Java Abstract Classes

What is an Abstract Class?

In Java, an abstract class is a class that cannot be instantiated on its own and is meant to be subclassed. Abstract classes are used to provide a base for subclasses to extend and implement the abstract methods. This allows for a common interface while enabling specific implementations in subclasses.

Abstract classes can include abstract methods (methods without a body) as well as regular methods with implementations. This dual nature allows abstract classes to define the structure and behavior of an object.

Defining an Abstract Class

To define an abstract class in Java, you use the abstract keyword. An abstract class can have both abstract and non-abstract methods. Here's an example of an abstract class definition:

Implementing Abstract Classes

To use an abstract class, you need to create a subclass that provides implementations for the abstract methods. Here's how you can implement the Animal class:

When to Use Abstract Classes

Abstract classes are best used when you have a parent class that should not be instantiated directly and you want to ensure that certain methods are implemented in any subclass. They are particularly useful when some common behavior or state is shared among subclasses, but the actual implementation of certain methods can vary.

Use abstract classes when:

• You want to share code among several closely related classes.
• You expect classes that extend your abstract class to have many common methods or fields.
• You want to declare non-static or non-final fields.

Abstract Classes vs Interfaces

While both abstract classes and interfaces can be used to define a contract for subclasses, they have some key differences:

• Abstract Classes: Can have instance variables, constructors, and methods with implementations.
• Interfaces: Cannot have instance variables (before Java 8), and all methods are implicitly abstract unless they have default implementations.

Choose abstract classes when you want to share code among several related classes, and interfaces when you want to define a common protocol for classes that are otherwise unrelated.