abstract Keyword in Java

The abstract keyword in Java is used to declare a class or a method that cannot be instantiated directly or must be implemented by subclasses, respectively. It is a key part of Java's abstraction mechanism, allowing developers to define abstract classes and methods that provide a blueprint for other classes.

Usage

Abstract Class

An abstract class is a class that cannot be instantiated on its own and may contain abstract methods that must be implemented by subclasses.

Abstract Method

An abstract method is a method that is declared without an implementation. Subclasses of the abstract class are required to provide an implementation for these methods.

Syntax

Abstract Class

abstract class ClassName {
    // Abstract methods
    abstract void methodName();
    
    // Concrete methods
    void concreteMethod() {
        // Method body
    }
}

Abstract Method

abstract void methodName();

Examples

Example 1: Abstract Class and Method

abstract class Animal {
    // Abstract method
    abstract void makeSound();

    // Concrete method
    void sleep() {
        System.out.println("Sleeping...");
    }
}

class Dog extends Animal {
    // Implementing the abstract method
    void makeSound() {
        System.out.println("Bark");
    }
}

public class AbstractExample {
    public static void main(String[] args) {
        Dog dog = new Dog();
        dog.makeSound(); // Outputs: Bark
        dog.sleep();     // Outputs: Sleeping...
    }
}

In this example, Animal is an abstract class with an abstract method makeSound() and a concrete method sleep(). The Dog class extends Animal and provides an implementation for the makeSound() method.

Example 2: Multiple Abstract Methods

abstract class Shape {
    abstract void draw();
    abstract double area();
}

class Circle extends Shape {
    private double radius;

    Circle(double radius) {
        this.radius = radius;
    }

    void draw() {
        System.out.println("Drawing Circle");
    }

    double area() {
        return Math.PI * radius * radius;
    }
}

public class ShapeExample {
    public static void main(String[] args) {
        Circle circle = new Circle(5);
        circle.draw(); // Outputs: Drawing Circle
        System.out.println("Area: " + circle.area()); // Outputs: Area: 78.53981633974483
    }
}

Here, Shape is an abstract class with two abstract methods: draw() and area(). Circle extends Shape and provides implementations for both abstract methods.

Tips and Best Practices

• Use Abstract Classes for Shared Code: Use abstract classes when you have shared code that should be inherited by multiple subclasses. Abstract classes can contain both abstract and concrete methods.
• Mandatory Implementation: Ensure that all abstract methods in an abstract class are implemented by its subclasses.
• Cannot Instantiate: Remember that you cannot create instances of an abstract class directly. You must create an instance of a subclass that implements all abstract methods.
• Use Abstract Methods for Interfaces: If you have a class that will serve as a base with methods that must be overridden, use abstract methods to enforce this contract.
• Combining with Interfaces: Abstract classes can implement interfaces and provide implementations for some of the methods, while leaving others as abstract for subclasses to implement.  

  abstract class Vehicle implements Movable {
      abstract void start();
  }

• Constructor in Abstract Class: Abstract classes can have constructors, which can be called during the instantiation of a subclass.  

  abstract class Base {
      Base() {
          System.out.println("Base Constructor");
      }
  }

• Abstract Classes vs Interfaces: Use abstract classes when you need to share code among closely related classes, and use interfaces when you need to define a contract that can be implemented by any class, regardless of its position in the class hierarchy.
• Static Methods and Fields: Abstract classes can have static fields and methods, which can be accessed without creating an instance of the class.

abstract class Utility {
    static void printMessage() {
        System.out.println("Utility Message");
    }
}