Java Tutorials || Java OOPS || Abstract class || by Durga sir

Summary

This video explains the concept of abstract classes and methods in object-oriented programming, particularly in Java. It discusses how abstract methods lack implementation, leaving only declarations, and how abstract classes are used when a class is only partially implemented. The video also covers the restriction of object creation for abstract classes and how declaring a class abstract is necessary when it contains abstract methods. Additionally, it highlights that a class can still be abstract even without abstract methods, providing examples and clarifying common misunderstandings about abstract classes in programming.

Takeaways

• 😀 Abstract classes are partially implemented classes that cannot be instantiated directly.
• 😀 An abstract method is a method without an implementation, requiring subclasses to provide the implementation.
• 😀 You cannot create objects of an abstract class directly. Attempting to do so results in a compile-time error.
• 😀 If a class contains at least one abstract method, the class must be declared abstract.
• 😀 If a class is abstract, it is not possible to create objects of that class, even if the class contains some implemented methods.
• 😀 An abstract class may contain concrete methods (methods with implementations), but it is still considered abstract if the overall class isn't fully implemented.
• 😀 A class that has no abstract methods can still be declared abstract to prevent object creation or serve as a base class for inheritance.
• 😀 Declaring a class as abstract without any abstract methods is possible if you want to prevent object creation or provide default functionality in subclasses.
• 😀 If a class does not provide a complete implementation, like adapter classes or classes with dummy methods, it should be declared abstract to restrict object instantiation.
• 😀 Even if a class has no abstract methods, declaring it as abstract ensures that no one can create an object from it and enforces the intended structure of inheritance.

Q & A

What is an abstract method in Java?

An abstract method in Java is a method that is declared but does not have an implementation. It only has a method signature (declaration) and is used to be overridden in subclasses.

What is an abstract class?

An abstract class in Java is a class that cannot be instantiated on its own. It is partially implemented, meaning it may have abstract methods (without implementation) and concrete methods (with implementation). The abstract class is meant to be extended by other classes.

Can we create an object of an abstract class in Java?

No, we cannot create an object of an abstract class in Java. Since abstract classes are not fully implemented, attempting to instantiate them will result in a compile-time error.

What happens if you try to instantiate an abstract class?

If you try to instantiate an abstract class, the compiler will give an error stating that the class cannot be instantiated because it is abstract.

What should be declared if a class contains at least one abstract method?

If a class contains at least one abstract method, the class must be declared as abstract. If not, the compiler will give an error.

Can an abstract class have concrete (non-abstract) methods?

Yes, an abstract class can have both abstract methods (without implementation) and concrete methods (with implementation). The concrete methods can be used as they are, while abstract methods need to be implemented by subclasses.

Is it possible for an abstract class to have zero abstract methods?

Yes, an abstract class can have zero abstract methods. Even if it doesn't have any abstract methods, it can still be declared as abstract if you want to prevent direct instantiation of the class.

Can you provide an example of an abstract class with no abstract methods?

Yes, an abstract class can still be useful even without abstract methods. For example, an adapter class with default methods might be declared as abstract to prevent object instantiation while still providing some base functionality.

What is the advantage of declaring a class as abstract even if it has no abstract methods?

Declaring a class as abstract, even if it has no abstract methods, allows you to prevent object creation of that class. This is useful when the class is only meant to be a base for other classes to extend and not to be used on its own.

What error occurs when a class with an abstract method is not declared abstract?

If a class contains at least one abstract method but is not declared abstract, the compiler will give an error, stating that the class is not abstract and does not implement the abstract method.