The final keyword is a non-access modifier used for classes, attributes and methods, which makes them non-changeable (impossible to inherit or override).
The final keyword is useful when you want a variable to always store the same value, like PI (3.14159...).
The final keyword is called a "modifier". You will learn more about these in the Java Modifiers Chapter.
Final Variables
When a variable is declared with final keyword, its value can’t be modified, essentially, a constant. This also means that you must initialize a final variable. If the final variable is a reference, this means that the variable cannot be re-bound to reference another object, but the internal state of the object pointed by that reference variable can be changed i.e. you can add or remove elements from the final array or final collection. It is good practice to represent final variables in all uppercase, using underscore to separate words.
Illustration:
// a final variable
final int THRESHOLD = 5;
// a blank final variable
final int THRESHOLD;
// a final static variable PI
static final double PI = 3.141592653589793;
// a blank final static variable
static final double PI;
Initializing a final Variable
We must initialize a final variable, otherwise, the compiler will throw a 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. A final variable is called a blank final variable if it is not initialized while declaration. Below are the two ways to initialize a blank final variable.
A blank final variable can be initialized inside an instance-initializer block or inside the constructor. If you have more than one constructor in your class then it must be initialized in all of them, otherwise, a compile-time error will be thrown.
A blank final static variable can be initialized inside a static block.
