Final Variable, Method, Class

Final Variable, Method, Class

• The final keyword in java is used to restrict the user. 
• The java final keyword can be used in many contexts. 
• Final can be: variable, method, class

1. Final Variable

• When a variable is declared with the final keyword, its value can’t be modified, essentially, a constant. 
• This also means that you must initialize a final variable. 
• 
  
• Initializing a final variable:
• We must initialize a final variable, otherwise compiler will throw compile-time error.
• A final variable can only be initialized once, either via an initializer or an assignment statement. 
• There are three ways to initialize a final variable :
• You can initialize a final variable when it is declared.This approach is the most common. 
• Syntax:

final datatype variable_name = value;

• Example:

final int age=10;

• A final variable is called blank final variable,if it is not initialized while declaration. 
• A blank final variable can be initialized inside instance-initializer block or inside constructor. 

• Syntax:

final datatype variable_name;

• Example:

final int age;
{
age=10;
}

• A blank final static variable can be initialized inside static block.
• Syntax:

final static datatype variable_name;

• Example:

final static int age;
static{
age=10;
}

• Full Example:

class Bike{ 
 final int speedlimit=90;
 void run(){ 
  speedlimit=400; 
 } 
 public static void main(String args[]){ 
 Bike obj=new  Bike(); 
 obj.run();  //Output:Compile Time Error
 } 
}

2. Final Method

• When a method is declared with final keyword, it is called a final method. 
• A final method cannot be overridden. 
• The Object class does this—a number of its methods are final.
• We must declare methods with final keyword for which we required to follow the same implementation throughout all the derived classes. 
• Example:

class A
{
    final void method()
    {
        System.out.println("This is a final method.");
    }
}

class B extends A
{
    void method()
    {
        // COMPILE-ERROR! Can't override.
        System.out.println("Illegal!");
    }
}

• Full Example:

class Bike{ 
  final void run()
{
System.out.println("running");
} 
} 
   
class Honda extends Bike{ 
   void run()
{
System.out.println("running safely with 100kmph");
} 
   public static void main(String args[])
{ 
   Honda honda= new Honda(); 
   honda.run();  //Output:Compile Time Error
    } 
} 

• Full Example:

class Bike{ 
  final void run()
{
System.out.println("running...");
} 
} 
class Honda2 extends Bike
{ 
   public static void main(String args[])
{ 
new Honda2().run(); 
} 
} 

3. Final Class

• When a class is declared with final keyword, it is called a final class. 
• A final class cannot be extended(inherited). 
• There are two uses of a final class :
• One is definitely to prevent inheritance, as final classes cannot be extended. 
• For example, all Wrapper Classes like Integer,Float etc. are final classes. 
• We can not extend them.
• Example:

final class A
{
     // methods and fields
}

class B extends A
{
    // COMPILE-ERROR! Can't subclass A
}

• The other use of final with classes is to create an immutable class like the predefined String class.
• You can not make a class immutable without making it final.
• Full Example:

final class Bike{} 
 
class Honda1 extends Bike
{ 
  void run()
{
System.out.println("running safely with 100kmph");
} 
  public static void main(String args[])
{ 
  Honda1 honda= new Honda1(); 
  honda.run(); 
} 
}