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.

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