I will give you an example first:
public interface LoginAuth{
public String encryptPassword(String pass);
public void checkDBforUser();
}
Suppose you have 3 databases in your application. Then each and every implementation for that database needs to define the above 2 methods:
public class DBMySQL implements LoginAuth{
// Needs to implement both methods
}
public class DBOracle implements LoginAuth{
// Needs to implement both methods
}
public class DBAbc implements LoginAuth{
// Needs to implement both methods
}
But what if encryptPassword() is not database dependent, and it's the same for each class? Then the above would not be a good approach.
Instead, consider this approach:
public abstract class LoginAuth{
public String encryptPassword(String pass){
// Implement the same default behavior here
// that is shared by all subclasses.
}
// Each subclass needs to provide their own implementation of this only:
public abstract void checkDBforUser();
}
Now in each child class, we only need to implement one method - the method that is database dependent.
Answer from Vimal Bera on Stack OverflowVideos
I will give you an example first:
public interface LoginAuth{
public String encryptPassword(String pass);
public void checkDBforUser();
}
Suppose you have 3 databases in your application. Then each and every implementation for that database needs to define the above 2 methods:
public class DBMySQL implements LoginAuth{
// Needs to implement both methods
}
public class DBOracle implements LoginAuth{
// Needs to implement both methods
}
public class DBAbc implements LoginAuth{
// Needs to implement both methods
}
But what if encryptPassword() is not database dependent, and it's the same for each class? Then the above would not be a good approach.
Instead, consider this approach:
public abstract class LoginAuth{
public String encryptPassword(String pass){
// Implement the same default behavior here
// that is shared by all subclasses.
}
// Each subclass needs to provide their own implementation of this only:
public abstract void checkDBforUser();
}
Now in each child class, we only need to implement one method - the method that is database dependent.
Nothing is perfect in this world. They may have been expecting more of a practical approach.
But after your explanation you could add these lines with a slightly different approach.
Interfaces are rules (rules because you must give an implementation to them that you can't ignore or avoid, so that they are imposed like rules) which works as a common understanding document among various teams in software development.
Interfaces give the idea what is to be done but not how it will be done. So implementation completely depends on developer by following the given rules (means given signature of methods).
Abstract classes may contain abstract declarations, concrete implementations, or both.
Abstract declarations are like rules to be followed and concrete implementations are like guidelines (you can use it as it is or you can ignore it by overriding and giving your own implementation to it).
Moreover which methods with same signature may change the behaviour in different context are provided as interface declarations as rules to implement accordingly in different contexts.
Edit: Java 8 facilitates to define default and static methods in interface.
public interface SomeInterfaceOne {
void usualAbstractMethod(String inputString);
default void defaultMethod(String inputString){
System.out.println("Inside SomeInterfaceOne defaultMethod::"+inputString);
}
}
Now when a class will implement SomeInterface, it is not mandatory to provide implementation for default methods of interface.
If we have another interface with following methods:
public interface SomeInterfaceTwo {
void usualAbstractMethod(String inputString);
default void defaultMethod(String inputString){
System.out.println("Inside SomeInterfaceTwo defaultMethod::"+inputString);
}
}
Java doesn’t allow extending multiple classes because it results in the “Diamond Problem” where compiler is not able to decide which superclass method to use. With the default methods, the diamond problem will arise for interfaces too. Because if a class is implementing both
SomeInterfaceOne and SomeInterfaceTwo
and doesn’t implement the common default method, compiler can’t decide which one to chose. To avoid this problem, in java 8 it is mandatory to implement common default methods of different interfaces. If any class is implementing both the above interfaces, it has to provide implementation for defaultMethod() method otherwise compiler will throw compile time error.
How about an analogy: when I was in the Air Force, I went to pilot training and became a USAF (US Air Force) pilot. At that point I wasn't qualified to fly anything, and had to attend aircraft type training. Once I qualified, I was a pilot (Abstract class) and a C-141 pilot (concrete class). At one of my assignments, I was given an additional duty: Safety Officer. Now I was still a pilot and a C-141 pilot, but I also performed Safety Officer duties (I implemented ISafetyOfficer, so to speak). A pilot wasn't required to be a safety officer, other people could have done it as well.
All USAF pilots have to follow certain Air Force-wide regulations, and all C-141 (or F-16, or T-38) pilots 'are' USAF pilots. Anyone can be a safety officer. So, to summarize:
- Pilot: abstract class
- C-141 Pilot: concrete class
- ISafety Officer: interface
added note: this was meant to be an analogy to help explain the concept, not a coding recommendation. See the various comments below, the discussion is interesting.
While your question indicates it's for "general OO", it really seems to be focusing on .NET use of these terms.
In .NET (similar for Java):
- interfaces can have no state or implementation
- a class that implements an interface must provide an implementation of all the methods of that interface
- abstract classes may contain state (data members) and/or implementation (methods)
- abstract classes can be inherited without implementing the abstract methods (though such a derived class is abstract itself)
- interfaces may be multiple-inherited, abstract classes may not (this is probably the key concrete reason for interfaces to exist separately from abtract classes - they permit an implementation of multiple inheritance that removes many of the problems of general MI).
As general OO terms, the differences are not necessarily well-defined. For example, there are C++ programmers who may hold similar rigid definitions (interfaces are a strict subset of abstract classes that cannot contain implementation), while some may say that an abstract class with some default implementations is still an interface or that a non-abstract class can still define an interface.
Indeed, there is a C++ idiom called the Non-Virtual Interface (NVI) where the public methods are non-virtual methods that 'thunk' to private virtual methods:
- http://www.gotw.ca/publications/mill18.htm
- http://en.wikibooks.org/wiki/More_C%2B%2B_Idioms/Non-Virtual_Interface
I understand that the interface is used to decouple the abstraction from the implementation. As a very common example you will see with a List and an ArrayList.
List<String> interfaceList = new ArrayList<>();
You can swap out the List implementation with any other class that implements the interface.
Same thing with abstract classes. So is there a reason why you don't see this?
AbstractList<String> abstractList = new ArrayList<>();
I wrote an article about that:
Abstract classes and interfaces
Summarizing:
When we talk about abstract classes we are defining characteristics of an object type; specifying what an object is.
When we talk about an interface and define capabilities that we promise to provide, we are talking about establishing a contract about what the object can do.
An abstract class can have shared state or functionality. An interface is only a promise to provide the state or functionality. A good abstract class will reduce the amount of code that has to be rewritten because it's functionality or state can be shared. The interface has no defined information to be shared
Note: please forgive the C# syntax, but the principle of the answer is the same for Java and C#.
Now when i started programming I noticed that in all subclasses i basically needed to do the exact same thing
Based on this, it seems like you think abstract classes are only allowed to declare abstract methods. This is not the case.
An abstract class is a class that cannot be instantiated directly (only its derivations can be instantiated). An abstract method is a method in an abstract class which must be implemented in the derived class.
But an abstract class can have non-abstract methods:
public abstract class Artikel
{
public int ArtikelId { get; set; }
public string SayHello()
{
return "Hi, I'm artikel " + ArtikelId;
}
}
When you derive Artikel into subclasses, you do not need to repeat the method body of the SayHello method. Its body has been declared in the base class and can be used by all of the derived classes.
I thought of making Artikel not abstract and put an interface between Artikel and the other classes
Interfaces prevent the ability to create a common method body. If you were to use an interface:
public interface IArtikel
{
string SayHello();
}
Then you will be required to implement this method separately in every class:
public class Book : IArtikel
{
public string SayHello()
{
// custom book logic
}
}
// And the same for all other derived classes.
It's also possible to make an seperate class which inherits from Artikel where I can put all the methods, but then there the methods would still be needed to made three times, one for each subclass right?
Don't take this the wrong way, but your attempts at solving this suggest you don't really master OOP. If this SeparateClass was created as another (4th) subclass from Artikel, how would you expect e.g. the Book class to rely on the methods found in SeparateClass?
Is it a bad design choice if I keep Artikel as abstract?
Keep Artikel abstract, but give it non-abstract methods (i.e. with method bodies) for each method that you are now copy/pasting between all of its subclasses.
You can have a base class as an abstract class which implements the Artikel interface. In the abstract class you can define the common implementation. Then you can derive LP, Book and Boardgame from that super class. In my opinion it is better to have a abstract class rather than copying the same code in all 3 sub classes.
I like to think of interfaces as contracts (this object will have property X & Y and method Z) but makes no assumptions about implementation (at least they did before Default Interface Implementations as of C# 8: https://devblogs.microsoft.com/dotnet/default-implementations-in-interfaces, although I use these sparingly if ever).
Abstract classes (and derived non-sealed classes) can be thought of as just a place to share implementation that's common between supersets of classes and their subsets. Abstract classes provide an object-oriented way of supporting the Don't Repeat Yourself (DRY) principle (https://www.c-sharpcorner.com/article/software-design-principles-dry-kiss-yagni) where the implementation you would otherwise repeat doesn't make sense outside of the context of your class hierarchy.
Interfaces & abstract classes are by no means mutually exclusive and can be used in conjunction with each other depending on the context.
abstract classes are to define a common base class for inheritance, without implementing any actual methods.
because c# does not support multiple inheritance, interfaces are the solution. a class can inherit from one base class, but it can implement many interfaces. interfaces can only be implemented. an interface can not inherit from another interface.
due to issues with inheritance chains, inheritance is often considered an anti-pattern, interface design is recommend instead. with interface design you define contacts that define properties and methods. then classes must implement. other languages use protocols/traits which are slight more feature rich than interfaces.
one past advantage with classes, was you could define default behavior, and interfaces were abstract (no implementations). But modern C# allows default implemations for interfaces.
so my recommendation is to avoid class inheritance (and never more than 1 deep, that is only inherit from base), but rather use interfaces and extension methods to extend classes.