Answers for "what is abstract class in java"

implementing an abstract class jaav

interface human {
    void dothings();
    void eatthings();
    void sleep();
}
abstract class student implements human {
    public void dothings()
    {
        System.out.println("doing something");
    }
    public void eatthings()
    {
        System.out.println("eating something");
    }
}
class basisstudent extends student {
    public void sleep()
    {
        System.out.println("no");
    }
}
public class Main {
    public static void main(String[] args)
    {
        basisstudent you = new basisstudent();
        you.dothings();
        you.eatthings();
        you.sleep();
    }
}

how to make abstract method in java

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;
        }
    }
}

abstract class example in java

//abstract parent class
abstract class Animal{
   //abstract method
   public abstract void sound();
}
//Dog class extends Animal class
public class Dog extends Animal{

   public void sound(){
	System.out.println("Woof");
   }
   public static void main(String args[]){
	Animal obj = new Dog();
	obj.sound();
   }
}

is it necessary for abstract class to have abstract method

Abstract classes CAN have non-abstract methods.

abstract class

Abstract class is used in defining a common super
class while writing Page Object Model layer of the
framework. We usually create an abstract class named
BasePage to have all common members for every page written
in this class example getPageTitle().
Then each Page class (HomePage, LoginPage, DashboardPage
etc.) inherit from BasePage.
Sometimes one may need to change the behavior of methods
implemented in superclass. So, subclass has freedom to
override that method where we use polymorphism.
This is how we use Abstract class in real projects.

.In my framework I have created my
BasePage class as super
class of the all page classes. 
I have collected all common elements
and functions into PageBase class and
all other page classes extent PageBase class.
By doing so, I don't have to locate very
common WebElements and it provides
reusability in my framework.
Also
1)Abstract classes cannot be instantiated
2)Abstract class meant to be inherited 
	so can not be final,static and private
2)An abstarct classes contains abstract method,
concrete methods or both.
3)Any class which extends abstarct class must
  override all methods of abstract class
4)An abstarct class can contain either
  0 or more abstract method.

abstract class java constructor

/**
 * 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