Abstract class in Java

What is abstract classs?

• An abstract class in Java is a class that cannot be instantiated(you cannot create an object of it directly). It is used as a blueprint for other classes.) and is meant to be extended by subclasses.
• It is declared using the abstract keyword.
• It can have both abstract methods (no body) and concrete methods (with body), Can have Constructor, Can have Variables & Fields, Can have Static Methods, final methods

Key Rules to Remember :

• abstract class cannot be instantiated.
• abstract class can have 0 or more abstract methods.
• If a class has even one abstract method it must be declared abstract.
• Subclass must override all abstract methods or be declared abstract itself.
• abstract + final = not allowed(because it can't be extended)
• abstract + private method = not allowed(because it can't be extended).
• abstract class can extend another abstract class.
• abstract class can implement an interface without implementing its methods.
• Constructors are allowed (called during subclass object creation).

Example :

public abstract class ATM {

    ATM()
    {
        System.out.println("ATM constructor called");
    }
    public abstract void withdraw();
    public abstract void withdraw(int a,int b);
    public void deposit(){
        System.out.println("ATM deposit method called");
    }

}


public class SBI extends ATM{

    SBI()
    {
        System.out.println("SBI constructor called");
    }

    public void withdraw(){
        System.out.println("SBI withdraw method called");
    }
    public void withdraw(int a,int b){
        System.out.println("SBI withdraw method called with parameters: " + a + " and " + b);
    }

    public static void main(String[] args) {

        SBI sbi = new SBI();
        sbi.withdraw();
        sbi.withdraw(1,2);
        sbi.deposit();

    }
}

output :
ATM constructor called
SBI constructor called
SBI withdraw method called
SBI withdraw method called with parameters: 1 and 2
ATM deposit method called