When to use an abstract class in Java - 038

In Java, an abstract class is a class definition where we don't provide all the implementation details. The class has the keyword "abstract" added to it here. We denote abstract classes in our diagrams by making the class name in italics.

We can also mark some of the methods as abstract. That means we just provide the method signature and return type, but we don't provide the Java block that implements the method. We mark the methods with the keyword "abstract". Note the method has no braces, no implementation, and just ends with a semi-colon. Again we use italics for abstract methods in our diagrams.

If we declare a class as abstract, the methods may or may not be abstract. We don't need to have any abstract methods. It would be weird, because what would be the point, but we can do it. The opposite isn't true. If we mark any method as abstract, we must mark the class as abstract.

Another note. We can’t have a method that is abstract and private. We couldn’t access it, and there’s really no point. If you’re protecting it for subclasses, you’ll want to use protected.

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Concepts: Java, classes, abstract classes, Compile-time safety

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[SirGibbels]

Came here to find out about JAVA, ended up discovering something far more important about bread...

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62 people failed their Intro to Object Oriented Programming class...

Bravo, excellent video. Using cooking recipes to explain Abstract classes is my most favorite way to describe them to the person who's interviewing me

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[guruprasad]

we can implement the methods using interface as well? I just wanted to know why abstract class should be chosen if we have interface?

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[ayushimathur]

Thank you so much!! I have gone through a lot of videos but i was really confused, now I finally understand. 😊

[HernisQ]

Thank you for your video!

I have a question. I want a simple employee hierarchy where testers, developers and designers, among others in the future, are all employees, and all employees have unique IDs and a state (meeting, working, break, etc...), among others, which could be modelled as abstract class with protected attributes and some methods, abstract and otherwise. In this case, it is better to go with abstract classes instead of interfaces since we need a state, that is going to be shared by all possible subclass instances of an employee; and there is no such a thing as a generic employee, which goes hand in hand with not being able to instantiate an employee directly.

I'd appreciate any thoughts on this.

Thanks!

[tobicipher]

My teacher doesnt teach us why we use certain things in Java so this is very useful, thank you for the simply and clear explanation!

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[jasper]

Can you please explain interfaces in more detail? I have seen your youtube video on Interfaces but I am not able to understand when to use Interfaces.