What is final in java?

Let’s explain final in Java in easy, everyday language — with clear examples so you fully get it.

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✅ What Does final Mean in Java?

In simple terms:

final means “you can’t change it later.”

It is used when you want to make something unchangeable — like a lock that prevents future changes.

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🛠️ Where Can You Use final in Java?

You can use final with:

1. Variables (can’t change the value)
2. Methods (can’t override them)
3. Classes (can’t extend/inherit them)

Let’s break them down one by one.

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🔹 1. final Variable

Once you assign a value to a final variable, you can’t change it.

📦 Example:

public class Demo {
    public static void main(String[] args) {
        final int speedLimit = 60;
        // speedLimit = 80; // ❌ Error: can't change a final variable
        System.out.println("Speed Limit is: " + speedLimit);
    }
}

✔️ speedLimit is final — you can set it once, but not change it.

This is useful when you want to declare constants like PI, tax rate, or max limit.

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🔹 2. final Method

You can mark a method as final so that no subclass can override it.

📦 Example:

class Vehicle {
    final void start() {
        System.out.println("Vehicle started");
    }
}

class Car extends Vehicle {
    // void start() { } // ❌ Error: can't override final method
}

✔️ Once a method is marked final, no class that inherits it can change its behavior.

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🔹 3. final Class

A final class cannot be extended (inherited).

📦 Example:

final class Animal {
    void sound() {
        System.out.println("Animal sound");
    }
}

// class Dog extends Animal {} // ❌ Error: can't extend a final class

✔️ You use a final class when you don’t want anyone to change or extend your class — it’s locked.

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🔄 Bonus: final with Objects

If you make an object final, you can’t change the object reference, but you can change the data inside.

📦 Example:

public class Demo {
    public static void main(String[] args) {
        final StringBuilder sb = new StringBuilder("Hello");
        sb.append(" World"); // ✅ Allowed
        // sb = new StringBuilder("Hi"); // ❌ Not allowed
        System.out.println(sb); // Output: Hello World
    }
}

✔️ The object is final — you can’t reassign it — but you can change what’s inside it.

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🧠 Summary

Use of final	What it does	Example
Variable	Can’t change its value	final int x = 10;
Method	Can’t override it in subclass	final void run()
Class	Can’t be extended (inherited)	final class Animal

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🎯 Think of final as:

• A lock 🔒 — once used, no one can change or override it.
• A way to protect values, methods, or class designs.