final keyword in java

final is a keyword in java which can be used with instance variables, local variables , methods and classes.

Use of final keyword in java:

1. final variable in java:

A variable declared with final keyword is known as final variable. It may be member variable or local variable. final variables are constants in java and they are generally declared with static keyword. As final variables are treated as constants they can’t reassign. They are initialised at the time of declaration.

Example:

FinalExample1.java

/**
 * This program is used to show that the value of 
 * final variable can't be change. 
 * @author W3spoint
 */
class Test{
	//final variable
	final int num = 100;
 
	//method for try to change the value of final variable.
	public void show(){
		//error because value of final variable can't be change.
		num = 200;
		System.out.println("Num = " + num);
	}
}
public class FinalExample1 {
	public static void main(String args[]){
		//creating object of Test Class
		Test obj = new Test();
		//method call
		obj.show();
	}
}

Output:

Exception in thread "main" java.lang.Error: 
Unresolved compilation problem:
The final field Test.num cannot be assigned
at com.w3spoint.business.Test.show(FinalExample1.java:15)
at com.w3spoint.business.FinalExample1.main
(FinalExample1.java:24)

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Note: Inside Anonymous classes only final variables are accessible.

2. final method in java:

A method declared with final keyword is known as final method.

Example:

FinalExample2.java

/**
 * This program is used to show that final method can't be override.
 * @author W3spoint
 */
class Show{
	public final void show(){
		System.out.println("Hello world.");
	}
}
 
class Display extends Show{
	//error because final method can't be override.
	public void show(){
		System.out.println("Hello w3spoint.com.");
	}
}
 
public class FinalExample2 {
	public static void main(String args[]){
		//creating object of Display class
		Display obj = new Display();
		//method call
		obj.show();
	}
}

Output:

Exception in thread "main" java.lang.VerifyError: 
class com.w3spoint.business.Display overrides final method show.()V
at java.lang.ClassLoader.defineClass1(Native Method)
at java.lang.ClassLoader.defineClass(Unknown Source)
at java.security.SecureClassLoader.defineClass(Unknown Source)
at java.net.URLClassLoader.defineClass(Unknown Source)
at java.net.URLClassLoader.access$100(Unknown Source)
at java.net.URLClassLoader$1.run(Unknown Source)
at java.net.URLClassLoader$1.run(Unknown Source)
at java.security.AccessController.doPrivileged(Native Method)
at java.net.URLClassLoader.findClass(Unknown Source)
at java.lang.ClassLoader.loadClass(Unknown Source)
at sun.misc.Launcher$AppClassLoader.loadClass(Unknown Source)
at java.lang.ClassLoader.loadClass(Unknown Source)
at com.w3spoint.business.FinalExample2.main
(FinalExample2.java:23)

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Note: A final method can be inherited but can’t be override.

3. final class in java:

A class declared with final keyword is known as final class. A final class can’t be inherited.

Example:

FinalExample3.java

/**
 * This program is used to show that final class can't be inherited.
 * @author W3spoint
 */
final class Show{
	public void show(){
		System.out.println("Hello world.");
	}
}
//error because final class can't be inherited.
class Display extends Show{
	public void display(){
		System.out.println("Hello w3spoint.com.");
	}
}
 
public class FinalExample3 {
	public static void main(String args[]){
		//creating object of Display class
		Display obj = new Display();
		//method call
		obj.display();
	}
}

Output:

Exception in thread "main" java.lang.Error: 
Unresolved compilation problem:
The type Display cannot subclass the final class Show
at com.w3spoint.business.Display.(FinalExample3.java:13)
at com.w3spoint.business.FinalExample3.main
(FinalExample3.java:22)

Download this example.
Note: final method can be inherited.

Example:

FinalExample4.java

/**
 * This program is used to show that final method can be inherited.
 * @author W3spoint
 */
class Show{
	public final void show(){
		System.out.println("Hello world.");
	}
}
 
class Display extends Show{
	public void display(){
		System.out.println("Hello w3spoint.com.");
	}
}
 
public class FinalExample4 {
	public static void main(String args[]){
		//creating object of Display class
		Display obj = new Display();
		//method call
		obj.show();
	}
}

Output:

Hello world.

Download this example.
Note: Abstract class and constructor can’t be final.

4. Blank final variable in java:

A variable declared with final keyword but not initialised at declaration time is known as blank final variable. They are initialised at the time of object creation in constructor and can’t change after that.

Example:

FinalExample5.java

/**
 * This class is used to show the example of blank final variable.
 * @author w3spoint
 */
class Test{
	//blank final variable which can only be 
        //initialize through constructor.
	final int num;
 
	Test(int n){
		num = n;
		System.out.println("Num = " + num);
	}
}
public class FinalExample5 {
	public static void main(String args[]){
		new Test(100);
	}
}

Output:

Num = 100

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5. static blank final variable in java:

A static variable declared with final keyword but not initialised at declaration time is known as static blank final variable. It can be initialised in a static block only.

Example:

FinalExample6.java

/**
 * This class is used to show the example of static blank final variable.
 * @author W3spoint
 */
class Test{
	//blank final variable which can only be initialize 
	//through static initializer block.
	static final int num;
 
	static{
		num = 100;
		System.out.println("Num = " + num);
	}
}
public class FinalExample6 {
	public static void main(String args[]){
		new Test();
	}
}

Output:

Num = 100

Download this example.

6. final parameter in java:

A method parameter declared with final keyword is known as final parameter. Its value can’t be changed.

Example:

FinalExample7.java

/**
 * This program is used to show that value of 
 * final parameter can't be changed.
 * @author W3spoint
 */
class Test{
	public void showDouble(final int num){
		//error because value of final parameter can't be changed.
		num = num * 2;
		System.out.println("Num * 2 = " + num);
	}
}
public class FinalExample7 {
	public static void main(String args[]){
		//creating object of Test class
		Test obj = new Test();
		//method call
		obj.showDouble(10);
	}
}

Output:

Exception in thread "main" java.lang.Error: 
Unresolved compilation problem:
The final local variable num cannot be assigned. 
It must be blank and not using a compound assignment
at com.w3spoint.business.Test.showDouble
(FinalExample7.java:11)
at com.w3spoint.business.FinalExample7.main
(FinalExample7.java:20)

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Advantages/Benefits of final keyword:

Performance: JVM kept in cache if variables, methods and classes are declared final.
Immutable classes: With the help of final keyword we can made immutable classes.

Interview Questions on final keyword

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