Object oriented design problem concerning abstract classes and interfaces

I have an abstract class (class A) that takes care of database connections. It cannot be made into an interface as other classes extend it and all these other classes require the functionality in the methods it has (i.e. I cannot make all the methods abstract and then make this class an interface).
I have a class that contains data (Customer class) that I will create from the data I extract from the database. This class will also be created by the User and submitted to the database portion of the program. The Customer class has functionality in its methods which is required by the rest of the program (i.e. I cannot make all the methods abstract and then make this class an interface).
I have a factory class (CustomerFactory) that extends the Customer class. This has been created to restrict access to the creation and manipulation of Customers.
I have a class (DatabaseQuery) that extends class A. But now that I have retrieved all of the information that comprises a Customer from the database, I cannot construct a Customer without making reference to UserFactory. But UserFactory is a class that I don't want the database portion of the program to know about.
What I would like to do is have my DatabaseQuery class extend both Customer class and A class. But they are both classes and Java won't allow that.
I can't make either of the two classes that I want to make parents of DatabaseQuery into interfaces... so what can I do other than just keep a reference to UserFactory in my DatabaseQuery class?
Thanks,
Tim

>
What I would like to do is have my DatabaseQuery class
extend both Customer class and A class. But they are
both classes and Java won't allow that.
I can't make either of the two classes that I want to
make parents of DatabaseQuery into interfaces... so
what can I do other than just keep a reference to
UserFactory in my DatabaseQuery class?Just a guess...
The description sounds a little vague but it sounds like the correct solution would be to refactor everything. The first clue is when I see "database connection" as an "abstract class". The only hierarchy that a database connection might exist in is in a connection pool and even that is probably shaky. It should never be part of data records, which is what your description sounds like.
That probably isn't what you want to hear.
The other solution, which is why refactoring is better (and which also makes it apparent why the original design is wrong) is to create an entire other hierarchy that mirrors your current data hierarchy and wraps it. So you now have "Customer", you will now have "Customer" and "DBCustomer". And all the code that currently uses "Customer" will have to start using DBCustomer. Actually it is easier than that since you can simply make the new class be "Customer" and rename the old class to "DBCustomer". Naturally that means the new class will have to have all of the functionality of the old class. Fortunately you can use the old class to do that. (But I would guess that isn't going to be easy.)

Similar Messages

Difference between abstract classes and interfaces

I actually wonder about what are the differences between abstract classes and interfaces may somebody give an example code about it?
and i have one more question how can i use interfaces like multiple inheritance ? i mean when i implement an interface like
class a extends b implements c,di have to use all c and d methods but what that methods means?
I mean as i know we cannot make implementations of methods in interfaces
but for example in runnable interface there is a method like run() and it has been defined somewhere because it knows what to do(i mean when it will run), i just write my code into that method .

Once you get past the starting point (I am referring to the OP here), there are a few salient differences:
You can only extend (or generalize) a single superclass; however, you can implement (or realize) multiple interfaces. As such, all things being equal, using an interface in lieu of an abstract class 'frees' your design. Later, if you want the implementor of an interface to inherit from another class, there is not issue.
Any abstract method specifies a contract. However, abstract classes allow you to also add common behavior to subclasses. This is an overused justification for abstract classes, IMO. You can achieve the same effect using delegation and still having interfaces.
Always program to interfaces wherever possible. This means that you define an interface and have an implementing class (usually at a minimum). Do not do this for all your classes, but rather the ones that make your system unique (the domain model or M in MVC architecture). This allows you to later change implementation with a minimal amount of refactoring. This is a core precept from the Group of Four and any number of decent programming books.Best of luck.
- Saish

Question about Classes, Abstract Classes and Interfaces.

I have been experimenting with Classes, Abstract Classes and Interfaces and wonder if anyone can explain this to me.
I was looking for a way to assign a value to a variable and then keep it fixed for the session and have devised this.
First I create an abstract class like this:
public abstract class DatabaseConnection {
private static String ServerName = null;
public static void setServerName(String serverName) {
          ServerName = serverName;
     public static String getServerName() {
          return ServerName;
}and then I created an interface
public interface DatabaseAccess {
     String servername = DatabaseConnection.getServerName();
}And finally the class itself with some test lines in it so I could see what was going on:
public class CreateDatabase extends DatabaseConnection implements DatabaseAccess {
     public static void main (String args[]){
          new CreateDatabase();
     public CreateDatabase(){
          setServerName("Server Name 1");
          System.out.println ("Before update ");
          System.out.println ("ServerName from Interface           = " + servername);
          System.out.println ("ServerName from Abstract Class = " + getServerName());
          System.out.println ("After update ");
          setServerName("Server Name 2");
          System.out.println ("ServerName from Interface           = " + servername);
          System.out.println ("ServerName from Abstract Class = " + getServerName());
          System.out.println ("==========================");
}The output I get from the above is:
Before update
ServerName from Interface           = Server Name 1
ServerName from Abstract Class = Server Name 1
After update
ServerName from Interface           = Server Name 1
ServerName from Abstract Class = Server Name 2
==========================I also tried this in another class which calls the above class to see if I get the same effect
public class CheckDatabaseAccess {
     public static void main (String args[]){
          new CreateDatabase();
          CreateDatabase.setServerName("Server 3");
          System.out.println("CreateDatabase "+CreateDatabase.servername);
          CreateDatabase.setServerName("Server 4");
          System.out.println("CreateDatabase "+CreateDatabase.servername);
          CreateDatabase.setServerName("Server 5");
          System.out.println("CreateDatabase "+CreateDatabase.servername);
}The output of which is this:
Before update
ServerName from Interface           = Server Name 1
ServerName from Abstract Class = Server Name 1
After update
ServerName from Interface           = Server Name 1
ServerName from Abstract Class = Server Name 2
==========================
CreateDatabase Server Name 1
CreateDatabase Server Name 1
CreateDatabase Server Name 1Can anyone explain why I appear to only be able to change or set the ServerName only the once?
Is this the correct way to do it? If it is it's exactly what I am looking for, a way to set the value of variable once in a session and then prevent it being changed.
Or is there a better way of doing this.
What I want to use this for is for example, storing the accesses to a database on a server. I won't know what server the database will be stored on nor what the database is called so I create an INI file which stores this information in encrypted format, which is set by the database administrator. It occurs to me I can use this method to then retrieve that data once and once only from the INI file and use that throughout the life of the session to access the database.
Any help appreciated
Regards
John

Not gonna read all of it, but this jumps out:
public abstract class DatabaseConnection {
private static String ServerName = null;
public interface DatabaseAccess {
     String servername = DatabaseConnection.getServerName();
}You have two completely separate variables (with two different names, for that matter, since you were inconsistent in your capitalization, but it wouldn't make a difference if they did have the same name with the same case). And the one in the interface is implicitly public, static, and final.
Anytime you refer to "servername" through a reference of type DatabaseAccess, it refers to the one declared in the interface.
Anytime you refer to "ServerName" inside the DatabaseConnection class, it refers to the one declared in that class.

What is the difference between Abstract class and Interface ?

Hi,
Could u plz tell me the difference between Abstract class and Interface?
Thanks in advance.
Gopi

Lots.
An abstract class can contain some method implementations, or indeed all the method implementations. It may contain methods with all the various access modifiers. It cannot be instantiated. A class may inherit from only a single abstract class.
An interface contains only public method stubs and constants. A class may implement multiple interfaces. An interface cannot (obviously) be instantiated.
Abstract classes are particularly useful when you need to provide a semi-complete implementation for reuse. Interfaces are used more like types.
Look at java.util.* for some good examples of the use of both.

Abstract class and interface????

hi all, i would like to know the difference between abstract class and interface....could anybody enlighten me on this....

Like the others have mentioned you can implement more than one interface, but only inherit from one abstract class. An abstract class can provide some implementation.
steve http:\\www.jamonapi.com

Abstract class and interface having same method

Hello,
Here is my problem. Suppose we have one abstarct class and one interface.Here is code-
//Abstarct class
abstract class X{
abstract void myMethod();
//Interface
public interface Y{
abstract void myMethod(){}
Now i have a class which extends both abstarct class X and interface Y.
If i call myMethod() from this class. Whose myMethod would be called.Will it be of abstract class or interface?
Many Thanks

Hello,
Here is my problem. Suppose we have one abstarct class
and one interface.Here is code-
//Abstarct class
abstract class X{
abstract void myMethod();
}OK, so far...
//Interface
public interface Y{
abstract void myMethod(){}
}An interface cannot have code (the {} part), so this won't work.
Lets pretend though, it read
//Interface
public interface Y{
abstract void myMethod();
However, the abstract class above can have code;
If you extended X and implemented Y (with no code in it), you would have to have a myMethod() implementation in your code. That's the one that would run.
Now, let's pretend the abstract class above did have code in it.
//Abstract class
abstract class X {
abstract void myMethod() { System.out.println("Hello"); }
Then, you wouldn't have to have a myMethod() implementation in your class which extends X and implements Y (it's defined in X). If you didn't have one, the method in X would run. If you defined your own myMethod() implementation in your class (which extends X and implements Y), then your own implementation would run.

Dealing with abstract classes and interfaces with XMLBeans

I'm quite new to XMLBeans and would like some help. I've got things working when
you are only saving concreate classes as XML, but when it comes to clesses containing
interfaces I am running into problems.
I basically have a collection of XY objects (see below) that I want to save as
XML.
I want to convert the following java classes to an xml schema:
public class XY{
X x;
Y y;
public interface X{
void setA(int a);
int getA();
public interface Y{
void setB(int b);
int getB();
public class X1 implements X{
void setA(int a);
int getA();
//other data to save
public class X2 implements X{
void setA(int a);
int getA();
//other data to save
public class Y1 implements Y{
void setB(int b);
int getB();
//other data to save
public class Y2 implements Y{
void setB(int b);
int getB();
//other data to save
What would be the best way to convert these to XML Schema? I've looked through
all the XMLBeans documentation, but it doesn't say anything about dealing with
interfaces/abstract classes. Is there anywhere else to look?
Thanks in advance,
Andrew

Anurag,
What I really wanted was a work-around to using substiutionGroups as substitution
groups are not supported in this release.
I want to convert the following schema:
<xsd:complexType name="PublicationType">
<xsd:sequence>
<xsd:element name="Title" type="xsd:string"/>
<xsd:element name="Author" type="xsd:string" minOccurs="0" maxOccurs="unbounded"/>
<xsd:element name="Date" type="xsd:gYear"/>
</xsd:sequence>
</xsd:complexType>
<xsd:complexType name="BookType">
<xsd:complexContent>
<xsd:extension base="PublicationType" >
<xsd:sequence>
<xsd:element name="ISBN" type="xsd:string"/>
<xsd:element name="Publisher" type="xsd:string"/>
</xsd:sequence>
</xsd:extension>
</xsd:complexContent>
</xsd:complexType>
<xsd:complexType name="MagazineType">
<xsd:complexContent>
<xsd:restriction base="PublicationType">
<xsd:sequence>
<xsd:element name="Title" type="xsd:string"/>
<xsd:element name="Date" type="xsd:gYear"/>
</xsd:sequence>
</xsd:restriction>
</xsd:complexContent>
</xsd:complexType>
<xsd:element name="Publication" type="PublicationType"/>
<xsd:element name="Book" substitutionGroup="Publication" type="BookType"/>
<xsd:element name="Magazine" substitutionGroup="Publication" type="MagazineType"/>
<xsd:element name="BookStore">
<xsd:complexType>
<xsd:sequence>
<xsd:element ref="Publication" maxOccurs="unbounded"/>
</xsd:sequence>
</xsd:complexType>
</xsd:element>
to produce an XML file like:
<?xml version="1.0"?>
<BookStore ¡Ä>
<Book>
<Title>Illusions: The Adventures of a Reluctant Messiah</Title>
<Author>Richard Bach</Author>
<Date>1977</Date>
<ISBN>0-440-34319-4</ISBN>
<Publisher>Dell Publishing Co.</Publisher>
</Book>
<Magazine>
<Title>Natural Health</Title>
<Date>1999</Date>
</Magazine>
<Book>
<Title>The First and Last Freedom</Title>
<Author>J. Krishnamurti</Author>
<Date>1954</Date>
<ISBN>0-06-064831-7</ISBN>
<Publisher>Harper & Row</Publisher>
</Book>
</BookStore>
What is the best way to do this without using substitution types?
Thanks,
Andrew
"Anurag" <[email protected]> wrote:
Andrew,
Could you please paste/attach a sample of the code you are using to convert
Java bean classes to XMLBeans?
In the current release the goal of XMLBeans technology was to convert
from
XML Schemas or XML files to XMLBeans. The focus was not towards conversion
of Java beans to XMLBeans.
Regards,
Anurag
"Andrew" <[email protected]> wrote in message news:[email protected]...
I'm quite new to XMLBeans and would like some help. I've got thingsworking when
you are only saving concreate classes as XML, but when it comes toclesses
containing
interfaces I am running into problems.
I basically have a collection of XY objects (see below) that I wantto
save as
XML.
I want to convert the following java classes to an xml schema:
public class XY{
X x;
Y y;
public interface X{
void setA(int a);
int getA();
public interface Y{
void setB(int b);
int getB();
public class X1 implements X{
void setA(int a);
int getA();
//other data to save
public class X2 implements X{
void setA(int a);
int getA();
//other data to save
public class Y1 implements Y{
void setB(int b);
int getB();
//other data to save
public class Y2 implements Y{
void setB(int b);
int getB();
//other data to save
What would be the best way to convert these to XML Schema? I've lookedthrough
all the XMLBeans documentation, but it doesn't say anything about dealingwith
interfaces/abstract classes. Is there anywhere else to look?
Thanks in advance,
Andrew

Abstract classes and Interfaces

Why would you use these? Why not make a concrete class and extend them? I see why JAVA doesn't use mutiple inheritance but I don't see how allowing interfaces correctes that, after all what happens if two interfaces implemented by one class have two fully defined methods with the same signature but differant outputs?
I tryed googleing this but just got articles on when to use Abstract over interface and interface over abstract, which should have helped some but it didn't.
Can someone explain or post a link for a good example.

Why would you use these? Why not make a concrete
class and extend them? Check out the JDBC interfac (java.sql package).
The core Java API defines what types and methods it wants to use for DB interactions. But because every DB and driver is so different, there's no concrete implementation that the core API could provide for those types. It's up to the vendors to provide all the "how"--all the concrete implementations of the methods.
interfaces correctes that, after all what happens if
two interfaces implemented by one class have two
fully defined methods with the same signature but
differant outputs?That rarely happens (it has never happened to me in about 8 years of Java programming) and if it does, you just have to find a different approach--you can't meet both contracts.

What's the difference between Abstract Class and Interface?

Dear all,
Can anyone give me some hints about this topic?
Thanks.
Leo

an abstract class may have some methods already implemented in the abstract class but an interface has no methods implemented
I think it's just that simple.
For your design needs, you just choose what you need : )
Cheers
Stephen

Difference between abstract class and interface

Hi everyone,
CAn anyone explain why the following happen????
Abstract class can have a constructor, Interface cannot.
Abstract class can have static methods, Interface cannot.

What are the advantages of defining constant
variables in Interfaces?Not many.
Effective Java - Joshua Bloch, Chapter 4, Item #17: Use interfaces only to define types.
The constant interface pattern is a poor use of interfaces. That a class uses some constants internally is an implementation detail. Implementing a constant interface causes this implementation detail to leak into the class's exported API. It is of no consequence to the users of a class that the class implements a constant interface. In fact, it may even confuse them. Worse, it represents a commitment: if in a future release the class is modified so that it no longer needs to use the constants, it still must implement the interface to ensure binary compatibility. If a nonfinal class implements a constant interface, all of its subclasses will have their namespaces polluted by the constants in the interface.
In summary, interfaces should only be used to define types. They should not be used to export constants.

Abstract class Vs Interface and importance of using

Hi All,
I want to know what is the difference between Abstract class and Interface.
and what is the importance of using them.
Moderator message: please search for available information/documentation.
Edited by: Thomas Zloch on Sep 27, 2011 1:55 PM

Please refer to this thread, already this is discussed in forums
[ABAP Objects.. Abastract & Interface Class;
Thanks
Pavan

What is the diff b/w Abstract class and an interface ?

Hey
I am always confused as with this issue : diff b/w Abstract class and an interface ?
Which is more powerful in what situation.
Regards
Vinay

Hi, Don't worry I am teach you
Abstract class and Interface
An abstract class can have instance methods that implement a default behavior. An Interface can only declare constants and instance methods, but cannot implement default behavior and all methods are implicitly abstract. An interface has all public members and no implementation. An abstract class is a class which may have the usual flavors of class members (private, protected, etc.), but has some abstract methods.
Edited by SASIKUMARA
SIT INNOVATIONS- Chennai
Message was edited by:
sasikumara
Message was edited by:
sasikumara

Difference between Abstarct Class and Interface

HI,
Here is a simple one for the gurus but quite important for me.
What is the difference b/w an Interface and an abstract class?
Why do we need each one of them?
I would appreciate if you people can give examples of each so that I amy understand fully
Thanks in advance...

A normal class (not abstract) has a special behaviour, like java.awt.Frame. A Frame is a frame no matter how you subclass it. If you create a subclass it will still be able to be displayed by calling it's show method. So by using a normal class you can create a type of class.
An abstract class does the same, but it leaves some of the code unwritten. For instance java.lang.Number. This class is abstract becuase it has no knowledge of how to store the number in memory. But it knows that it is a number, and any subclass will still be a number. You could create a subclass of Number that can hold the time in milliseconds and checks your system clock to see what the time is, it could also have methods that return the time in another country, but it would still be just a Number.
An interface is a way to describe what an object can do, not what it is. So with java.lang.Comparable as an example you can make any class comparable. This means that no matter what type of object you have, it can be compared with other objects. So you can have a subclass of Frame that can be compared with other windows. Or a subclass of Number that can be compared with other Numbers. You can even compare those two different types if you like. So you could compare a window with a number.
That is the difference between abstract classes and interfaces.
I hope you could follow my arguments, it isn't an easy subject,
Daniel

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 X Interfaces

1.The Java language has two similar concepts : Abstract Classes and Interfaces. So, When its better to use one or other technique and the advantages and disadvantages of each one ?

Classes can implement any number of interfaces but can extend only one class. Interfaces are used when a class might want to implement that interface but extend another class. However, interfaces cannot have implementation, so that's what abstract classes are used for.

Object oriented design problem concerning abstract classes and interfaces

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