final keyword in java

Final keyword can be applied on

• variable
  • Blank final variable
  • Blank final static variable
• method
• class
• objects

Final keyword cannot be applied on :

• Constructor
• abstract class and interface

Final variable :

• A final variable once assigned a value cannot be changed.
• A final variable that is not initialized at the time of declaration is known as blank final variable.
• A static final variable that is not initialized at the time of declaration is known as static blank final variable. It can be initialized only in static block.

When a compilation error is thrown in final instance variable ?

• When an final instance method is not initialized, this instance variable can be initialized by
  • Directly assigning a value.
  • Initializing it via constructor (If once done should be done is all constructors else compilation error)
  • Initializing it in the instance initializer block.
• Note : If final instance variable is assigned value in any one of the above three ways then it cannot be reassigned Example : If the final instance variable is assigned value in instance initializer block then it cannot be reassigned in the constructor block

When a compilation error is thrown in final static variable ?

• When an final instance method is not initialized, this instance variable can be initialized by
  • Directly assigning a value.
  • Initializing inside a static block
• Note : If final static variable is assigned value in any one of the above two ways then it cannot be reassigned Example : If the final static variable is assigned value directly then in static block then it cannot be reassigned.

Final method :

• Once you make a method final you cannot override it. (Note but it can be overloaded)
• Once you declare a parameter of a method as final then you cannot change the value of that parameter in that method

Final class :

Once you make a class final you cannot extend it

Final Object (reference):

• You can modify the properties (instance variable data) of an object but you cannot reassign a new object to that reference.

class Employee {
	public int value = 5;
}

class Demo {
	public static void FinalReference(String args[]) {

		final Employee example = new Employee(); // declaration
		example.value = 6; // Modifying the objects properties creates no disturbance

		Employee another = new Employee();
		example = another; // Attempting to change the objects reference creates a compilation error
	}
}

To do :

• final keyword can be applied on which all keyword
• how to make a immutable class
• final interview questions
• can you overload a final method
• blank or uninitialized final variable and usefulness in memory
• static blank final variable
• can we declare a final constructor
• can a inner class be final