public abstract class Shape { // Abstract method without implementation public abstract double area(); // Concrete method with implementation public void display() { System.out.println("This is a shape."); } }Sometimes we may come across a situation where we cannot provide implementation to all the methods in a class. We want to leave the implementation to a class that extends it. In such case we declare a class as abstract.To make a class abstract we use key word abstract. Any class that contains one or more abstract methods is declared as abstract. If we don’t declare class as abstract which contains abstract methods we get compile time error. 1)Abstract classes cannot be instantiated 2)An abstract classes contains abstract method, concrete methods or both. 3)Any class which extends abstract class must override all methods of abstract class 4)An abstract class can contain either 0 or more abstract method. public abstract class Animal { private String name; public Animal(String name) { this.name = name; } public void eat() { System.out.println(name + " is eating."); } public abstract void makeSound(); // Abstract method }public abstract class Account { //abstract class //perent class protected int accountNumber; protected Customer customerObj; protected double balance; //constructor public Account(int saccountNumber, Customer scustomerObj,double sbalance){ accountNumber = saccountNumber; customerObj = scustomerObj; balance = sbalance; } // abstract Function public abstract boolean withdraw(double amount); } public class SavingsAccount extends Account { // child class private double minimumBalance; // constructor public SavingsAccount(int saccountNumber, Customer scustomerObj, double sbalance, double sminimumBalance) { super(saccountNumber, scustomerObj, sbalance); minimumBalance = sminimumBalance; } // Implementation of abstract function in child class public boolean withdraw(double amount) { if (balance() > minimumBalance && balance() - amount > minimumBalance) { super.setBalance(balance() - amount); return true; } else { return false; } } } An abstract method is the method which does’nt have any body. Abstract method is declared with keyword abstract and semicolon in place of method body. public abstract void <method name>(); Ex : public abstract void getDetails(); It is the responsibility of subclass to provide implementation to abstract method defined in abstract class/*Abstract class: A class which is declared as abstract is known as an abstract class. It can have abstract and non-abstract methods. It needs to be extended and its method implemented. It cannot be instantiated. */ // Example of abstract class abstract class A{} // Example of Abstract class that has an abstract method abstract class Bike{ abstract void run(); } class Honda4 extends Bike{ void run(){System.out.println("running safely");} public static void main(String args[]){ Bike obj = new Honda4(); obj.run(); } } /** * Simple Java program to prove that abstract class can have constructor in Java. * @author http://java67.blogspot.com */ public class AbstractConstructorTest { public static void main(String args[]) { Server server = new Tomcat("Apache Tomcat"); server.start(); } } abstract class Server{ protected final String name; public Server(String name){ this.name = name; } public abstract boolean start(); } class Tomcat extends Server{ public Tomcat(String name){ super(name); } @Override public boolean start() { System.out.println( this.name + " started successfully"); return true; } } Output: Apache Tomcat started successfully interface methods{ public void hey(); public void bye(); } //unable to implement all the abstract methods in the interface so // the other class automatically becomes abstract abstract class other implements methods{ public void hey(){ System.out.println("Hey"); } } //able to implement all the methods so is not abstract class scratch implements methods { public void hey(){ System.out.println("Hey"); } public void bye() { System.out.println("Hey"); } }abstract class Language { // method of abstract class public void display() { System.out.println("This is Java Programming"); } } class Main extends Language { public static void main(String[] args) { // create an object of Main Main obj = new Main(); // access method of abstract class // using object of Main class obj.display(); } }abstract class scratch{ }Abstract classes have some special features: it's impossible to create an instance of an abstract class; an abstract class can contain abstract methods that must be implemented in non-abstract subclasses; it can contain fields and non-abstract methods (including static); an abstract class can extend another class, including abstract; it can contain a constructor.