Abstract class vs Interface in Java

I was asked a question, I wanted to get my answer reviewed here.

Q: In which scenario it is more appropriate to extend an abstract class rather than implementing the interface(s)?

A: If we are using template method design pattern.

Am I correct ?

I am sorry if I was not able to state the question clearly.
I know the basic difference between abstract class and interface.

1. use abstract class when the requirement is such that we need to implement the same functionality in every subclass for a specific operation (implement the method) and different functionality for some other operations (only method signatures)

2. use interface if you need to put the signature to be same (and implementation different) so that you can comply with interface implementation

3. we can extend max of one abstract class, but can implement more than one interface

Reiterating the question: Are there any other scenarios, besides those mentioned above, where specifically we require to use abstract class (one is see is template method design pattern is conceptually based on this only)?

Interface vs. Abstract class

Choosing between these two really depends on what you want to do, but luckily for us, Erich Gamma can help us a bit.

As always there is a trade-off, an interface gives you freedom with regard to the base class, an abstract class gives you the freedom to add new methods later. – Erich Gamma

You can’t go and change an Interface without having to change a lot of other things in your code, so the only way to avoid this would be to create a whole new Interface, which might not always be a good thing.

Abstract classes should primarily be used for objects that are closely related. Interfaces are better at providing common functionality for unrelated classes.

When To Use Interfaces

An interface allows somebody to start from scratch to implement your interface or implement your interface in some other code whose original or primary purpose was quite different from your interface. To them, your interface is only incidental, something that have to add on to the their code to be able to use your package. The disadvantage is every method in the interface must be public. You might not want to expose everything.

When To Use Abstract classes

An abstract class, in contrast, provides more structure. It usually defines some default implementations and provides some tools useful for a full implementation. The catch is, code using it must use your class as the base. That may be highly inconvenient if the other programmers wanting to use your package have already developed their own class hierarchy independently. In Java, a class can inherit from only one base class.

When to Use Both

You can offer the best of both worlds, an interface and an abstract class. Implementors can ignore your abstract class if they choose. The only drawback of doing that is calling methods via their interface name is slightly slower than calling them via their abstract class name.

> reiterating the question: there is any other scenario besides these > mentioned above where specifically we require to use abstract class > (one is see is template method design pattern is conceptually based on > this only)

Yes, if you use JAXB. It does not like interfaces. You should either use abstract classes or work around this limitation with generics.

From a personal blog post:

Interface:

1. A class can implement multiple interfaces
2. An interface cannot provide any code at all
3. An interface can only define public static final constants
4. An interface cannot define instance variables
5. Adding a new method has ripple effects on implementing classes (design maintenance)
6. JAXB cannot deal with interfaces
7. An interface cannot extends or implement an abstract class
8. All interface methods are public

In general, interfaces should be used to define contracts (what is to be achieved, not how to achieve it).

Abstract Class:

1. A class can extend at most one abstract class
2. An abstract class can contain code
3. An abstract class can define both static and instance constants (final)
4. An abstract class can define instance variables
5. Modification of existing abstract class code has ripple effects on extending classes (implementation maintenance)
6. Adding a new method to an abstract class has no ripple effect on extending classes
7. An abstract class can implement an interface
8. Abstract classes can implement private and protected methods

Abstract classes should be used for (partial) implementation. They can be a mean to restrain the way API contracts should be implemented.

Interface is used when you have scenario that all classes has same structure but totally have different functionality.

Abstract class is used when you have scenario that all classes has same structure but some same and some different functionality.

Take a look the article : http://shoaibmk.blogspot.com/2011/09/abstract-class-is-class-which-cannot-be.html

There are a lot of great answers here, but I often find using BOTH interfaces and abstract classes is the best route. Consider this contrived example:

You're a software developer at an investment bank, and need to build a system that places orders into a market. Your interface captures the most general idea of what a trading system does,

1) Trading system places orders
2) Trading system receives acknowledgements

and can be captured in an interface, ITradeSystem

public interface ITradeSystem{

     public void placeOrder(IOrder order);
     public void ackOrder(IOrder order);

}

Now engineers working at the sales desk and along other business lines can start to interface with your system to add order placement functionality to their existing apps. And you haven't even started building yet! This is the power of interfaces.

So you go ahead and build the system for stock traders; they've heard that your system has a feature to find cheap stocks and are very eager to try it out! You capture this behavior in a method called findGoodDeals(), but also realize there's a lot of messy stuff that's involved in connecting to the markets. For example, you have to open a SocketChannel,

public class StockTradeSystem implements ITradeSystem{    

    @Override 
    public void placeOrder(IOrder order);
         getMarket().place(order);

    @Override 
    public void ackOrder(IOrder order);
         System.out.println("Order received" + order);    

    private void connectToMarket();
       SocketChannel sock = Socket.open();
       sock.bind(marketAddress); 
       <LOTS MORE MESSY CODE>
    }

    public void findGoodDeals();
       deals = <apply magic wizardry>
       System.out.println("The best stocks to buy are: " + deals);
    }

The concrete implementations are going to have lots of these messy methods like connectToMarket(), but findGoodDeals() is all the traders actually care about.

Now here's where abstract classes come into play. Your boss informs you that currency traders also want to use your system. And looking at currency markets, you see the plumbing is nearly identical to stock markets. In fact, connectToMarket() can be reused verbatim to connect to foreign exchange markets. However, findGoodDeals() is a much different concept in the currency arena. So before you pass off the codebase to the foreign exchange wiz kid across the ocean, you first refactor into an abstract class, leaving findGoodDeals() unimplmented

public abstract class ABCTradeSystem implements ITradeSystem{    

    public abstract void findGoodDeals();

    @Override 
    public void placeOrder(IOrder order);
         getMarket().place(order);

    @Override 
    public void ackOrder(IOrder order);
         System.out.println("Order received" + order);    

    private void connectToMarket();
       SocketChannel sock = Socket.open();
       sock.bind(marketAddress); 
       <LOTS MORE MESSY CODE>
    }
  

Your stock trading system implements findGoodDeals() as you've already defined,

public class StockTradeSystem extends ABCTradeSystem{    

    public void findGoodDeals();
       deals = <apply magic wizardry>
       System.out.println("The best stocks to buy are: " + deals);
    }

but now the FX whiz kid can build her system by simply providing an implementation of findGoodDeals() for currencies; she doesn't have to reimplement socket connections or even the interface methods!

public class CurrencyTradeSystem extends ABCTradeSystem{    

    public void findGoodDeals();
       ccys = <Genius stuff to find undervalued currencies>
       System.out.println("The best FX spot rates are: " + ccys);
    }

Programming to an interface is powerful, but similar applications often re-implement methods in nearly identical ways. Using an abstract class avoids reimplmentations, while preserving the power of the interface.

Note: one may wonder why findGreatDeals() isn't part of the interface. Remember, the interface defines the most general components of a trading system. Another engineer may develop a COMPLETELY DIFFERENT trading system, where they don't care about finding good deals. The interface guarantees that the sales desk can interface to their system as well, so it's preferable not to entangle your interface with application concepts like "great deals".

Which should you use, abstract classes or interfaces?

Consider using abstract classes if any of these statements apply to your use case:

You want to share code among several closely related classes.

You expect that classes that extend your abstract class have many common methods or fields, or require access modifiers other than public (such as protected and private).

You want to declare non-static or non-final fields. This enables you to define methods that can access and modify the state of the object to which they belong.

Consider using interfaces if any of these statements apply to your use case:

You expect that unrelated classes would implement your interface. For example, the interfaces Comparable and Cloneable are implemented by many unrelated classes.

You want to specify the behavior of a particular data type, but not concerned about who implements its behavior.

You want to take advantage of multiple inheritance of type.

http://docs.oracle.com/javase/tutorial/java/IandI/abstract.html

Things have been changed a lot in last three years with addition of new capabilities to interface with Java 8 release.

From oracle documentation page on interface:

> An interface is a reference type, similar to a class, that can contain only constants, method signatures, default methods, static methods, and nested types. Method bodies exist only for default methods and static methods.

As you quoted in your question, abstract class is best fit for template method pattern where you have to create skeleton. Interface cant be used here.

One more consideration to prefer abstract class over interface:

You don't have implementation in base class and only sub-classes have to define their own implementation. You need abstract class instead of interface since you want to share state with sub-classes.

Abstract class establishes "is a" relation between related classes and interface provides "has a" capability between unrelated classes.

---

Regarding second part of your question, which is valid for most of the programming languages including java prior to java-8 release

> As always there is a trade-off, an interface gives you freedom with regard to the base class, an abstract class gives you the freedom to add new methods later. – Erich Gamma > > You can’t go and change an Interface without having to change a lot of other things in your code

If you prefer abstract class to interface earlier with above two considerations, you have to re-think now as default methods have added powerful capabilities to interfaces.

> Default methods enable you to add new functionality to the interfaces of your libraries and ensure binary compatibility with code written for older versions of those interfaces.

To select one of them between interface and abstract class, oracle documentation page quote that:

> Abstract classes are similar to interfaces. You cannot instantiate them, and they may contain a mix of methods declared with or without an implementation. However, with abstract classes, you can declare fields that are not static and final, and define public, protected, and private concrete methods. > >With interfaces, all fields are automatically public, static, and final, and all methods that you declare or define (as default methods) are public. In addition, you can extend only one class, whether or not it is abstract, whereas you can implement any number of interfaces.

Refer to these related questions fore more details:

https://stackoverflow.com/questions/761194/interface-vs-abstract-class-general-oo/33963650#33963650

https://stackoverflow.com/questions/18777989/how-should-i-have-explained-the-difference-between-an-interface-and-an-abstract/34978606#34978606

In summary : The balance is tilting more towards interfaces now.

> Are there any other scenarios, besides those mentioned above, where specifically we require to use abstract class (one is see is template method design pattern is conceptually based on this only)?

Some design patterns use abstract classes (over interfaces) apart from Template method pattern.

Creational patterns:

Abstract_factory_pattern

Structural patterns:

Decorator_pattern

Behavioral patterns:

Mediator_pattern

You are not correct. There are many scenarios. It just isn't possible to reduce it to a single 8-word rule.

The shortest answer is, extend abstract class when some of the functionalities uou seek are already implemented in it.

If you implement the interface you have to implement all the method. But for abstract class number of methods you need to implement might be fewer.

In template design pattern there must be a behavior defined. This behavior depends on other methods which are abstract. By making sub class and defining those methods you actually define the main behavior. The underlying behavior can not be in a interface as interface does not define anything, it just declares. So a template design pattern always comes with an abstract class. If you want to keep the flow of the behavior intact you must extend the abstract class but don't override the main behavior.

In my opinion , the basic difference is that an interface can't contain non abstract methods while an abstract class can. So if subclasses share a common behaviour, this behaviour can be implemented in the super class and thus inherited in the subclasses

Also I quoted the following from "software architecture design ppatterns in java" book

" In the Java programming language there is no support for multiple inheritance. That means a class can inherit only from one single class. Hence inheritance should be used only when it is absolutely necessary. Whenever possible, methods denoting the common behavior should be declared in the form of a Java interface to be implemented by different implementer classes. But interfaces suffer from the limitation that they cannot provide method implementations. This means that every implementer of an interface must explicitly implement all methods declared in an interface, even when some of these methods represent the invariable part of the functionality and have exactly the same implementation in all of the implementer classes. This leads to redundant code. The following example demonstrates how the Abstract Parent Class pattern can be used in such cases without requiring redundant method implementations."

Abstract classes are different from interfaces in two important aspects

• they provide default implementation for chosen methods (that is covered by your answer)
• abstract classes can have state (instance variables) - so this is one more situation you want to use them in place of interfaces

This is a good question The two of these are not similar but can be use for some of the same reason, like a rewrite. When creating it is best to use Interface. When it comes down to class, it is good for debugging.

This is my understanding, hope this helps

Abstract classes:

1. Can have member variables that are inherited (can’t be done in interfaces)
2. Can have constructors (interfaces can’t)
3. Its methods can have any visibility (ie: private, protected, etc - whereas all interface methods are public)
4. Can have defined methods (methods with an implementation)

Interfaces:

1. Can have variables, but they are all public static final variables
   • constant values that never change with a static scope
   • non static variables require an instance, and you can’t instantiate an interface
2. All methods are abstract (no code in abstract methods)

• all code has to be actually written in the class that implements the particular interface

Usage of abstract and interface:

One has "Is-A-Relationship" and another one has "Has-A-Relationship"

The default properties has set in abstract and extra properties can be expressed through interface.

Example: --> In the human beings we have some default properties that are eating, sleeping etc. but if anyone has any other curricular activities like swimming, playing etc those could be expressed by Interface.

Abstract classes should be extended when you want to some common behavior to get extended. The Abstract super class will have the common behavior and will define abstract method/specific behavior which sub classes should implement.

Interfaces allows you to change the implementation anytime allowing the interface to be intact.

I think the answers here are missing the main point:

Java interfaces (the question is about Java but there are similar mechanisms in other languages) is a way to partially support multiple inheritance, i.e. method-only inheritance.

It is similar to PHP's traits or Python's duck typing.

Besides that, there is nothing additional that you truly need an interface for --and you cannot instantiate a Java interface.