Null Object

It uses a special instance of a class that acts as a default or null object, eliminating the need for null checks in the code. Instead of returning null or an empty value, the pattern provides a neutral object that implements the same interface as the other objects, allowing methods to be called without checking for null.

Structure

• Subject Interface: Defines the behavior that concrete objects must implement.
• Real Object: A concrete implementation of the subject interface that performs the intended behavior.
• Null Object: A concrete implementation of the subject interface that does nothing or provides default behavior.

Example

// Subject Interface
interface Logger {
    void log(String message);
}

// Real Object
class ConsoleLogger implements Logger {
    @Override
    public void log(String message) {
        System.out.println("ConsoleLogger: " + message);
    }
}

// Null Object
class NullLogger implements Logger {
    @Override
    public void log(String message) {
        // Do nothing
    }
}

// Client Class
class Application {
    private Logger logger;

    // Constructor that accepts a logger
    public Application(Logger logger) {
        this.logger = logger != null ? logger : new NullLogger();  // Use NullLogger if logger is null
    }

    public void run() {
        logger.log("Application is starting...");
        // Other application logic
        logger.log("Application is running...");
    }
}

// Main Class
public class Main {
    public static void main(String[] args) {
        // Using a real logger
        Logger consoleLogger = new ConsoleLogger();
        Application appWithLogger = new Application(consoleLogger);
        appWithLogger.run();

        // Using a null logger
        Application appWithoutLogger = new Application(null);
        appWithoutLogger.run();
    }
}