The final Keyword

Part of the Object Oriented Programming section of Coddy's Java journey — lesson 13 of 87.

The final keyword in Java prevents modification. It can be applied to variables, methods, and classes, each with a different effect.

When applied to a variable, final makes it a constant - once assigned, its value cannot change:

public class Circle {
    private final double PI = 3.14159;
    private final double radius;
    
    public Circle(double radius) {
        this.radius = radius;  // Can assign once in constructor
    }
    
    public void setRadius(double r) {
        // this.radius = r;  // Error! Cannot reassign final variable
    }
}

A final variable must be initialized either at declaration or in the constructor. After that, any attempt to reassign it causes a compilation error.

For reference types, final means the reference cannot point to a different object, but the object's contents can still change:

private final StringBuilder name = new StringBuilder("John");

// name = new StringBuilder("Jane");  // Error! Can't reassign
name.append(" Doe");  // OK! Object contents can change

Using final with fields communicates intent clearly - it tells other developers that this value should never change after initialization. This is especially useful for configuration values, IDs, or any data that must remain constant throughout an object's lifetime.

Challenge

Easy

Let's build a Student registration system that uses the final keyword to protect data that should never change after a student is registered.

You'll create two files to organize your code:

Student.java: Create a Student class where certain information is permanently locked once set:
- A final field studentId (String) - assigned at creation and can never change
- A final field enrollmentYear (int) - the year the student enrolled, also permanent
- A regular private field name (String) - can be updated if the student changes their name
- A constructor that takes all three values and initializes the fields
- A getter getStudentId() for the student ID
- A getter getEnrollmentYear() for the enrollment year
- A getter getName() and setter setName() for the name
- A method getInfo() that returns: "ID: [studentId] | Name: [name] | Enrolled: [enrollmentYear]"
Main.java: Create a Student object, then update their name and display their information. You'll receive four inputs: the student ID, initial name, enrollment year, and a new name to update to. Print two lines:
- The student info before the name change
- The student info after the name change

You will receive four inputs in order: studentId (String), name (String), enrollmentYear (int), and newName (String).

Notice how the final fields protect the student's ID and enrollment year - these are permanent records that shouldn't change, while the name can be updated through its setter.

Cheat sheet

The final keyword prevents modification in Java. It can be applied to variables, methods, and classes.

When applied to a variable, final makes it a constant - once assigned, its value cannot change:

public class Circle {
    private final double PI = 3.14159;
    private final double radius;
    
    public Circle(double radius) {
        this.radius = radius;  // Can assign once in constructor
    }
    
    public void setRadius(double r) {
        // this.radius = r;  // Error! Cannot reassign final variable
    }
}

A final variable must be initialized either at declaration or in the constructor. After that, any attempt to reassign it causes a compilation error.

For reference types, final means the reference cannot point to a different object, but the object's contents can still change:

private final StringBuilder name = new StringBuilder("John");

// name = new StringBuilder("Jane");  // Error! Can't reassign
name.append(" Doe");  // OK! Object contents can change

Using final with fields communicates intent - it indicates that a value should never change after initialization, useful for configuration values, IDs, or constant data.

Try it yourself

import java.util.Scanner;

class Main {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        
        // Read inputs
        String studentId = scanner.nextLine();
        String name = scanner.nextLine();
        int enrollmentYear = scanner.nextInt();
        scanner.nextLine(); // consume newline
        String newName = scanner.nextLine();
        
        // TODO: Create a Student object with the initial values
        
        // TODO: Print the student info before the name change
        
        // TODO: Update the student's name using the setter
        
        // TODO: Print the student info after the name change
    }
}