Introduction
When a class in java is declared with a keyword i.e. ‘abstract’, the class becomes an abstract class. It can contain non-abstract methods (methods with no body) and abstract. Before diving into the abstract classes let us have a look at the abstraction part of the java.
Abstraction
In Java, the process of only showing the functionality to the user and hiding the implementation part is called abstraction.
Another way to understand abstraction is taking the example of sending an email, you compose an email, attaching files etc then you send it. You are not aware of the processes occurring internally.
Mainly, abstraction lets the user only focus on the functionality of the object not how the object works.
What is Abstract class?
In java, the class which is declared with an abstract keyword is called the abstract keyword. It can contain non-abstract as well as abstract methods. Its methods need to be extended and then needs to be implemented. It can’t be instantiated.
Some important points about abstract class
An abstract class needs to be declared with an abstract keyword to be declared as an abstract class.
- It can contain non-abstract and abstract methods.
- It can’t be instantiated.
- It can also contain constructors and static methods.
- It can also contain final methods.
NOTE: Final Methods force the subclass so that it does not change the body of the method.
Syntax
abstract class infinite{} |
Abstract Method in Java
When a method is declared with an abstract keyword and also does not contain implementation is an abstract method.
Syntax
abstract void infinite();//no method body and abstract |
Examples of abstract class in Java
The abstract class having abstract methods
Code
abstract class Car{ //abstract class car
abstract void On(); //abstract method on
}
class BMW extends Car{ //class BMW extending Car
void On(){
System.out.println("Welcome to BMW Experience."); //implementing the abstract method On
}
public static void main(String args[]){
Car obj = new BMW(); //Car object
obj.On();
}
} |
Output
Welcome to BMW Experience. |
Abstract class having constructor, data member and methods
Code
abstract class vehicle{ //creating abstract class named vehicle
vehicle(){ //constructor of class vehicle
System.out.println("Drive Safe.");
}
abstract void on(); //abstract method named on
void Warn(){ // another non-abstract method
System.out.println("Do not drink and drive.");
}
}
class BMW extends vehicle{ //child class wxtending parent class
void on(){ //implementation of abstract method
System.out.println("Do not speed up.");
}
}
class Test{ //another class to run the abstract and non abstract methods
public static void main(String args[]){
vehicle obj = new BMW();
obj.on();
obj.Warn();
}
} |
Output
Drive Safe.
Do not speed up.
Do not drink and drive. |
Another example
Code
abstract class Hospital{ // abstract class named Hospital
abstract String getName(); // abstract method named getName
}
class H1 extends Hospital{ // child class extending abstract class
String getName(){ // implementing abstract method
return "H1";
}
}
class H2 extends Hospital{ // another child class extending parent class
String getName(){ // implementing abstract method
return "H2";
}
}
class Test{ // test class to run abstract method
public static void main(String args[]){
Hospital a; // object a
a = new H1();
System.out.println("The name of Hospital is: " + a.getName());
a = new H2();
System.out.println("The name of Hospital is: "+ a.getName());
}
} |
Output
The name of Hospital is: H1
The name of Hospital is: H2 |
