Abstract Class

Abstract class:

• Any class that have at least one or more methods  are abstract then that class must be declare as abstract class.
• A class with abstract keyword is called abstract class.
• It can have both abstract and non-abstract methods
• It can not be instantiated means object of abstract class can not be created
• It can have Constructor
• Implementation of abstract methods should be provided by sub-class that inherited the abstract class
• Create abstract class using abstract keyword
• Make abstract class where you want to provide the implementation of abstract methods in the sub-class
• To restrict the class to have certain methods

Syntax:
AccessSpecifier abstract class ClassName {
      //data members, abstract methods and non-abstract methods
}

Example: Create an  abstract class Shape which have two abstract methods area() and perimeter() and one non-abstract method display() then create 6 classes Circle, Triangle, Rectangle, Square, Cuboid and Cube all will extends Shape abstract class?

Shape.java

package com.java.abst;

public abstract class Shape {
    abstract double area();
    abstract double perimeter();
    public void display(){
        System.out.println("Display method of abstract class Shape");
    }
}

Circle.java

package com.java.abst;

public class Circle extends Shape{
    double radius;
    public Circle(){
        radius = 1.0;
    }
    public Circle(double r){
        radius = r;
    }
    double area() {
        return 22.0 / 7.0 * radius * radius;
    }
    double perimeter() {
        return 2.0 * 22.0 / 7.0 * radius ;
    }
}

Triangle.java

package com.java.abst;

public class Triangle extends Shape {
    double a, b, c, s;
    
    public Triangle(double a, double b, double c) {
        this.a = a;
        this.b = b;
        this.c = c;
        s = perimeter()/2;
    }
    double area() {
        return Math.sqrt(s*(s-a)*(s-b)*(s-c));
    }
    double perimeter() {
        return a + b + c;
    }
}

Rectangle.java

package com.java.abst;

public class Rectangle extends Shape {
    double length, breadth;
    
    public Rectangle(double length, double breadth) {
        this.length = length;
        this.breadth = breadth;
    }
    double area() {
        return length * breadth;
    }
    double perimeter() {
        return 2 * (length * breadth);
    }
}

Square.java

package com.java.abst;

public class Square extends Shape {
    double side;

    public Square(double side) {
        this.side = side;
    }
    double area() {
        return side * side;
    }
    double perimeter() {
        return 4 * side;
    }
}

Cuboid.java

package com.java.abst;

public class Cuboid extends Shape {
    double l, b, h;
    
    public Cuboid(double l, double b, double h) {
        this.l = l;
        this.b = b;
        this.h = h;
    }
    double area() {
        return 2*(l*b + b*h + h*l);
    }
    double perimeter() {
        return 2*(l*b + b*h + h*l);
    }
    double volume() {
        return l*b*h;
    }
}

Cube.java

package com.java.abst;

public class Cube extends Shape {
    double side;
    
    public Cube(double side) {
        this.side = side;
    }
    double area() {
        return 6 * side * side;
    }
    double perimeter() {
        return 6 * side * side;
    }
    double volume() {
        return side * side * side;
    }
}

Test.java

package com.java.abst;

public class Test {
    public static void main(String[] args) {
        //Shape s = new Shape();   //Compiler error because abstract class cannot be Instantiated
        
        Circle c1 = new Circle();
        c1.display();
        System.out.println("-------------------------------Circle-----------------------------------");
        System.out.println("Area of Circle of " + "radius " + c1.radius + " is: " + c1.area());
        System.out.println("Perimeter of Circle of " + "radius " + c1.radius + " is: " + c1.perimeter());

        Circle c2 = new Circle(10.0);
        System.out.println("Area of Circle of " + "radius " + c2.radius + " is: " + c2.area());
        System.out.println("Perimeter of Circle of " + "radius " + c2.radius + " is: " + c2.perimeter());
        
        System.out.println("-------------------------------Triangle-----------------------------------");
        Triangle t = new Triangle(3, 4, 5);
        System.out.printf("Area of Triangle of sides %f, %f and %f is: %f \n",t.a, t.b, t.c, t.area());
        System.out.printf("Perimeter of Triangle of sides %f, %f and %f is: %f \n",t.a, t.b, t.c, t.perimeter());
        
        System.out.println("-------------------------------Rectangle-----------------------------------");
        Rectangle r = new Rectangle(5, 6);
        System.out.printf("Area of Square of length %f and breadth %f is %f \n ", r.length, r.breadth, r.area());
        System.out.printf("Perimeter of length %f and breadth %f is %f \n ", r.length, r.breadth, r.perimeter());
        
        System.out.println("-------------------------------Square-----------------------------------");
        Square s = new Square(6);
        System.out.println("Area of Square of side " + s.side + " is " + s.area());
        System.out.println("Perimeter of Square of side " + s.side + " is " + s.perimeter());
        
        System.out.println("-------------------------------Cuboid-----------------------------------");
        Cuboid cb = new Cuboid(4, 5, 6);
        System.out.printf("Area of Cuboid of dimensions %f, %f and %f is: %f \n",cb.l, cb.b, cb.h, cb.area());
        System.out.printf("Perimeter of Cuboid of dimensions %f, %f and %f is: %f \n",cb.l, cb.b, cb.h, cb.perimeter());
        System.out.printf("Volume of Cuboid of dimensions %f, %f and %f is: %f \n",cb.l, cb.b, cb.h, cb.volume());
    
        
        System.out.println("-------------------------------Cube-----------------------------------");
        Cube c = new Cube(5);
        System.out.printf("Area of Cube of side %f is: %f \n",c.side, c.area());
        System.out.printf("Perimeter of Cube of side %f is: %f \n",c.side, c.perimeter());
        System.out.printf("Volume of Cube of side %f is: %f \n",c.side, c.volume());
    }
}

Output:

Display method of abstract class Shape
-------------------------------Circle-----------------------------------
Area of Circle of radius 1.0 is: 3.142857142857143
Perimeter of Circle of radius 1.0 is: 6.285714285714286
Area of Circle of radius 10.0 is: 314.2857142857143
Perimeter of Circle of radius 10.0 is: 62.857142857142854
-------------------------------Triangle-----------------------------------
Area of Triangle of sides 3.000000, 4.000000 and 5.000000 is: 6.000000
Perimeter of Triangle of sides 3.000000, 4.000000 and 5.000000 is: 12.000000
-------------------------------Rectangle-----------------------------------
Area of Square of length 5.000000 and breadth 6.000000 is 30.000000
 Perimeter of length 5.000000 and breadth 6.000000 is 22.000000
 -------------------------------Square-----------------------------------
Area of Square of side 6.0 is 36.0
Perimeter of Square of side 6.0 is 24.0
-------------------------------Cuboid-----------------------------------
Area of Cuboid of dimensions 4.000000, 5.000000 and 6.000000 is: 148.000000
Perimeter of Cuboid of dimensions 4.000000, 5.000000 and 6.000000 is: 148.000000
Volume of Cuboid of dimensions 4.000000, 5.000000 and 6.000000 is: 120.000000
-------------------------------Cube-----------------------------------
Area of Cube of side 5.000000 is: 150.000000
Perimeter of Cube of side 5.000000 is: 150.000000
Volume of Cube of side 5.000000 is: 125.000000