final Keyword

The final keyword in Java is used to restrict modification. It is applied to variables, methods, and classes to enforce immutability, prevent overriding, and stop inheritance.

This is a high-frequency interview topic and very important for writing safe, predictable Java code.

What Does final Mean?

• final means cannot be changed
• Enforces restrictions at compile time
• Improves security, performance, and design clarity

Uses of final Keyword in Java

The final keyword can be used with:

1. Variables
2. Methods
3. Classes

1. final Variable

What Is a Final Variable?

• Value cannot be changed once assigned
• Acts like a constant

final int MAX_LIMIT = 100;
          

Rules for Final Variables

• Must be initialized only once
• Can be initialized:
  • At declaration
  • In constructor (for instance final variables)

Example: Final Variable

final int x = 10;
x = 20;   // ❌ compilation error
          

Final Instance Variable

class Test {
    final int value;
    Test(int value) {
        this.value = value; // allowed
    }
}
          

Final Static Variable (Constant)

class Config {
    static final String APP_NAME = "SoftwareTips4U";
}
          

Best Practice: Use UPPER_CASE naming for constants.

2. final Method

What Is a Final Method?

• A method declared as final cannot be overridden
• Inheritance is allowed, overriding is not

class Parent {
    final void display() {
        System.out.println("Parent display");
    }
}

class Child extends Parent {
    void display() {   // ❌ compilation error
    }
}
          

Why Use Final Methods?

• Prevent modification of critical logic
• Improve security
• Used in framework/base classes

Example: Security-related validation methods.

3. final Class

What Is a Final Class?

• A final class cannot be inherited
• No subclassing allowed

final class Utility {
    void help() {}
}

class MyUtil extends Utility {  // ❌ compilation error
}
          

Why Use Final Classes?

• Prevent unwanted inheritance
• Improve immutability and security

Real-Time Example: String class is final.

Important Clarification (Very Common Interview Trap)

final vs Immutability

final StringBuilder sb = new StringBuilder("Java");
sb.append(" Rocks");   // ✅ allowed
          

• final → reference cannot change
• Object can change (if mutable)

final with Parameters

void calculate(final int x) {
    x = 20;   // ❌ not allowed
}
          

Why it matters: Prevents accidental modification inside methods.

final vs finally vs finalize

Common Beginner Mistakes

• Assuming final makes objects immutable
• Forgetting to initialize final variables
• Trying to override final methods
• Confusing final with finally
• Overusing final unnecessarily

Interview-Ready Answers

Short Answer

The final keyword is used to restrict modification of variables, methods, and classes in Java.

Detailed Answer

In Java, final variables cannot be reassigned, final methods cannot be overridden, and final classes cannot be inherited. It is used to enforce immutability, protect logic, and improve code safety.

Key Takeaway

The final keyword is a design and safety tool. It helps write secure, predictable, and maintainable Java code when used appropriately.