In Java, abstract class is declared with the abstract keyword. It may have both abstract and non-abstract methods(methods with bodies). An abstract is a Java modifier applicable for classes and methods in Java but not for Variables. In this article, we will learn the use of abstract classes in Java.
What is Abstract Class in Java?
Java abstract class is a class that can not be instantiated by itself, it needs to be subclassed by another class to use its properties. An abstract class is declared using the "abstract" keyword in its class definition.
Illustration of Abstract class
abstract class Shape
{
int color;
// An abstract function
abstract void draw();
}
In Java, the following some important observations about abstract classes are as follows:
- An instance of an abstract class can not be created.
- Constructors are allowed.
- We can have an abstract class without any abstract method.
- There can be a final method in abstract class but any abstract method in class(abstract class) can not be declared as final or in simpler terms final method can not be abstract itself as it will yield an error: "Illegal combination of modifiers: abstract and final"
- We can define static methods in an abstract class
- We can use the abstract keyword for declaring top-level classes (Outer class) as well as inner classes as abstract
- If a class contains at least one abstract method then compulsory should declare a class as abstract
- If the Child class is unable to provide implementation to all abstract methods of the Parent class then we should declare that Child class as abstract so that the next level Child class should provide implementation to the remaining abstract method
Examples of Java Abstract Class
1. Example of Abstract Class that has Abstract method
Below is the implementation of the above topic:
Java
// Abstract class
abstract class Sunstar {
abstract void printInfo();
}
// Abstraction performed using extends
class Employee extends Sunstar {
void printInfo()
{
String name = "avinash";
int age = 21;
float salary = 222.2F;
System.out.println(name);
System.out.println(age);
System.out.println(salary);
}
}
// Base class
class Base {
public static void main(String args[])
{
Sunstar s = new Employee();
s.printInfo();
}
}
2. Abstract Class having constructor, data member, and methods
Elements abstract class can have
- data member
- abstract method
- method body (non-abstract method)
- constructor
- main() method.
Below is the implementation of the above topic:
Java
// Java Program to implement Abstract Class
// having constructor, data member, and methods
import java.io.*;
abstract class Subject {
Subject() {
System.out.println("Learning Subject");
}
abstract void syllabus();
void Learn(){
System.out.println("Preparing Right Now!");
}
}
class IT extends Subject {
void syllabus(){
System.out.println("C , Java , C++");
}
}
class GFG {
public static void main(String[] args) {
Subject x=new IT();
x.syllabus();
x.Learn();
}
}
OutputLearning Subject
C , Java , C++
Preparing Right Now!
Properties of Abstract class
Let us elaborate on these observations and do justify them with help of clean java programs as follows.
Observation 1
In Java, just like in C++ an instance of an abstract class cannot be created, we can have references to abstract class type though. It is as shown below via the clean Java program.
Example
Java
// Java Program to Illustrate
// that an instance of Abstract
// Class can not be created
// Class 1
// Abstract class
abstract class Base {
abstract void fun();
}
// Class 2
class Derived extends Base {
void fun()
{
System.out.println("Derived fun() called");
}
}
// Class 3
// Main class
class Main {
// Main driver method
public static void main(String args[])
{
// Uncommenting the following line will cause
// compiler error as the line tries to create an
// instance of abstract class. Base b = new Base();
// We can have references of Base type.
Base b = new Derived();
b.fun();
}
}
OutputDerived fun() called
Observation 2
Like C++, an abstract class can contain constructors in Java. And a constructor of an abstract class is called when an instance of an inherited class is created. It is as shown in the program below as follows:
Example:
Java
// Java Program to Illustrate Abstract Class
// Can contain Constructors
// Class 1
// Abstract class
abstract class Base {
// Constructor of class 1
Base()
{
// Print statement
System.out.println("Base Constructor Called");
}
// Abstract method inside class1
abstract void fun();
}
// Class 2
class Derived extends Base {
// Constructor of class2
Derived()
{
System.out.println("Derived Constructor Called");
}
// Method of class2
void fun()
{
System.out.println("Derived fun() called");
}
}
// Class 3
// Main class
class GFG {
// Main driver method
public static void main(String args[])
{
// Creating object of class 2
// inside main() method
Derived d = new Derived();
d.fun();
}
}
OutputBase Constructor Called
Derived Constructor Called
Derived fun() called
Observation 3
In Java, we can have an abstract class without any abstract method. This allows us to create classes that cannot be instantiated but can only be inherited. It is as shown below as follows with help of a clean java program.
Example:
Java
// Java Program to illustrate Abstract class
// Without any abstract method
// Class 1
// An abstract class without any abstract method
abstract class Base {
// Demo method. This is not an abstract method.
void fun()
{
// Print message if class 1 function is called
System.out.println(
"Function of Base class is called");
}
}
// Class 2
class Derived extends Base {
// This class only inherits the Base class methods and
// properties
}
// Class 3
class Main {
// Main driver method
public static void main(String args[])
{
// Creating object of class 2
Derived d = new Derived();
// Calling function defined in class 1 inside main()
// with object of class 2 inside main() method
d.fun();
}
}
OutputFunction of Base class is called
Observation 4
Abstract classes can also have final methods (methods that cannot be overridden)
Example:
Java
// Java Program to Illustrate Abstract classes
// Can also have Final Methods
// Class 1
// Abstract class
abstract class Base {
final void fun()
{
System.out.println("Base fun() called");
}
}
// Class 2
class Derived extends Base {
}
// Class 3
// Main class
class GFG {
// Main driver method
public static void main(String args[])
{
{
// Creating object of abstract class
Base b = new Derived();
// Calling method on object created above
// inside main method
b.fun();
}
}
}
Observation 5
For any abstract java class we are not allowed to create an object i.e., for an abstract class instantiation is not possible.
Java
// Java Program to Illustrate Abstract Class
// Main class
// An abstract class
abstract class GFG {
// Main driver method
public static void main(String args[])
{
// Trying to create an object
GFG gfg = new GFG();
}
}
Output:
Observation 6
Similar to the interface we can define static methods in an abstract class that can be called independently without an object.
Java
// Java Program to Illustrate
// Static Methods in Abstract
// Class Can be called Independently
// Class 1
// Abstract class
abstract class Helper {
// Abstract method
static void demofun()
{
// Print statement
System.out.println("Geeks for Geeks");
}
}
// Class 2
// Main class extending Helper class
public class GFG extends Helper {
// Main driver method
public static void main(String[] args)
{
// Calling method inside main()
// as defined in above class
Helper.demofun();
}
}
Observation 7
We can use the abstract keyword for declaring top-level classes (Outer class) as well as inner classes as abstract
Java
import java.io.*;
abstract class B {
// declaring inner class as abstract with abstract
// method
abstract class C {
abstract void myAbstractMethod();
}
}
class D extends B {
class E extends C {
// implementing the abstract method
void myAbstractMethod()
{
System.out.println(
"Inside abstract method implementation");
}
}
}
public class Main {
public static void main(String args[])
{
// Instantiating the outer class
D outer = new D();
// Instantiating the inner class
D.E inner = outer.new E();
inner.myAbstractMethod();
}
}
OutputInside abstract method implementation
Observation 8
If a class contains at least one abstract method then compulsory that we should declare the class as abstract otherwise we will get a compile-time error ,If a class contains at least one abstract method then, implementation is not complete for that class, and hence it is not recommended to create an object so in order to restrict object creation for such partial classes we use abstract keyword.
Java
/*package whatever //do not write package name here */
import java.io.*;
// here if we remove the abstract
// keyword then we will get compile
// time error due to abstract method
abstract class Demo {
abstract void m1();
}
class Child extends Demo {
public void m1()
{
System.out.print("Hello");
}
}
class GFG {
public static void main(String[] args)
{
Child c = new Child();
c.m1();
}
}
Observation 9
If the Child class is unable to provide implementation to all abstract methods of the Parent class then we should declare that Child class as abstract so that the next level Child class should provide implementation to the remaining abstract method.
Java
// Java Program to demonstrate
// Observation
import java.io.*;
abstract class Demo {
abstract void m1();
abstract void m2();
abstract void m3();
}
abstract class FirstChild extends Demo {
public void m1() {
System.out.println("Inside m1");
}
}
class SecondChild extends FirstChild {
public void m2() {
System.out.println("Inside m2");
}
public void m3() {
System.out.println("Inside m3");
}
}
class GFG {
public static void main(String[] args)
{
// if we remove the abstract keyword from FirstChild
// Class and uncommented below obj creation for
// FirstChild then it will throw
// compile time error as did't override all the
// abstract methods
// FirstChild f=new FirstChild();
// f.m1();
SecondChild s = new SecondChild();
s.m1();
s.m2();
s.m3();
}
}
OutputInside m1
Inside m2
Inside m3
In C++, if a class has at least one pure virtual function, then the class becomes abstract. Unlike C++, in Java, a separate keyword abstract is used to make a class abstract.
Conclusion
Points to remember from this article are mentioned below:
- An abstract class is a class that can not be initiated by itself, it needs to be subclassed by another class to use its properties.
- An abstract class can be created using "abstract" keywords.
- We can have an abstract class without any abstract method.
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