In this tutorial, we will learn about Java abstract classes and abstract methods and we will also learn about the concept of abstraction in Java programming language.
Abstraction in Java
In Object oriented programming paradigm, abstraction is a process of hiding the implementation details from the user, only the needed information will be provided to the user without exposing internal implementation details. The user will have the information on what the object does but user will never know about how it does.A real life example of abstraction is car steering. As a car driver, we know that when we turn the car steeting left, car will turn left side and when we turn car steering right, car will turn right side. However, we don't know the details of how car steering system works internally. By defining the abstract turning behaviour of car steering, car manufacture abstracted the internal complexities of steering system.
With abstraction in place, car manufacturer can implement different steering technology, without changing the expected behaviour of car steering system. As long as car turns right or left when driver turns steering right or left respectively, driver need not worry about internal steering technology of the car.
Abstract class and Abstract method in java are used for hiding certain details and showing only essential information to the user. In Java, keyword abstract is used to declare abstract class and abstract methods. abstract keyword is a non-access modifier applicable to classes and methods but not for Variables.
Abstract class in Java
A class which is declared with abstract keyword is known as abstract class. It can have abstract and non-abstract methods. For example:
abstract class Shape {
abstract void draw();
}
We cannot create objects of abstract classes in Java.
abstract class Shape {
abstract void draw();
}
public class AbstractClass {
public static void main(String[] args) {
// Error: We cannot create an
// object of abstract class
Shape shape = new Shape();
}
}
Output
Error: Shape is abstract; cannot be instantiated
In above program, we are trying to create an object of abstract class Shape. As we discussed earlier that we cannot instantiate an abstract class. Hence, it generated a compile time error.
- An instance of an abstract class can not be created.
- It can have abstract and non-abstract methods.
- It can have constructors and static methods.
- We can have an abstract class without any abstract method.
- If a class contains at least one abstract method then it must be declare as abstract class.
- Like inheritence, we can create subclasses from an abstract cass using extends keyword. A subclass object can access members of the abstract class.
Abstract method in Java
A method which is declared with abstract keyword and doesn't have its body is known as an abstract method. If you want a class to contain a method but you want the actual implementation of that method to be determined by child classes, you can declare the method in the parent class as an abstract.
Example of abstract method
abstract void draw();
Here, draw() is an abstract method. The body of draw() method is missing and is replaced by ;
- The "abstract" keyword is used to declare the an abstract method. abstract keyword is used before the method name in the method declaration.
- An abstract method contains a method signature, but no method body.
- Instead of curly braces, an abstract method will have a semicolon (;) at the end.
- The class containing an abstract method must be declared as abstract.
- Any class inheriting the abstract class must implement abstract method or declare itself as abstract class.
Java program for abstract class and abstract method
// Abstract class
abstract class Shape {
// Abstract method
abstract void draw();
// Non-Abstract method
void printShapeType() {
System.out.println("Geometric Shape");
}
}
class Circle extends Shape {
void draw() {
System.out.println("Drawing a circle");
}
}
public class Paint {
public static void main(String[] args) {
Circle cir = new Circle();
cir.draw();
cir.printShapeType();
}
}
Output
Drawing a circle Geometric Shape
In above program, we create an abstract class named "Shape" containing an abstract method "draw()" and a non-abstract method "printShapeType()". Class Circle is inheriting Shape class by providing implementation of abstract method draw(). Inside main method of Paint class, we can access abstract as well as non-abstract method of Shape class using an object of subclass Circle.
If the subclass Circle doesn't provide implementation of abstract method draw(), then Circle class must be declared as abstract.