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Is it possible to use abstract and final both with a method in Java?

Here, we are going to learn that can you use abstract and final both with a method in Java? Is it possible to use abstract and final both with a method in Java?
Submitted by Preeti Jain, on September 17, 2019

No, it is not possible to use abstract and final both with a method simultaneously.

• We can use abstract only with a method without final and it is valid and the code will execute without error.
• Similarly, we can use final only with a method without abstract and it is valid and the code will execute without error.
• As we know that, we can't use abstract and final both with a method simultaneously because there are two things to discuss:
• First, if we declare abstract with method then the abstract method need to override (i.e. abstract method must be overridden in its implementation class).
• Second, if we declare final with method then the final method is final and we can't override the method (i.e. final method must not be overridden in its implementation class).
• Few points need to remember about abstract and final method:
  1. What will happen, if we use abstract only with a method?
  2. What will happen, if we use final only with a method?
  3. What will happen, if we use abstract and final both with a method?

We will see each of the above cases one by one with the help of an example...

1) Using "abstract" keyword with a method in Java

Example:

// Java program to demonstrate the example of
// using "abstract" keyword with a method 

abstract class AbstractMethodClass {
    // Abstract Method Declaration
    abstract void display();
}

public class Main extends AbstractMethodClass {
    // override display() of AbstractMethodClass
    public void display() {
        System.out.print("abstract specifiers are allowed" + " ");
        System.out.print("for Methods");
    }

    public static void main(String[] args) {
        // class instantiation
        Main m = new Main();
        // calling display() of Main class
        m.display();
    }
}

Output

abstract specifiers are allowed for Methods

Conclusion: We can use only abstract without final for the methods.

2) Using "final" keyword with a method in Java

Example:

// Java program to demonstrate the example of
// using "final" keyword with a method 

class FinalMethodClass {
    // Final Method Definition
    final void display() {
        System.out.print("final specifier is allowed" + " ");
        System.out.print("for Methods");
    }
}

public class Main extends FinalMethodClass {
    // show() method definition
    public void show() {
        System.out.print("final method is not overridable");
    }

    public static void main(String[] args) {
        // class instantiation
        Main m = new Main();
        FinalMethodClass fmc = new FinalMethodClass();

        // calling display() of FinalMethodClass 
        fmc.display();

        System.out.println();
        // calling display() of Main

        m.show();
    }
}

Output

final specifier is allowed for Methods
final method is not overridable

Conclusion: We can use only final without abstract for the methods.

3) Using "abstract" and "final" keyword both with a method in Java

Example:

// Java program to demonstrate the example of
// using "abstract" and "final" keyword both 
// with a method 

class AbstractFinalMethodClass {
    // Abstract Method Declaration
    abstract void display();
    // Final Method Definition
    final void show() {
        System.out.print("final method is not overridable");
    }
}

public class Main extends AbstractFinalMethodClass {
    // override display() of AbstractFinalMethodClass
    public void display() {
        System.out.println("abstract method is overridable");
    }

    public static void main(String[] args) {
        // Main class object instantiation
        Main m = new Main();

        // AbstractFinalMethodClass object instantiation
        AbstractFinalMethodClass afmc = new AbstractFinalMethodClass();

        // calling display() of Main class
        m.display();

        // calling show() of AbstractFinalMethodClass 
        afmc.show();
    }
}

Output

/Main.java:5: error: AbstractFinalMethodClass is not abstract and 
does not override abstract method display() in AbstractFinalMethodClass
class AbstractFinalMethodClass {
^
1 error

Conclusion: We cannot use both abstract and final with the methods because the abstract method overrides in its implementation class but the final method cannot override in its implementation class.