Abstract class vs Interface in Java

By | July 17, 2019

Question

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.

Solution

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.

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