are virtually the same as in the previous version. One small change is that getWinner() now returns a String instead of an int. This makes the method more generally useful as a way of identifying the winner for all TwoPlayerGames.
Similarly, the getGamePrompt() and reportGameState() methods merely encapsulate functionality that was present in the earlier version of the game. In our earlier version the prompts to the user were generated directly by the main program. By encapsulating this information in an inherited method, we make it more generally useful to all TwoPlayerGames.
Inheritance and generality
The major change to OneRowNim comes in the play() method, which controls the playing of OneRowNim (Fig. 8.22). Because this version of the game incorporates computer players, the play loop is a bit more complex than in earlier versions of the game. The basic idea is still the same: The method loops until the game is over. On each iteration of the loop, one or the other of the two players, PLAYER_ONE or PLAYER_TWO, takes a turn making a movethat is, deciding how many sticks to pick up. If the move is a legal move, then it becomes the other player's turn.
Figure 8.22. The revised OneRowNim class, Part II.
(This item is displayed on page 386 in the print version)
/** From CLUIPlayableGame */
public String getGamePrompt() {
return "\nYou can pick up between 1 and " +
Math.min(MAX_PICKUP,nSticks) + " : ";
} // getGamePrompt()
public String reportGameState() {
if (!gameOver())
return ("\nSticks left: " + getSticks() +
" Who's turn: Player " + getPlayer());
else
return ("\nSticks left: " + getSticks() +
" Game over! Winner is Player " + getWinner() +"\n");
} // reportGameState()
public void play(UserInterface ui) { // From CLUIPlayableGame interface
int sticks = 0;
ui.report(getRules());
if (computer1 != null)
ui.report("\nPlayer 1 is a " + computer1.toString());
if (computer2 != null)
ui.report("\nPlayer 2 is a " + computer2.toString());
while(!gameOver()) {
IPlayer computer = null; // Assume no computers
ui.report(reportGameState());
switch(getPlayer()) {
case PLAYER_ONE: // Player 1's turn
computer = computer1;
break;
case PLAYER_TWO: // Player 2's turn
computer = computer2;
break;
} // cases
if (computer != null) {                           // If computer's turn
sticks = Integer.parseInt(computer.makeAMove(""));
ui.report(computer.toString() + " takes " + sticks + " sticks.\n");
} else {                                          // otherwise, user's turn
ui.prompt(getGamePrompt());
sticks =
Integer.parseInt(ui.getUserInput()); // Get user's move
if (takeSticks(sticks)) // If a legal move
changePlayer();
} // while
ui.report(reportGameState()); // The game is now over
} // play()
} // OneRowNim class
Let's look now at how the code decides whether it is a computer's turn to move or a human player's turn. Note that at the beginning of the while loop, it sets the computer variable to null. It then assigns computer a value of either computer1 or computer2, depending on whose turn it is. But recall that one or both of these variables may be null, depending on how many computers are playing the game. If there are no computers playing the game, then both variables will be null. If only one computer is playing, then computer1 will be null. This is determined during initialization of the game, when the addComputerPlayer() is called. (See above.)
In the code following the switch statement, if computer is not null, then we call computer.makeAMove(). As we know, the makeAMove() method is part of the IPlayer interface. The makeAMove() method takes a String parameter that is meant to serve as a prompt, and returns a String that is meant to represent the IPlayer's move:
public interface IPlayer {
public String makeAMove(String prompt);
[Page 385]
In OneRowNim the "move" is an integer, representing the number of sticks the player picks. Therefore, in play() OneRowNim has to convert the String into an int, which represents the number of sticks the IPlayer picks up.
On the other hand, if computer is null, this means that it is a human user's turn to play. In this case, play() calls ui.getUserInput(), employing the user interface to input a value from the keyboard. The user's input must also be converted from String to int. Once the value of sticks is set, either from the user or from the IPlayer, the play() method calls takeSticks(). If the move is legal, then it changes whose turn it is, and the loop repeats.
[Page 386]
There are a couple of important points about the design of the play() method. First, the play() method has to know what to do with the input it receives from the user or the IPlayer. This is game-dependent knowledge. The user is inputting the number of sticks to take in OneRowNim. For a tic-tac-toe game, the "move" might represent a square on the tic-tac-toe board. This suggests that play() is a method that should be implemented in OneRowNim, as it is here, because OneRowNim encapsulates the knowledge of how to play the One-Row Nim game.
Encapsulation of game-dependent knowledge
[Page

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.

Abstract Classes vs Private Constructor

Hopefully this doesn't make me sound to naive
If we have a super class called food, and subclasses chocolate, chicken, cereal and so on then its clear we dont want to make instances of the super class food correct? So we can make this class an abstract class
What are the benefits of making a class abstract over making a constructor private and in what circumstance would it be beneficial to do one over the other?
Thanks, and sorry for a poor example!
Edited by: compSciUndergrad on Apr 22, 2009 4:53 AM

wrybread wrote:
Also, Corlettk -
Excellent point regarding prudent use of interfaces; thank you. If you read this, I'd appreciate any comment you have on how conservative programmers need to be in designing an interface. It seems to me that if I'm uncertain whether or not to include a certain member in the interface, I should omit it or include it elsewhere (perhaps in an interface all its own, or in an abstract super if that's applicable).
Let me know if you want me to open this as a new thread; I'm just looking for your take on this topic. -- Thank you!Obviously, there's no right answer. Each deisgn must be assessed on a case-by-case basis. Every design presents new and interesting problems.
I'm no OO guru... what I do know is that you can make some very nasty mistakes, thanks to experience of maintaining a fairly poorly written (first in the organisation) large java project.
I'm of the opinion that interface should define an aspect of behaviour... If the Sun GODS had there time again they'd do java.io would be full of interfaces... Closeable, Openable, Readable, Writable, Printable... etc etc etc.
Please remember that a class (or group of classes) can implement multiple interfaces... and an interface can extend multiple interfaces... So java supports "multiple type inheritance" as apposed to C++'s "multiple implementation inheritance".
If you need an "aglomerate" interface as the type for a collection then assemble one from your premade "set of aspects"... eg: Input extends Openable, Closeable, Readable...
package forums;
interface AnInterface
public void a();
interface AnotherInterface
public void b();
interface AglomerateInterface extends AnInterface, AnotherInterface
public void c();
class AglomerateImpl implements AglomerateInterface
public static void main(String[] args) {
    try {
      System.out.println("Hello World!");
    } catch (Exception e) {
      e.printStackTrace();
public void a() {}
public void b() {}
public void c() {}
}PS: You wouldn't normally define multiple interfaces and an implementation all in one file... sort of defeats the whole purpose really... and in fact you can't as soon as you make the interfaces public, which they need to be to be any use.
Make sense?

Implement method inside abstract class?

hello everyone:
I have a question regarding implementation of method inside a abstract class. The abstract class has a static method to swamp two numbers.
The problem ask if there is any output from the program; if no output explain why?
This is a quiz question I took from a java class. I tried the best to recollect the code sample from my memory.
if the code segment doesn't make sense, could you list several cases that meet the purpose of the question. I appreciate your help!
code sample
public abstract class SwampNumber
   int a = 4;
   int b = 2;
   System.out.println(a);
   System.out.println(b);
   swamp(a, b);
   public static void swamp(int a, int b)
     int temp = a;
         a = b;
         b = a;
     System.out.println(a);
     System.out.println(b);

It won't compile.
You can't instantiate an abstract class before you know anything.
//somewhere in main
SwampNumber myNum = new SwampNumber();
//Syntax ErrorQuote DrClap
This error commonly occurs when you have code that >>is not inside a method.
The only statements that can appear outside methods >>in a Java class are >>declarations.Message was edited by:
lethalwire

Using the UI editor for a class that extends an abstract class

Hi,
At the moment it's unfortunately impossible to use the UI editor to edit a class that extends a certain abstract class. There is a way to circumvent this problem, by using a proxy class. Does anyone know how to create a proxy class for this purpose and how to register it?
Hopefully a future JDeveloper release will solve this problem. Is this a planned feature?
Regards,
Peter

I hope it works for you now (why using , but not indentation?).

Purpose/Advantage of Abstract Class

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