Lookinf for a "flexible" abstract class mechanism

When writing an abstract class, or an interface, is there a way to force the classes who extends it to implement at least one method in a set of 2 ?
For instance,
public abstract class PermanentCard {
public void concretePlayByPlayer(Player parPlayer);
public void concretePlayByPlayer(Player parPlayer, Location parLocation);
}In this case, I'd like to say that all class who implement PermanentCard must implement at least one these two methods.
Is there a way to avoid multiplying the number of abstract I have?

bestam wrote:
To BigDaddy :
Consider I have an abstract class who expects its implementations to implement a concretePlay method, whatever is its signature. Of course, java is only able to distinguish methods via their signatures.
I was just looking for non-direct way to let the implementations implement one or another, but at least one.
Supposing you did that, how is the code using the interface to know which method(s) are available in this particular implementation? An interface is a contract, and the user is entitled to expect that contract to be fulfilled.
If you have some classes which do things one way, and some the other, it would be better to put the two method signatures in different interfaces, in which case the code using the class could us instanceof to test which method signature was actually available.
The two interfaces could extend a common, possibly empty one for references that might hold either.
Of course in this particular case using the same signature with a null argument is more satisfactory.
Edited by: malcolmmc on May 14, 2009 9:20 AM

Similar Messages

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.

How to use function of an abstract class??

Hi all,
I want to use the format(object obj) function of the package java.text.Format. But as the Format class is an abstract class i could not instantiate it, so that i can call the format(object obj) method using the dot(.) operator.
I know what is an abstract class. i've studied and tried to understand. as everybody knows, studied the perfect example(because i've read it in all the abstract class tutorial, books) of the Shape class. But still i dont understand how am i going to use that format(object obj) method. Please help me..

Instead of using the abstract class Format use the
concrete classes DecimalFormat
SimpleDateFormat etc check java.text APIS
http://java.sun.com/j2se/1.4.2/docs/api
Hi!! Thanks both of you.
If Sun has a abstract class then there must be a
concrete class which extends that abstract class .It
is always better to check the APIWhat do you mean by this line. Is it true for all the abstract classes in the jdk?

Abstract class & final class

Which is more correct... Or which is the more preferred way.
abstract class A
final method1(){}
final method2(){}
final method30(){}
}or
final class A
private A(){}
method1(){}
method2(){}
method3(){}
}My understanding is that both classes cannot be instantiated. The first one requires writing 'final' for EACH method. The second one involves writing a private constructor.

It depends on what you are trying to do. If you are trying to make a class that can be sub-classed but has some methods that cannot be overriden, then "abstract class A" with final methods is the way to go. If you want a class that cannot be sub-classed or instantiated then "final class A" with private constructor is the way to go.
classes cannot be instantiatedOnly true for "final class A" because you made the only constructor private. Not true for the abstract one.// you forgot return values for the methods
abstract class A
final static /*void*/ method1(){} // package private
final static /*void*/ method2(){} // package private
final static /*void*/ method30(){} // package private
// this would work
A a = new A(){};
// if I am in the same package as A, then this would work
a.method1();
// or this
public class B extends A
public B(String whatever)
    // Although, I cannot override the super methods
    // becuase they are all declared as final
    // I can only invoke them if I am in the same package. You
    // declared them as package private instead of class "private"
}Using final as a class modifier disables the ability to sub-class it but does not disable the ability to create an object of that class. You must make a private constructer. If the only constructor is "private" then you can't subclass or instantiate, so making the class final is uneeded.

Having trouble understanding Abstract class. Help!!!!!!

Having trouble understanding Abstract class. when is Abstract class used and for what.

Having trouble understanding Abstract class. when is
Abstract class used and for what.An abstract class is used to force the developer to provide a subclass, to implement the abstract methods, while still keeping the methods that were provided.
� {�

JSF annotations on interface or abstract class

Hello,
Can we use JSF 2 annotations like @ManagedBeans or @SessionScoped for interfaces and abstract classes? I mean, will it works if I implement an interface which declares some annotation(s) and the managed bean class itself doesn't have annotations?
Thanks a lot!
Oleg.

Okey, an example:
@ManagedBean
public abstract class MyAbstractClass {
public class MyClass extends MyAbstractClass {
Does it work if I access MyClass by ValueExpression #{myAbstractClass} ? Will be MyClass automatically annotated with @ManagedBean?
Best regards.
Oleg.

When we will go for an abstract class and when we will go for an interface?

it's always some what confusing to choose an abstract class and an interface,can anybody post a suitable answer for this.

jwenting wrote:
with experience and the insight it brings, you will know which to use when.
Without it, it can't be explained.
More often than not there's no X OR Y anyway.It's fortunate that there are posters here who possess the insight and experience necessary to explain this. The principal differences between an abstract class and an interface are,
1. An abstract class can carry implementation, whereas an interface cannot.
2. An abstract class is singly inherited, wheras an interface is multiply inherited.
So use an abstract class when the implementation it can carry outweights the fact that it cannot be multiply inherited That's the gist of it.
The inheritance relationship where this happens is when the supertype is a general concept of which all potential subtypes are special cases. This is called a specialization (or sometimes a generalization) relationship. For example Apple and Banana are Fruit. Or Car and Bike are Vechicle. The Fruit and Vechicle supertypes are general concepts of which their subtypes are special cases. In this case make Fruit and Vechicle abstract classes because the subtypes will benefit from a shared implementation.
If you don't have a clearcut specialization/generalization relationship make the supertype an interface. An example could be the Comparable supertype. The potential subtypes aren't supposed to be specializations of the Comparable concept, they're suppose to become Comparable (and make this property an integral part of their being). This is not a specialization/generalization relationship. Instead the supertype is intended to add character to the subtypes. The subtypes are unlikely to benefit from an inherited implementation. So make Comparable an interface.

OC4J 9.0.4 migrate to 10.1.3.1 JSP fails using abstract class for ResultSet

Consider the following JSP code snipet:
<%
com.class.SQLDataSource detail = ((com.class.SQLDataSource)request.getAttribute("AcctList"));
%>
<%while detail.next()) {detail.getRow();%>
<tr ...>
<td ...><%= detail.getString("ACCT") %></td>
</tr>
<%}%>
blah, blah, blah
class snipet looks like this:
public abstarct class SQLDataSource extends serializable {
private ResultSet resultSet = null;
private PreparesStatement stmt = null;
public boolean next() throws SQLException {
boolean result = getResultSet().next();
return (result);
public String getString(String columnName) throws SQLExcpetion {
return getResultSet().getString(columnName);
public void execute() throws SQLException {
-- checks stmt
-- if null generates resultSet from "AcctList.sql" query
-- if not null, re-executes as-is stmt for a query only
protected ResultSet getResultSet() throws SQLException {
execute(); // see above
return resultSet;
The problem:
<%= detail.getString("ACCT") %> generates a NullPointerException
Yet, if I add debug statements in the SQLDataSouce class, they conclusively show that resultSet has 1902 rows and I can diplay the contents on these rows (on the console, of course, the JSP still generates a NullPointerException).
If I "hardcode" the creation of a ResultSet in the JSP, the pages displays properly (not the desired method).
PLEASE TAKE NOTE: This JSP/Class combination works PERFECTLY in OC4J 9.0.4 using JDeveloper 9.0.5.2. When I deploy the **EXACT SAME** JSP/Class combination in JDeveloper/OC4J 10.1.3.1, I receive the NullPointerException.
So what this feels like to me (and my VERY limited J2EE experinece) is that the abstract class SQLDataSource has somehow lost/closed/dropped/corrupted resultSet when "returning" to the JSP for display of the contents of resultSet. This is, of course, a laymans explanation of the experienced effect of running this code.
It makes me wonder if under 10.1.3.1 "something more" must be done so that the (abstract) SQLDataSource class can operate in the same way it did under 9.0.4.
Last note: this NullPointerException happens whether I deploy to standalone OC4J 10.1.3.1 or if I execute my application withing JDevloper's Embedded OC4J instance.
Any Assistance on this MOST aggrivating problem would be greatly appreciated.
Others have yet to solve it in similar posts of mine. Hopefully this more definitive description will help YOU be my personal HERO.
Ed.

repost to pique interest

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?).

Documentation for abstract classes (Behavior and Binding) ?

Hi,
I am looking at the beautiful sample-app made by Jasper Potts at the www.fxexperience.com. ("Javafx 2.0 Audio Player")
There some abstract classes are used, for Behavior and Binding.
It turns out that I have difficulties finding documentation for those classes. The classes are:
com.sun.javafx.scene.control.behavior.BehaviorBase;
com.sun.javafx.scene.control.behavior.KeyBinding;
Could anybody give me hint, pls, where I could find documentation for those classes.
They are all in the JavaFx-Runtime-Jar-file, together with all the Javafx-classes.
The Javafx-classes are pretty well documented in the meantime.
But, allthough in the same "package", the com.sun... classes are still black boxes to me.
Appreciate a link from somebody who knows, pls.
Hans

Hi Hans,
the classes in the com.sun.* packages are internal classes, in other words they are meant to be black boxes for you. :-)
A developer should not use these classes, because they can change anytime without warning, even between minor releases. But a developer should also not need to use the internal classes. If Jasper needed them in the demo, it is a clear indicator that something is missing in the public API.
The classes seem to be part of the UI controls, which were already open-sourced. If you just want to play with the classes, you can study the sources. As a long term solution, I suggest to add a feature request in JIRA for the parts you are missing in the public API (http://javafx-jira.kenai.com).

Documentation formats for interfaces, classes, abstract classes

I am writing docs and want to make sure it conforms to best practices. Here is what I am doing:
interfaces = [italics, not bold]
classes = [not italics, bold]
abstract classes = [italics, bold]
I am most unsure of abstract classes?
That is important to me, so please help me get this straight. thanks.

Everybody knows that interfaces are blue, classes are red, and abstract classes use jumpy text.
(that was a joke by the way)
To my mind, using bold for abstract classes would be misleading. Just on an intuitive level, bold seems more concrete. But that's just me. I think the best thing to do is just to decorate the names with "<<interface>>" or "<<abstract>>" so there's no confusion.

What are abstract classes/methods and what are they for?

Hi,
I've just heard about abstract classes and methods and I'm just wondering what exactly they're used for, and why are they there for the Graphics class for example?
Cheers.

raggy wrote:
bastones_ wrote:
Hi,
I've just heard about abstract classes and methods and I'm just wondering what exactly they're used for, and why are they there for the Graphics class for example?
Cheers.Hey bro, I'll try to solve your problemYou have to know two important concepts for this part. 1 is Abstract classes and the other is Interface classes. Depends on the nature of the project, you need to set certain level of standards and rules that the other developers must follow. This is where Abstract classes and Interface classes come into picture.
Abstract classes are usually used on small time projects, where it can have code implementation like general classes and also declare Abstract methods (empty methods that require implementation from the sub-classes).Wrong, they are used equally among big and small projects alike.
Here are the rules of an Abstract class and method:
1. Abstract classes cannot be instantiatedRight.
2. Abstract class can extend an abstract class and implement several interface classesRight, but the same is true for non-abstract classes, so nothing special here.
3. Abstract class cannot extend a general class or an interfaceWrong. Abstract classes can extend non-abstract ones. Best example: Object is non-abstract. How would you write an abstract class that doesn't extend Object (directly or indirectly)?
4. If a class contains Abstract method, the class has to be declared Abstract classRight.
5. An Abstract class may or may not contain an Abstract methodRight, and an important point to realize. A class need not have abstract methods to be an abstract class, although usually it will.
6. Abstract method should not have any code implementations, the sub-classes must override it (sub-class must give the code implementations). An abstract method must not have any implementation code code. It's more than a suggestion.
7. If a sub-class of an Abstract class does not override the Abstract methods of its super-class, than the sub-class should be declared Abstract also.This follows from point 4.
9. Abstract classes can only be declared with public and default access modifiers.That's the same for abstract and non-abstract classes.

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.
[Page 376]
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.
[Page 377]
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.
[Page 378]
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:
[Page 379]
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
[Page 380]
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.
[Page 381]
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.
[Page 382]
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.
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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.
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/** 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);
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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.
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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

Why use an Abstract Class ?

I am new to Java and for some reason I can't get my head around why to use an abstract class. I understand that an abstract class is something like:
public abstract class Food{ // abstract class
public void eat(){
// stub
public class Apple extends Food{
public void eat(){
// Eat an apple code
}So basically the idea above is that you can eat an "apple" but you can't eat "food" because you can't instantiate an abstract class.
I understand what an abstract class is and how to write one. What I don't understand is why you would use it? It looks to me like I could have just created a normal class called "Food" and just not instantiated it. What are the benefits of using an abstract class?

807479 wrote:
I am new to Java and for some reason I can't get my head around why to use an abstract class.One of the first books I ever read about Object-Oriented design contained the following quote from [url http://en.wikipedia.org/wiki/Lucius_Cary,_2nd_Viscount_Falkland]Lord Falkland:
"When it is not necessary to make a decision, it is necessary +not+ to make a decision."
It took me quite a while to understand, but it's all about flexibility: As soon as you cast something in stone, you lose the ability to change it later on if something better/more appropriate comes along. Interfaces and abstract classes are all about delaying that decision.
As jverd said, interfaces allow you to specify what is required without defining the how; and as ErasP said, abstract classes are usually incomplete: ie, they define some of the 'how', but not all of it.
What is most important about abstract classes though is that they cannot exist on their own: They must be extended by a concrete class that completes the 'how' before they can be instantiated and, as such, they declare the intent of the designer.
One of the most important uses of abstract classes is as "skeleton implementations" of interfaces, and there are a lot of examples of these in the Java Collections hierarchy. My favourite is probably AbstractList, which contains a skeleton implementation of a List. Because it exists, I can create a class that wraps an array as a List with very little code, viz:public final class ArrayAsList<T>()
   extends AbstractList<T>
   private final T[] values;
   public ArrayAsList(T... values) {
      this.values = values;
   @Override
   public T get(int index) {
      return values[index];
   @Override
   public T set(int index, T element) {
      T value = get(index);
      values[index] = element;
      return value;
   @Override
   public int size() {
      return values.length;
};and somewhere else, I can use it:   List<String> letters =
      new ArrayAsList<String>("a", "b", "c");or perhaps, more practically:   List<String> words = new ArrayAsList<String>(
      bigTextString.split(" +") );Now that may not seem like a big deal to you, but given all that Lists can do, it's actually a very powerful bit of code. The above example is from "Effective Java" (p.95).
HIH
Winston

When to use abstract classes instead of interfaces with extension methods in C#?

"Abstract class" and "interface" are similar concepts, with interface being the more abstract of the two. One differentiating factor is that abstract classes provide method implementations for derived classes when needed. In C#, however,
this differentiating factor has been reduced by the recent introduction of extension methods, which enable implementations to be provided for interface methods. Another differentiating factor is that a class can inherit only one abstract class (i.e., there
is no multiple inheritance), but it can implement multiple interfaces. This makes interfaces less restrictive and more flexible. So, in C#, when should we use abstract classes
instead of interfaces with extension methods?
A notable example of the interface + extension method model is LINQ, where query functionality is provided for any type that implements IEnumerable via
a multitude of extension methods.

Hi
Well I believe Interfaces have more uses in software design. You could decouple your component implementing against interfaces so that
you have more flexibility on changing your code with less risk. Like Inversion of Control patterns where you can use interfaces and then when you decide you can change the concrete implementation that you want to use. Or other uses for interfaces is you could
use Interceptors using interfaces (Unity
Interceptor) to do different things where not all of these is feasible or at least as straightforward using abstract classes.
Regards
Aram

Lookinf for a "flexible" abstract class mechanism

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