What is the final Keyword in Java?

The final keyword is used to create constants (variables), prevent method overriding, and prevent inheritance (classes).

1. final Variables (Constants)

Once initialized, value cannot be changed.

class Constants {
    final double PI = 3.14159;
    final int MAX_VALUE;
    
    // Blank final variable - must initialize in constructor
    Constants(int max) {
        MAX_VALUE = max;
    }
    
    void demonstrate() {
        // PI = 3.14; // ERROR: Cannot reassign final variable
        System.out.println("PI: " + PI);
        System.out.println("MAX: " + MAX_VALUE);
    }
}

2. final Methods (Prevent Overriding)

Cannot be overridden by subclasses.

class Parent {
    final void display() {
        System.out.println("Parent display - cannot override");
    }
    
    void normalMethod() {
        System.out.println("Normal method");
    }
}

class Child extends Parent {
    // void display() { } // ERROR: Cannot override final method
    
    @Override
    void normalMethod() { // OK - can override non-final
        System.out.println("Child normal method");
    }
}

3. final Classes (Prevent Inheritance)

Cannot be extended/subclassed.

final class MathConstants {
    public static final double PI = 3.14159;
    public static final double E = 2.71828;
    
    public static double circleArea(double radius) {
        return PI * radius * radius;
    }
}

// class ExtendedMath extends MathConstants { } // ERROR: Cannot extend final class

final Parameters

class Calculator {
    int add(final int a, final int b) {
        // a = 10; // ERROR: Cannot modify final parameter
        return a + b;
    }
}

final vs Immutability

Important: final means reference cannot change, but the object itself can be mutable.

final StringBuilder sb = new StringBuilder("Hello");
sb.append(" World"); // OK - modifying object content
// sb = new StringBuilder("New"); // ERROR: Cannot reassign final reference

Quick Reference

Master Java keywords with Online Learner!