abstract class and Interface

[Muni K Reddy]

Hi Guys,
I know this question might sound trivial. I know the differences between Abstract classes and Interfaces. Could you please provide me with real world examples. Examples like cats and dogs makes sense in books but when I start to do a project how do I decide when I need an Interface and when I need an Abstract class? I would really be glad if someone who is in the Industry can give me examples of few real world Interfaces and Abstract classes
Many Thanks
Muni

[Peter Chase]

An interface is just a contract that an object can fulfill. It includes no implementation.

An abstract class is best used when you can provide, in a base class, the vast majority of the implementation, but you need to defer to subclasses for implementation of a small proportion.

Because a class can implement any number of interfaces, but can inherit only one superclass, using interfaces makes it easier to extend and maintain your code later. So, choose an interface over an abstract class, if there isn't some other big reason for a different choice.

[Raghavan Muthu]

Thats a very good and precise information Peter.

[Maneesh Godbole]

There is another use for the interface concept, called the "marker" interface.
You can write an interface which is absolutely empty!Yes. Really!
Since a java class can implement multiple interfaces, any class can implement such a "marker" interface.

Imagine some code which has a method call say allow(Object object). This method is going to allow only certain type of objects to be processed. So you can have something like

public boolean allow(Object object){
return object instanceof IMarker
}    

Since you know what kind of object are legal objects, you just declare these objects to be implementing the IMarker interface.

Such approach is typically used to keep the code loosely coupled. Since the arguments are of Object type, which as you know is the cosmic super class and IMarker is an interface, you dont need to change the allow method at all in case you change any of the objects which you are passing as arguments.

Does this make sense or would you like some code to make things clear?

[Muni K Reddy]

Thanks for taking time to explain mate!

I would love to make this concept clearer and some code would be gratefully appreciated!

Just wondering under what scenarios would you make a loosely coupled method you mentioned in the example.

Regards
Muni

[Maneesh Godbole]

Ok. I am posting some sample code which should clear up things. And yes it is not a "cat and dog" example. I hate those kind of examples too!

This is the "marker interface.

public interface IMarker {
//This is a marker interface without even one single method signature
//On its own it does absolutely nothing
}    

This is another interface which supplies the method to compute the size.

public interface IMarkerSizeInfo {

//Method to compute the size of the object.
//Notice the argument is of type Object to make it absolutely generic.
//Now what is a size? Well it depends on the object doesnt it?
public Long computeSizeFor(Object object);
 
}    

Now we have a concrete implementation of the IMarkerSizeInfo for the FILE object.

public class MyFileMarkerSizeInfo implements IMarkerSizeInfo {
 
public MyFileMarkerSizeInfo() {
//Class which is designed to compute the size of Files and Folders
//Size of file is file.length();
//Size of a folder needs to be explicitly computed by traversing the
//folders and files inside it
}
 
public Long computeSizeFor(Object object) {
//First level check. Accept objects only if they are of the type IMarker
if(!(object instanceof IMarker))
return null;
else {
//Type cast it to file object.
File file=(File) object;
//File is actually a file and not a directory
if(!file.isDirectory())
return new Long(file.length());
else {
long size=0;
File files[]=file.listFiles();
for(File currentFile:files) {
if(currentFile.isDirectory())
size+=getSizeOf(currentFile);
else
size+=currentFile.length();
}
return new Long(size);
}
}

}
 
private long getSizeOf(File currentFile) {
long size=0;
//Use recursion in case of nested directories
if(currentFile.isDirectory())
size+=getSizeOf(currentFile);
else
size+=currentFile.length();

return size;
}
 
}    

Now we have your class which is extending the File but with some added information.

public class MyFile extends File implements IMarker {

//Some extra info not available in the standard file object
private long lastAccessedDate;
private String lastAccessedUser;

public MyFile(String name) {
super(name);
}
 
public long getLastAccessedDate() {
return lastAccessedDate;
}
 
public void setLastAccessedDate(long lastAccessedDate) {
this.lastAccessedDate = lastAccessedDate;
}
 
public String getLastAccessedUser() {
return lastAccessedUser;
}
 
public void setLastAccessedUser(String lastAccessedUser) {
this.lastAccessedUser = lastAccessedUser;
}
 

}    

[Maneesh Godbole]

Sorry to post the reply like this but the editor window is too small for me and scrolling is hell.

We have another concrete implementation which accepts the Rectangle object instead of File

public class MyRectangleMarkerSize implements IMarkerSizeInfo {
 
public MyRectangleMarkerSize() {
//Class designed to return the size of the Rectangle
//Size is defined as the area for example's sake
}
 
public Long computeSizeFor(Object object) {
//First level check. Accept objects only if they are of the type IMarker
if(!(object instanceof IMarker))
return null;
else {
Rectangle rect=(Rectangle) object;
return new Long(rect.width*rect.height);
}
}
 
}    

As you can see the first level of checking is always for the IMarker interface. Only then the computation takes place.

Of course in reality, one should check the IMarker type before invoking the compute size for method. In that case there will be a factory which will return the corresponding concrete implementation class depending on the type of object. However to make things easier to figure out, I have coded it this way.

Hope this helps.

[Muni K Reddy]

Thats a brilliant example. Thanks a ton! If you live around Hyderabad let me know,I owe you a nice chilled beer

[Maneesh Godbole]

Beer! Now you are talking man!
Unfortunately I am from Pune. So I will have to hold you on to it for the future