Abstract Classes in Java

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. An abstract is a java modifier applicable for classes and methods in java but not for Variables. 

Illustration: Abstract class

abstract class Shape 
{
    int color;
    // An abstract function
    abstract void draw();
}

In java, following are some important observations about abstract classes are as follows:

1. An instance of an abstract class can not be created.
2. Constructors are allowed.
3. We can have an abstract class without any abstract method.
4. 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 simper terms final method can not be abstract itself as it will yield an error: “Illegal combination of modifiers: abstract and final”
5. We are not allowed to create objects for an abstract class.
6. We can define static methods in an abstract class
7. We can use the abstract keyword for declaring top-level classes (Outer class) as well as inner classes as abstract
8. If a class contains at least one abstract method then compulsory should declare a class as abstract 
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

Let us elaborate on these observations and do justify them with help of clean java programs as follows.

Observation 1: In Java, just likely 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();
    }
}

Output

Derived 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();
    }
}

Output

Base 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();
    }
}

Output

Function 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()
        b.fun();
    }
}

Output

Base fun() called

Observation 5: For any abstract java class we are not allowed to create an object i.e., for abstract class instantiation is not possible. 

Example

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. 

Example 

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();
    }
}

Output

Geeks for Geeks

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();
    }
}

Output:

Inside abstract method implementation

Observation 8: If a class contains at least one abstract method then compulsory we should declare the class as abstract otherwise we will get a compile-time error because 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();
    }
}

Output:

Hello

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

/*package whatever //do not write package name here */

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();
    }
}

Output:

Inside m1
Inside m2
Inside m3

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