Interview #225: Java - Difference between final, finally and finalize?

final, finally, and finalize in Java — all of which may sound similar but serve very different purposes in the Java programming language.

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✅ 1. final (Keyword)

Purpose: To declare constants, prevent inheritance or method overriding.

The final keyword is used in several contexts in Java, and it has different meanings depending on where it is applied:

🔹 a. Final Variable:

A final variable can only be initialized once and its value cannot be changed after that.

final int MAX_VALUE = 100;
MAX_VALUE = 200; // Compile-time error        

• If it’s a primitive type, the value cannot be reassigned.
• If it’s a reference, the object reference cannot be changed, but the object’s contents can still be modified (unless it's immutable).

🔹 b. Final Method:

A final method cannot be overridden by subclasses.

class A {
    final void show() {
        System.out.println("This is a final method.");
    }
}

class B extends A {
    void show() {  // Compile-time error
        System.out.println("Trying to override.");
    }
}        

🔹 c. Final Class:

A final class cannot be subclassed/inherited.

final class Vehicle {
    // class code
}

class Car extends Vehicle { // Compile-time error
}        

💡 Common Use Case: Making classes like String final to ensure immutability and prevent unwanted modification via inheritance.

✅ 2. finally (Block)

Purpose: To define a block of code that is always executed after a try-catch block — whether or not an exception is thrown or caught.

try {
    int data = 50 / 0;
} catch (ArithmeticException e) {
    System.out.println("Exception caught: " + e);
} finally {
    System.out.println("This will always be executed.");
}        

🔹 Key Points:

• The finally block is always executed, even if the try block exits via a return, break, or continue.
• It is generally used to release resources like closing file streams, database connections, etc.
• Even if no catch block is present, finally can be used with try.

try {
    // risky code
} finally {
    // cleanup code
}        

💡 Best Practice: Always use finally for cleanup code that must execute regardless of exceptions.

✅ 3. finalize() (Method)

Purpose: Acts as a clean-up mechanism before an object is garbage collected.

🔹 Signature:

protected void finalize() throws Throwable {
    // cleanup code
}        

• It belongs to the Object class and can be overridden by any class.
• It is called by the Garbage Collector (GC) before reclaiming memory from the object.
• It is not guaranteed to be called promptly or at all — hence, it is not reliable for releasing critical resources like files or sockets.

class MyClass {
    protected void finalize() throws Throwable {
        System.out.println("Object is being garbage collected");
    }
}        

⚠️ Note: As of Java 9, finalize() is deprecated because it causes performance issues and is unreliable. Developers are encouraged to use try-with-resources or explicit resource management instead.

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