What is ‘final’ keyword in Java?

In Java, the keyword ‘final’ serves as a non-access modifier applicable to classes, methods, and variables. A ‘final’ class cannot be subclassed, a ‘final’ method cannot be overridden, and a ‘final’ variable cannot be reassigned once initialized. It’s a fundamental component in Java’s type system, ensuring that the semantics of the code are clear and predictable.

Introduction to the ‘final’ Keyword in Java

Java is a statically-typed, object-oriented programming language where control over the class hierarchy and immutability is often crucial. The ‘final’ keyword is a tool in Java that helps to achieve this control. Its presence indicates that the programmer’s intent is to create an unchangeable reference, method, or class.

Basic Usage of ‘final’

The ‘final’ keyword has several uses in Java, each contributing to different aspects of the language’s robustness and security. It can be applied to primitive types, objects, methods, and classes.

Declaring Final Variables

When declaring a variable as final, you’re creating a constant. For primitive types, the value of a final variable cannot change. For object references, the reference itself cannot change, but the object it points to can still be modified, unless the object itself is immutable. Final variables must be initialized when declared or within the constructor if they are instance variables.

final int maxUsers = 10;

final User currentUser = new User();

Immutability vs. Unmodifiable References

Immutability in Java refers to the property of an object to remain unchanged after its creation. Unmodifiable references, on the other hand, mean that the reference cannot point to a different object once set, although the object itself can change if it’s mutable. The ‘final’ keyword ensures the latter.

final List<String> unmodifiableList = new ArrayList<>();

unmodifiableList.add("codedamn is awesome"); // Allowed

unmodifiableList = new ArrayList<>(); // Compile-time error

‘final’ with Methods

Applying ‘final’ to methods means that the method cannot be overridden or hidden by subclasses, which can be useful when you want to lock down the method’s behavior in a class hierarchy.

Preventing Method Overriding

By marking a method as final, you secure its implementation, ensuring that the expected behavior remains consistent across all instances of the class, even in derived classes.

public class User {

    public final void login() {

        // ... login code that should not be overridden

    }

}

Implications on Inheritance

Using ‘final’ with methods sends a clear message to other developers and to the JVM that the method is complete in its current form and should not be extended further. It can be used to ensure consistency, security, and prevent a change in the contract of the method in subclass implementations.

‘final’ with Classes

When you apply ‘final’ to a class definition, it prevents the class from being subclassed. This is particularly useful when you want to create an immutable class or to ensure the security and integrity of your class’s behavior.

Preventing Class Inheritance

A final class is a declaration that the class is complete and should not be extended or modified through inheritance. This is often used in conjunction with immutable classes to ensure that the immutability guarantee is not weakened by subclassing.

public final class Constants {

    public static final int MAX_USERS = 10;

}

Examples of Final Classes in Java API

In the Java API, there are several examples of final classes, such as java.lang.String and wrapper classes like java.lang.Integer and java.lang.Double. These are designed to be immutable and secure.

‘final’ vs. ‘static’

While both ‘final’ and ‘static’ are non-access modifiers, they serve different purposes. A ‘static’ variable or method belongs to the class, rather than any instance, while ‘final’ implies immutability or the prevention of change.

Understanding the Difference

A static variable is shared among all instances of the class, whereas a final variable can have a different value for each instance. However, a static final variable combines both concepts, representing a constant value shared across all instances.

When to Use ‘final’, ‘static’, or Both

Use ‘final’ when you want to protect the variable from being re-assigned, ‘static’ when you want to share a field or method across all instances, and ‘static final’ when you have a constant that should be shared across all instances. Understanding when to use each, or both, depends on the specific requirements of your application design.

For those who wish to dive deeper into the subtleties of ‘final’, the Java Language Specification provides comprehensive details, and for practical examples, the Java API documentation is an invaluable resource. Exploring these
In Java, the final keyword is a non-access modifier used for classes, methods, and variables, which makes them non-changeable after initialization. Its use can have several performance implications, particularly when it comes to compiler optimizations.

Performance Implications

When a variable is declared with final, its value can’t be modified post-initialization, which may lead to performance benefits. Since final variables are immutable, the Java compiler can make certain assumptions that enable it to optimize code execution. For instance, inlining final variables can save time on variable lookup and reduce overhead, particularly in loops or frequently called methods.

Impact on Compiler Optimizations

The Java compiler can apply aggressive optimizations for final fields, such as lock elision in synchronized blocks when it’s guaranteed that the field is only set once. Furthermore, final variables can be treated as compile-time constants, and their actual values can be used instead of references, which can significantly improve performance.

Best Practices and Common Pitfalls

While final can be beneficial, it’s essential to understand when and where to use it effectively to avoid common pitfalls.

When and Where to Use final Effectively

final should be used when you want to ensure an object’s state remains constant throughout its lifecycle, like with constants (static final). It’s also helpful when an immutable object is required, or to enforce the immutability of method arguments to prevent side effects. In classes, marking methods as final can prevent them from being overridden, which can be vital for maintaining consistency in behavior, especially when designing APIs.

Common Mistakes and Misconceptions

A common misconception is that final always improves performance. While final can help in some scenarios, its misuse can lead to a rigid codebase that’s difficult to test and extend. For example, overusing final on methods can impede subclassing and polymorphism, restricting the flexibility of the code.

final in Advanced Scenarios

In more complex programming scenarios, understanding the nuances of final becomes even more critical.

Usage in Concurrent Programming

In concurrent programming, final fields have the advantage of being thread-safe without requiring synchronization, provided they are correctly constructed. This is because the final field values are guaranteed to be visible to other threads after the constructor has completed.

Impact on Serialization and Deserialization

Regarding serialization, final fields are not inherently problematic, but care must be taken when modifying classes with final fields, as this can break compatibility with previously serialized objects. Deserialization restores the values of final fields, but it does not use the class constructors, which can lead to unexpected behaviors if not handled correctly.

Comparative Study

Comparing final in Java with similar constructs in other programming languages can provide a broader perspective on immutability.

Final-like Constructs in Other Languages

Languages like C++ have const, while C# uses readonly to impose similar restrictions. Kotlin has val for declaring read-only properties, and Scala uses val for declaring immutable variables.

Real-world Examples and Case Studies

Real-world applications of final are vast. For instance, String class in Java uses final to ensure immutability, which is crucial for security and string pool functionality.

Code Snippets

Here’s a snippet demonstrating the use of final with variables:

final int threshold = 10;

// threshold = 20; // This line would cause a compilation error.

And a final method:

public final void connect() {

    // Connection code that should not be overridden.

}

Conclusion

The final keyword is a powerful tool in Java programming, enabling immutability, offering potential performance optimizations, and aiding in concurrent programming. However, it should be used judiciously to maintain code flexibility and readability.