A Singleton variable in an Abstract Class

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

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

Similar Messages

Abstract classes and OO theory...

I have an OO theory question related to abstract classes and their member variables. I realize opinions will vary from person to person but I'll ask anyway...
If I'm creating an abstract class, should member variables that will be used by subclasses be private or protected? Why? Either way, they will have accessor methods for classes that instantiate them (where appropriate) but I'm debating whether or not to give subclasses direct access to these variables or not.
To me, it seems odd to make variables in the abstract class private but I'm curious what your opinions are.
Thanks...

I knew you'd say that. I'd like to play along with a for-instance, if you will allow ... here's an example parent class:
public abstract class ProvideMenu   extends JFrame {
protected              JMenuBar       jmbar;
protected              JMenu          jmfile,
                                        jmedit,
                                        jmhelp;
protected              JMenuItem      jmfnew,
                                        jmfopen,
                                        jmfsave,
                                        jmfsaveas,
                                        jmfprint,
                                        jmfexit,
                                        jmecopy,
                                        jmepaste,
                                        jmhabout,
                                        jmhcontents;
public ProvideMenu() {
    jmbar       = new JMenuBar();
    jmfile      = new JMenu( FILE_MENU );
    jmedit      = new JMenu( EDIT_MENU );
public abstract void aMeth();
}Now the subclass does not need to provide setProvideMenuFont( new Font( ... ) ) and Font getProvideMenuFont() methods, setProvideMenuEnabled(), etc. but can use the standard methods setFont, getFont setBackground(), setEnabled, etc, etc.
Mind you I'm not advocating this ... what I am doing is trying to open the discussion up a bit - I hope no one minds ... what better design - just a for-instance - would you advocate?
~Bill

Abstract class reference itself

How do I reference an abstract class to itself.
public abstract class SomeClass {
   public static SomeType SomeMethod(){
         SomeClass mine = new SomeClass();
   }It gives me an error saying it can not instantiate the class.

Possible source of confusion: you can declare a variable of type abstract class and assign it an object of a derived class that fully implements the abstract class. For instance:
abstract class SomeClass
     public static void SomeMethod(){}
     public abstract void greeting();
class AnotherClass extends SomeClass
     public void greeting()
          System.out.println("hello");
public class DemoAbstractClasses
     public static void main(String args[])
          SomeClass A = new AnotherClass(); // A's type is an abstract class
          A.greeting(); //polymorphism
}

How to read data from abstract class

Hi,
I have an abstract class which has a try block in the run method which looks like this:
public void run()
throws exception
try
ArrayList xCoord = new ArrayList();
Double xValue = null;
for (j=0; j<10; j++)
xValue = new Double (table.getX(j).toDouble());
xCoord.add(xValue); // to populate arraylist
i also have the required catch statements, i have not written them here to keep in concise.
My question is: How do I read the values stored in the xCoord arraylist from another 'concrete' class derived from the above abstract class.
I know that for this, I first have to be able to execute the above "run" method, because that is how the arraylist will get populated with values. But how do I do this from the concrete class?
Then, once the arraylist is formed, how do I read the values from the concrete class, without copying all the code?
Any guidance will be really helpful. Thanks -- Vinay.

Add a public or protected accessor (getter) method to retrieve the value calculated in run(). Modify run() to store the results in an instance variable in the abstract class.
- Saish

Abstract class with set and get methods

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

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

Implementing Comparable in an abstract class

Hi all,
I am making my first sortie with abstract classes. I have had a good look around, but would still appreciate some advice with the following problem.
In my application I have several classes that have many things in common. I have concluded therefore, that if I create and then inherit from an abstract super class, I can reduce and improve my code. I created this abstract class:
public abstract class YAbstractObject implements Comparable {
    public YAbstractObject(int projectId, YObject object, String objectName) {
        this.projectId = projectId; // Always the same parameters
        this.object = object;
        this.objectName = objectName;
    // This is abstract as it must always be present for sub classes but differant processing will take place
    public abstract void resolveObject();
    // These three methods will always be the same for all sub classes
    public String getName() {
        return objectName;
    public YObject getObject() {
        return object;
    public boolean isValid() {
        return isValid;
// Overridden and always the same for all sub classes
    public String toString() {
        return objectName;
    // implemented abstract method
    public int compareTo(Object thatObject) {
        // Issue here! I would like something as follows:
        // return this.getName().compareToIgnoreCase(thatObject.getName());
// Variable decleration
    private int projectId;
    private YObject object;
    private String objectName;
    private boolean isValid;As I have commented in the compareTo() method, I would like it to be able to use the getName() method for comparison objects and compare them. But it does not like this, as it does not know that "thatObject" is of the same class as this object - I hope that made sense.
in essence, I want to inherit this method for different classes and have it work with each.
Is there a way to do this? Generics?
Any observations, greatly appreciated,
Steve

You can use also generics (if applicable: java -version >= 1.5).
public abstract class Test implements Comparable<Test> {
     String name;
     public Test(String name) {
          this.name = name;
     public String getName() {
          return name;
     public int compareTo(Test obj) {
          return this.getName().compareTo(obj.getName());
public class Other extends Test {
     public Other(String name) {
          super(name);
public class Tester extends Test {
     public Tester(String name) {
          super(name);
     public static void main(String[] args) {
          Test t = new Tester("t");
          Test a = new Tester("a");
          Test o = new Other("t");
          System.out.println(t.compareTo(a));
          System.out.println(t.compareTo(new Object())); //compile error
          System.out.println(t.compareTo(o));
}Without the compile error line it will give the following result:
19
0

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

Abstract Class and polymorphism - class diagram

Hi,
]Im trying to draw a class diagram for my overall system but im not too sure how to deal with classes derived from abstract classes. I'll explain my problem using the classic shape inheritance senario...
myClass has a member of type Shape, but when the program is running shape gets instantiated to a circle, square or triangle for example -
Shape s = new circle();
since they are shapes. But at no stage is the class Shape instantiated.
I then call things like s.Area();
On my class diagram should there be any lines going from myClass directly to circle or triangle, or should a line just be joining myClass to Shape class?
BTW - is s.Area() polymorphism?
Thanks,
Conor.

Sorry, my drawing did not display very well.
If you have MyClass, and it has a class variable of type Shape, and the class is responsible for creating MyClass, use Composition on your UML diagram to link Shape and MyClass.
If you have MyClass, and it has a class variable of type Shape, and the class is created elsewhere, use Aggregation on your UML diagram to link Shape and MyClass.
If you have MyClass, and it is used in method signatures, but it is not a class variable, use Depedency on your UML diagram to link Shape and MyClass. The arrow will point to Shape.
Shape and instances of Circle, Triangle, Square, etc. will be linked using a Generalization on your UML diagram. The arrow will always point to Shape.
Anything that is abstract (class, method, variable, etc.) should be italicized. Concrete items (same list) should be formatted normally.
BTW, the distinction between Composition, Aggregation and Dependency will vary from project to project or class to class. It's a gray area. Just be consistent or follow whatever guidelines have been established.
- Saish

Final field in abstract class

Hi ... consider the code:abstract class It {
     protected final boolean
          USE;
public class Test extends It {
     public Test(){
          USE = true;
} upon compiling there is an error (infact, two errors).
final may not be assigned, can't assign to final.
of course, if "It" was not abstract, the first error might be true ... but it is abstract, so
shouldn't this checking of assignment to final be delayed until the class is inheritied
as concrete ? it would seem to make sense ... i can't see a reason for this not being
the case.
if it was the case, we could delay assignment until our constructor of "Test" where it
would assign to the final variable of "It".
it shouldn't be an error, should it ?
java version:java version "1.4.2_03"
Java(TM) 2 Runtime Environment, Standard Edition (build 1.4.2_03-b02)
Java HotSpot(TM) Client VM (build 1.4.2_03-b02, mixed mode)

is it because references to the final fields are resolved at compile-time ?
such that the compiler must know the value of USE in:if(USE){
// a
// b at compile time so it can remove either "b" or "a". I think so. hmm.

Accessing a Sub class variable in a Super Class

Hi ,
Is there any easiest way to access a Subclass Variable in a Super Class.
Class Super1{
Class sub extends Super1
private String substring1;
In my application the 'substring1' values is not null .But all fields in Super1 class are null .
How can i access the value of the Subclass Variable in Super class .
Thanks

This would be a way to make the superclass dependent on subclass behavior. Of course this only makes sense if getSubString() is likely to have multiple different implementations in different subclasses.
public abstract class Super {
public String getString() {
   return "SuperString" + getSubString();
protected abstract String getSubString();
public class Sub extends Super {
private String substring;
protected String getSubString() {
   return substring;
}Using this just to access a variable whose contents differ from subclass to subclass is overkill. If you want each subclass to provide a different substring, create a constructor with a substring parameter in the superclass instead:public class Super {
private String substring;
protected Super(String substring) {
   this.substring = substring;
public String getString() {
   return "SuperString" + substring;
public class Sub extends Super {
public Sub() {
   super("substring");
}

A question about Abstract Classes

Hello
Can someone please help me with the following question.
Going through Brian's tutorials he defines in one of his exercises an 'abstract' class as follows
<ClassType ID="MyMP.MyComputerRoleBase" Base="Windows!Microsoft.Windows.ComputerRole" Accessibility="Internal" Abstract="true" Hosted="true" Singleton="false">
Now I am new to authoring but get the general concepts, and on the surface of it, it appears odd to have an 'abstract' class that is also 'hosted' I understand and abstract class to be a template with one or more properties defined which is then used
as the base class for one or more concrete classes. Therefore you do not have to keep defining the properties on these concrete classes over and over as they inherit the properties from their parent abstract class, is this correct so far?
if the above is correct then it seems odd that the abstract class would be hosted, unless (and this is the only way it makes sense to me) that ultimately (apart from singleton classes) any class that is going to end up being discovered and
thereby an instance of said class instigated in the database must ultimately be hosted somewhere, is that correct?
in other words if you has an abstract class, which acts as the base class for one or more concrete classes (by which I mean ones for which you are going to create lets say WMI discoveries ), if this parent abstract class is not ultimately
hosted to a concrete class of its own then these child concrete classes (once discovered) will never actually create instances in the database.
Is that how it is?
Any advise most welcome, thanks
AAnotherUser__
AAnotherUser__

Hi,
Hosting is the only relationship type that doesn't require you to discover it with a discovery rule. OpsMgr will create a relationship instance automatically for you using a 'hosted' properties 'chain' in a class\model definition. In an abstract class definition
you need to have this property set to 'true' or you will 'brake' a hosting 'chain'.
http://OpsMgr.ru/

Question about Abstract Classes and Class Inheritance

The semester is winding down, and the assignments are getting more complicated....
Prof has given us a project involving both an interface and an abstract class. There's a class Person, then an abstract class Employee that extends Person, and two types of Employees (Hourly and Salaried) that extend Employee. The finished assignment is a type of payroll program that's supposed to be able to store both types of Employees and related info (name, salary, etc). One thing the prof suggested was to store both HourlyEmployees and SalariedEmployees in an array of Employees. But you can't instantiate an array of Employees directly, of course, since it's an abstract class.
So is it possible to create an array of Persons, cast either HourlyEmployees or SalariedEmployees into Employee objects, and then store the Employee objects in the Person array? And if I do that, can I still use methods particular to the SalariedEmployees and/or HourlyEmployees once they are in the Person array? Or can I store SalariedEmployee and HourlyEmployee directly in an array of Persons, without having to cast them into anything else? (In that case, I'm not sure what the point of having the abstract Employee class is, though). Do they become just generic "Persons" if I do this?
Thanks for any help!

But you
can't instantiate an array of Employees directly, of
course, since it's an abstract class.Sure you can. You just can't instantiate Employee (the abstact class itself, as opposed to its subclasses) objects.
Employee[] employees = new Employee[20];
employees[0] = new HourlyEmployee();That should work.
So is it possible to create an array of Persons, cast
either HourlyEmployees or SalariedEmployees into
Employee objects, and then store the Employee objects
in the Person array?You could do that as well, but you shouldn't need to cast it.
Given the type hierarchy you describe, an HourlyEmployee is a Person, so you should be able to assign an HourlyEmployee directly to a Person-valued variable.
And if I do that, can I still use
methods particular to the SalariedEmployees and/or
HourlyEmployees once they are in the Person array?No. If the method doesn't exist in Person, then you can't call it on a Person variable, even if the method does exist in the class implementing Person.
But if the method exists in Person, but is implemented and possibly overridden in HourlyEmployee, you can still invoke it, by just invoking the Person method.
public interface Person {
public void feed();
public abstract class Employee implements Person {
public abstract void hire();
public class HourlyEmployee extends Employee {
// then:
Person persons = new Person[20];
// add HourlyEmployees or SalariedEmployees to persons array...
persons[0].feed(); // OK!
persons[0].hire(); // NOT OK!

Difference between Abstract Classes Vs Interface

Hi,
Can u pls mention all the differences between Abstract Classes and Interface.? I've mentioned the differences I've known here.
Known Differences:
(*) An interface cannot implement any methods, whereas an abstract class can.
(*) A class can implement many interfaces but can have only one superclass
Can U pls mention at what situation(practical situation) we've to go for abstract class or Interface?
Tell me the situation when we have to go for abstract class?
Tell me the situation when we have to go for interface?
Please Reply me
Thanks & Regards
Venkatesh

There are more differences, and one really important is that abstract classes can also define class variables, while interfaces cannot. I think the question of when to use interfaces or abstract classes is not always easy to answer, but yourself have pointed some tips you should be aware of :
If you need that some funcionality of the class is derived by more than one "parent" then you should use interfaces, since you cannot extend more than one class.
If your "superclass" needs to define some class variables then the choice must be made to have a superclass and then extend it. Also this is applicable if there is a method that can be programmed at a higher level (in interfaces you cannot program methods).
But the answer to the question is still not easy. And remember, you can always mix both tipes, you can extend one class and implement some interfaces.
Examples or that are very common in the Java API for AWT or Swing components, for example javax.swing.JLabel extends javax.swing.JComponent (that is beacuse a JLabel IS a JComponent and it uses some variables and methods programmed at the JComponent "level") and it also implements some interfaces: Accessible, ImageObserver, MenuContainer, Serializable & SwingConstants.
I hope this helps.
Zerjillo

Abstract class and RMI

Hi!
I have a desgin where I have an abstract class that in its constructor registers itself in a registry and define three abstract methods. Then I have a class that extends the abstract class and implements the three methods.
One of the methods inserts som text in a textArea defined in the child of the abstract class but when I call this method remotly I get a nullpointer exception saying that the textArea has not been initialized. The child ofcause calls super in its constructor.
So my question is can you not extend an abstract class that register it self in the constructor and then call methods remotly that uses variables that have been initialized in a childs constructor?
Have your any idea to why I get the nullpointer exception?
Peter

>
This sounds like there might be a misunderstanding
about the nature of methods and variables. If, for
instance, your client looks something like this:
RObject roo = ...// get via RMI
TextArea text = roo.getTextArea();
text.setText("...");
My design is more like this:
public interface Subscriber extends Remote {
public void notifySubscriber();
public abstract class A implements Subscriber {
a() {
    server.addSubscriber(this);
public abstract method1();
// some more abstract methods
public void notifySubscriber() {
    // do some stuff
    method1();
public class B extends A {
JTextArea text;
A() {
    super();
    text = new JTextArea();
public void method1() {
    // do some stuff
    // then text is null when called remotly
    text.setText("Hello");
}Hope this makes my design more clear and you can see the problem.
Thanks in advance

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
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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
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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.
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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
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