abstract class in java

1Sometimes we may come across a situation where we cannot provide 
2implementation to all the methods in a class. We want to leave the 
3implementation to a class that extends it. In such case we declare a class
4as abstract.To make a class abstract we use key word abstract. 
5Any class that contains one or more abstract methods is declared as abstract. 
6If we don’t declare class as abstract which contains abstract methods we get 
7compile time error.
8  
9  1)Abstract classes cannot be instantiated
10  2)An abstarct classes contains abstract method, concrete methods or both.
11  3)Any class which extends abstarct class must override all methods of abstract
12    class
13  4)An abstarct class can contain either 0 or more abstract method.    

1An abstract method is the method which does’nt have any body. 
2Abstract method is declared with
3keyword abstract and semicolon in place of method body.
4
5  public abstract void <method name>();
6Ex : public abstract void getDetails();
7It is the responsibility of subclass to provide implementation to 
8abstract method defined in abstract class

1
2interface human {
3    void dothings();
4    void eatthings();
5    void sleep();
6}
7abstract class student implements human {
8    public void dothings()
9    {
10        System.out.println("doing something");
11    }
12    public void eatthings()
13    {
14        System.out.println("eating something");
15    }
16}
17class basisstudent extends student {
18    public void sleep()
19    {
20        System.out.println("no");
21    }
22}
23public class Main {
24    public static void main(String[] args)
25    {
26        basisstudent you = new basisstudent();
27        you.dothings();
28        you.eatthings();
29        you.sleep();
30    }
31}
32

1public abstract class Account {		//abstract class //perent class
2    protected int accountNumber;
3    protected Customer customerObj;
4    protected double balance;
5  	//constructor
6  	public Account(int saccountNumber, Customer scustomerObj,double sbalance){
7        accountNumber = saccountNumber;
8        customerObj = scustomerObj;
9        balance = sbalance;
10    }
11  	// abstract Function
12    public abstract boolean withdraw(double amount); 
13}   
14
15public class SavingsAccount extends Account { // child class
16    private double minimumBalance;
17  	// constructor
18    public SavingsAccount(int saccountNumber, Customer scustomerObj, double sbalance, double sminimumBalance) {
19        super(saccountNumber, scustomerObj, sbalance);
20        minimumBalance = sminimumBalance;
21    }
22	// Implementation of abstract function in child class
23    public boolean withdraw(double amount) {
24        if (balance() > minimumBalance && balance() - amount > minimumBalance) {
25            super.setBalance(balance() - amount);
26            return true;
27        } else {
28            return false;
29        }
30    }
31}
32
33

1Sometimes we may come across a situation
2where we cannot provide implementation to
3all the methods in a class. We want to leave the 
4implementation to a class that extends it.
5  In that case we declare a class
6as abstract by using abstract keyword on method
7signature.In my framework I have created my
8PageBase class as super
9class of the all page classes. 
10I have collected all common elements
11and functions into PageBase class and
12all other page classes extent PageBase class.
13By doing so, I don't have to locate very
14common WebElements and it provides
15reusability in my framework.
16Also
171)Abstract classes cannot be instantiated
182)An abstarct classes contains abstract method,
19concrete methods or both.
203)Any class which extends abstarct class must
21  override all methods of abstract class
224)An abstarct class can contain either
23  0 or more abstract method.   

1public abstract class GraphicObject {
2
3   abstract void draw();
4}