final Keyword in Java

The final keyword in Java is a non-access modifier used to prevent modification. It can be applied to variables (value cannot change), methods (cannot be overridden) and classes (cannot be extended).

The following are different contexts where the final is used:

1. Variable
2. Method
3. Class

Characteristics of final keyword in Java

In Java, the final keyword is used to indicate that a variable, method or class cannot be modified or extended. Here are some of its characteristics:

• Final variables: Declaring a variable as final prevents its value from being changed after initialization, useful for constants.
• Final methods: When a method is declared as final, it cannot be overridden by a subclass. This is useful for methods that are part of a class's public API and should not be modified by subclasses.
• Final classes: When a class is declared as final, it cannot be extended by a subclass. This is useful for classes that are intended to be used as is and should not be modified or extended.
• Initialization: Final variables must be initialized either at the time of declaration or in the constructor of the class. This ensures that the value of the variable is set and cannot be changed.
• Performance: The use of a final can sometimes improve performance, as the compiler can optimize the code more effectively when it knows that a variable or method cannot be changed.
• Security: The final can help improve security by preventing malicious code from modifying sensitive data or behavior.

Java Final Variable

When a variable is declared with the final keyword, its value can't be changed, essentially, a constant. This also means that you must initialize a final variable..

Example:

public class Geeks {
    public static void main(String[] args) {
        
        // Define a constant variable PI
        final double PI = 3.14159;

        // Print the value of PI
        System.out.println("Value of PI: " + PI);
    }
}

Value of PI: 3.14159

Different Methods of Using Final Variable

Let's see different methods that we can use final variable in Java.

1. final Variable

Example:

// Final variable
final int THRESHOLD = 5;

2. Blank final Variable

Example:

// Blank final variable
final int THRESHOLD;

3. Static final Variable

Example:

// Final static variable PI
static final double PI = 3.141592653589793;

4. Static Blank final Variable

Example:

// Blank final static variable
static final double PI;

Initializing a final Variable 

A final variable in Java must be initialized exactly once. If not initialized at the time of declaration, it is known as a blank final variable. There are three ways to initialize a final variable:

• At declaration: the most common approach.
• In a constructor or instance initializer block: required for blank final instance variables. If multiple constructors exist, all must initialize the variable.
• In a static block: used for blank final static variables.

Let us see these two different ways of initializing a final variable

Example:

class Geeks {
  
    // a final variable direct initialize
    final int THRESHOLD = 5;
    
    // a blank final variable
    final int CAPACITY;
    
    // another blank final variable
    final int  MINIMUM;
    
    // a final static variable PI direct initialize
    static final double PI = 3.141592653589793;
    
    // a  blank final static  variable
    static final double EULERCONSTANT;
    
    // instance initializer block for initializing CAPACITY
    {
        CAPACITY = 25;
    }
    
    // static initializer block for initializing EULERCONSTANT
    static{
        EULERCONSTANT = 2.3;
    }
    
    // constructor for initializing MINIMUM
    public Geeks() 
    {
        MINIMUM = -1;
    }
        
}

 
There was no main method in the above code as it was simply for illustration purposes to get a better understanding to draw conclusions:

Observation 1: When to use a final variable?

The only difference between a normal variable and a final variable is that we can re-assign the value to a normal variable but we cannot change the value of a final variable once assigned. Hence final variables must be used only for the values that we want to remain constant throughout the execution of the program.

Observation 2: Reference final variable. 

When a final variable is a reference to an object, then this final variable is called the reference final variable. For example, a final StringBuffer variable looks defined below as follows:

final StringBuffer sb;

As we all know a final variable cannot be re-assign. But in the case of a reference final variable, the internal state of the object pointed by that reference variable can be changed. Note that this is not re-assigning. 

This property of the final is called non-transitivity. To understand what is meant by the internal state of the object as shown in the below example as follows:

Example 1:

// Main class
class Geeks {

    // Main driver method
    public static void main(String[] args)
    {
        // Creating an object of StringBuilder class Final reference variable
        final StringBuilder sb = new StringBuilder("Geeks");

        // Printing the element in StringBuilder object
        System.out.println(sb);

        // changing internal state of object reference by final reference variable sb
        sb.append("ForGeeks");

        // Again printing the element in StringBuilder object after appending above element in it
        System.out.println(sb);
    }
}

Geeks
GeeksForGeeks

The non-transitivity property also applies to arrays, because arrays are objects in Java. Arrays with the final keyword are also called final arrays.

Note: As discussed above, a final variable cannot be reassign, doing it will throw compile-time error.

Example 2:

class Geeks {

    // Declaring and customly initializing static final variable
    static final int CAPACITY = 4;

    // Main driver method
    public static void main(String args[])
    {

        // Re-assigning final variable will throw compile-time error
        CAPACITY = 5;
    }
}

Output: 

Remember: When a final variable is created inside a method/constructor/block, it is called local final variable and it must initialize once where it is created. See below program for local final variable.

Example 3:

class Geeks {

    // Main driver method
    public static void main(String args[])
    {
        // Declaring local final variable
        final int i;

        // Now initializing it with integer value
        i = 20;

        // Printing the value on console
        System.out.println(i);
    }
}

20

Remember these key points as perceived before moving forward as listed below as follows:

Note: the difference between C++ const variables and Java final variables. const variables in C++ must be assigned a value when declared. For final variables in Java, it is not necessary as we see in the above examples. A final variable can be assigned value later, but only once.

final with foreach loop: final with for-each statement is a legal statement.

Example:

class Geeks {

    // Main driver method
    public static void main(String[] args)
    {

        // Declaring and initializing custom integer array
        int arr[] = { 1, 2, 3 };

        // final with for-each statement legal statement
        for (final int i : arr)
            System.out.print(i + " ");
    }
}

1 2 3 

Output explanation: Since the "i" variable goes out of scope with each iteration of the loop, it is re-declared each iteration, allowing the same token (i) to be used to represent multiple variables. 

Example:

public class Geeks {
    public static void main(String[] args) {
        final int VALUE = 10; // declaring a final variable
        
        System.out.println("The value is: " + VALUE);
        
        final String MESSAGE = "Hello, world!"; // declaring a final variable
        System.out.println(MESSAGE);
        
        MyClass myObj = new MyClass();
        myObj.printMessage();
        myObj.printFinalMessage();
        
    }
}

class MyClass {
    final String message = "Hello!"; // declaring a final instance variable
    
    void printMessage() {
        System.out.println(message);
    }
    
    void printFinalMessage() {
        final String finalMessage = "Hello, final!";
        System.out.println(finalMessage);
    }
}

final class MyOtherClass { // declaring a final class
    // ...
}

The value is: 10
Hello, world!
Hello!
Hello, final!

Java Final classes

When a class is declared with the final keyword in Java, it is called a final class. A final class cannot be extended(inherited). 

There are two uses of a final class: 

Usage 1: 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.

final class A
{
     // methods and fields
}
// The following class is illegal
class B extends A 
{ 
    // COMPILE-ERROR! Can't subclass A
}

Usage 2: The other use of final with classes is to create an immutable class like the predefined String class. One can not make a class immutable without making it final.

Java Final Method

When a method is declared with final keyword, it is called a final method in Java. A final method cannot be overridden. 

The Object class does this a number of its methods are final. We must declare methods with the final keyword for which we are required to follow the same implementation throughout all the derived classes. 

Illustration: Final keyword with a method 

class A 
{
    final void m1() 
    {
        System.out.println("This is a final method.");
    }
}
class B extends A 
{
    void m1()
    { 
        // Compile-error! We can not override
        System.out.println("Illegal!");
    }
}

Advantages of final Keyword in Java

The final keyword in Java provides several advantages, including:

• Ensuring immutability: When a variable or reference is marked as final, its value cannot be changed once it is assigned. This helps ensure that data is immutable and cannot be accidentally or maliciously modified.
• Improving performance: The use of the final can sometimes help improve performance, as the Java Virtual Machine (JVM) can optimize code more effectively when it knows that certain values or references cannot be changed.
• Making code easier to understand: By declaring variables, methods or classes as final, developers can make their code easier to understand and reason about. When a value or reference is marked as final, it is clear that it will not change, which can simplify code analysis and debugging.
• Promoting code reuse: By declaring methods as final, developers can prevent subclasses from overriding them. This can help promote code reuse and reduce duplication, as subclasses must use the parent class's implementation of the method.
• Enhancing security: The use of final can help enhance security by preventing malicious code from modifying sensitive data or behavior.

Related Interview Question(Important): Difference between final, finally and finalize in Java

Final Keyword

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