Abstract Class

An abstract class in Java is a class that cannot be instantiated directly and is designed to be subclassed. It serves as a blueprint for other classes and may contain one or more abstract methods, which are methods declared without a body. Here are some key points about abstract classes:

1. Cannot be instantiated: An abstract class cannot be instantiated on its own. Instead, it must be subclassed by other classes, which provide concrete implementations for its abstract methods.

2. May contain abstract methods: An abstract class may contain abstract methods, which are declared using the abstract keyword and do not have a body. Subclasses of the abstract class must provide concrete implementations for these abstract methods.

3. Can contain concrete methods: In addition to abstract methods, an abstract class can also contain concrete methods with implementations. These methods are inherited by subclasses but can be overridden if needed.

4. Can contain fields: Abstract classes can have fields, constructors, and other members like a regular class. These members are inherited by subclasses and can be accessed or overridden as needed.

5. Used for code reusability and polymorphism: Abstract classes are useful for defining common behavior and ensuring consistency across related classes. They promote code reusability by providing a common base for subclasses to build upon. They also enable polymorphism, allowing objects of different subclasses to be treated uniformly through their common abstract superclass.

Example:

abstract class Shape {

    protected String color;

    public Shape(String color) {

        this.color = color;

    }

    // Abstract method

    abstract double area();

    // Concrete method

    public void setColor(String color) {

        this.color = color;

    }

    // Concrete method

    public String getColor() {

        return color;

    }

}

class Circle extends Shape {

    private double radius;

    public Circle(String color, double radius) {

        super(color);

        this.radius = radius;

    }

    @Override

    double area() {

        return Math.PI * radius * radius;

    }

}

class Rectangle extends Shape {

    private double length;

    private double width;

    public Rectangle(String color, double length, double width) {

        super(color);

        this.length = length;

        this.width = width;

    }

    @Override

    double area() {

        return length * width;

    }

}

In this example, `Shape` is an abstract class with an abstract method `area()` and concrete methods `setColor()` and `getColor()`. The `Circle` and `Rectangle` classes extend `Shape` and provide concrete implementations for the `area()` method.