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

Summary

This video provides an in-depth explanation of abstract methods and classes in Java. The speaker covers essential concepts, such as the purpose and syntax of abstract methods, which only have a declaration and no implementation. They also emphasize the importance of declaring a class as abstract when it contains abstract methods. Common mistakes, such as providing method bodies for abstract methods or forgetting to declare a class as abstract, are highlighted. Through examples like vehicle types and loan systems, viewers gain a clear understanding of when and how to use abstract methods and classes in Java programming.

Takeaways

• 😀 Abstract methods in Java are methods that have only a declaration, but no implementation.
• 😀 The abstract modifier can be applied to methods and classes, but not to variables.
• 😀 The abstract modifier is used when the implementation of a method or class is unknown or incomplete at the time of declaration.
• 😀 An abstract method must end with a semicolon (;) and not curly braces ({}).
• 😀 When declaring an abstract method, the implementing class is responsible for providing the method's implementation.
• 😀 Abstract classes can contain abstract methods, but the class itself must be declared as abstract if it contains any abstract methods.
• 😀 A method declared as abstract cannot have a method body. If a body is provided, it becomes invalid.
• 😀 The abstract modifier ensures that the method has no body, and its implementation is left to the subclasses.
• 😀 A class with at least one abstract method must also be declared abstract.
• 😀 If a method does not provide an implementation and is not declared abstract, a compilation error will occur.
• 😀 Abstract methods are often used when defining a common interface for different subclasses, allowing each subclass to define its own specific behavior.

Q & A

What is the primary purpose of the abstract modifier in Java?

The abstract modifier in Java is used to declare methods or classes that do not have complete implementations. It signifies that the method or class is meant to be implemented or extended by subclasses.

Can abstract methods have a method body in Java?

No, abstract methods cannot have a body. They can only have a declaration, and the implementation is provided by subclasses that extend the abstract class.

When must a class be declared as abstract in Java?

A class must be declared as abstract if it contains at least one abstract method. If a class has an abstract method, it cannot be instantiated directly and must be extended by a subclass that provides implementations for the abstract methods.

What happens if you forget to declare an abstract method with the 'abstract' keyword?

If you forget to declare an abstract method with the 'abstract' keyword, it will be treated as a regular method, and you must provide an implementation for it. This will cause a compilation error if you try to leave the body empty.

Can an abstract method be declared in a non-abstract class?

No, an abstract method cannot be declared in a non-abstract class. If a class contains at least one abstract method, the class itself must also be declared as abstract.

What is the correct syntax for declaring an abstract method in Java?

The correct syntax for declaring an abstract method is: `public abstract returnType methodName();`. This method must not have a body, and the implementation will be provided by subclasses.

What is the purpose of using the abstract modifier in a method when the implementation is unknown?

The abstract modifier allows the declaration of a method when the implementation is not yet known. This is particularly useful for creating base classes with method templates that will be implemented by subclasses.

What would happen if you provide an implementation for an abstract method in the abstract class?

Providing an implementation for an abstract method in the abstract class would result in a compilation error because abstract methods are meant to only have a declaration, not an implementation. The implementation should be provided by concrete subclasses.

What are the key characteristics of an abstract method?

An abstract method has no body, only a declaration with a semicolon. It must be implemented by concrete subclasses. If a class has any abstract methods, it must also be declared as abstract.

Can an abstract class have non-abstract methods?

Yes, an abstract class can have both abstract and non-abstract methods. The non-abstract methods can have a complete implementation, while abstract methods only have a declaration and must be implemented by subclasses.