Abstract Classes

In object-oriented programming, an abstract class is a class that cannot be instantiated on its own and serves as a blueprint or template for other classes to inherit from. It defines a set of methods and properties that its subclasses must implement or override, but it does not provide a complete implementation of these methods and properties.

An abstract class typically contains one or more abstract methods, which are methods that do not have a body or implementation in the abstract class itself. Instead, they are declared with the "abstract" keyword and are meant to be implemented by the subclasses. Concrete methods, which have a complete implementation in the abstract class, can also be included.

Abstract classes in Java are a powerful tool for creating flexible, extensible, and maintainable code by establishing a common foundation for related classes, while preventing direct instantiation of the base class. They promote code reuse, abstraction, and polymorphism - key principles of object-oriented programming

An abstract class in Java is like a blueprint for a house that isn't complete yet. It defines the structure of the house, but leaves some details for others to fill in. Here's how it works in simple terms:

Partial Blueprint: An abstract class is like a blueprint for a building that has some parts detailed out and some parts left vague. It tells you the general layout and structure but doesn’t specify everything.
Cannot Be Built As-Is: Just like you can’t build a house directly from an incomplete blueprint, you can’t create an object of an abstract class in Java. It’s not fully defined.
Inheritance and Completion: To use an abstract class, other classes (known as concrete classes) must inherit from it and fill in the missing details. This is like a builder who takes the partial blueprint and adds the missing parts to complete the house.
Contains Abstract Methods: An abstract class can have abstract methods. These are like parts of the blueprint that say “something goes here, but we don’t know what.” Classes that inherit from the abstract class must provide the specific details of these methods.
Can Have Concrete Methods Too: An abstract class can also have fully defined (concrete) methods. These are like parts of the blueprint that are fully detailed and won’t change, no matter who builds the house.
Why Use Abstract Classes? They are used when you want to define a common blueprint for a group of subclasses. This ensures that all subclasses share certain methods, but also have their unique implementations of some methods.

An abstract class in Java is like a partial blueprint that outlines a structure but doesn’t provide all the details, leaving it to the inheriting classes to fill in the specifics. It’s a way to define a common template while enforcing certain structure and behavior in subclasses.

Abstract Classes

An abstract class in object-oriented programming is a class that cannot be instantiated on its own and is designed to be subclassed. It often includes abstract methods as well as concrete methods. Abstract methods are methods that are declared in the abstract class but must be implemented by subclasses.

Here are some key points about abstract classes:

Purpose: Abstract classes are used to represent general concepts (e.g., Shape, Animal), which can be used as a basis for more specific subclasses (e.g., Circle, Square, Dog, Cat). They provide a template for other classes to inherit from.
Abstract Methods: These are declared without an implementation. They only provide the method signature (return type, name, and parameters). The implementation of these methods must be provided by non-abstract subclasses.
Instantiation: Abstract classes cannot be instantiated directly. This means you cannot create an object of an abstract class using the new keyword. They are designed to be extended by subclasses.
Subclassing: To use an abstract class, a subclass must be created that extends the abstract class. The subclass usually provides implementations for all of the abstract methods in its parent abstract class.
Concrete Methods: Abstract classes can also have concrete methods (methods with an implementation). This allows you to define some common behavior that can be reused by multiple subclasses.
Constructors: Abstract classes can have constructors, and these constructors can be called from the subclass using the super keyword in most object-oriented languages like Java or C#.
Partial Abstraction: Unlike interfaces, which define a full contract with no implementations, abstract classes can provide a partial implementation. This means that an abstract class can define both what is to be done and how some of it should be done.

Here's a simple example in Java:

public abstract class Animal {
    private String name;

    public Animal(String name) {
        this.name = name;
    }

    public abstract void makeSound(); // Abstract method with no body

    public void eat() {
        System.out.println(name + " is eating.");
    }
}

public class Dog extends Animal {
    public Dog(String name) {
        super(name);
    }

    @Override
    public void makeSound() {
        System.out.println(getName() + " barks.");
    }
}

// You cannot do this: Animal a = new Animal("Some Animal");
// But you can do this: Animal a = new Dog("Buddy");

In this example, Animal is an abstract class that cannot be instantiated on its own. It includes an abstract method makeSound() that must be implemented by any non-abstract subclass that extends Animal. The Dog class provides the implementation of makeSound(). The eat() method is a concrete method and can be used by any subclass of Animal without providing its own implementation.