空对象设计模式

// Step 1: Define the User interface
interface User {
    void showDetails();
}

// Step 2: Create the RealUser class that implements User
class RealUser implements User {
    private String name;

    public RealUser(String name) {
        this.name = name;
    }

    @Override
    public void showDetails() {
        System.out.println("User: " + name);
    }
}

// Step 3: Create the NullUser class that also implements User
class NullUser implements User {
    @Override
    public void showDetails() {
        System.out.println("No user found.");
    }
}

// Step 4: Implement a User Factory to return RealUser or NullUser
class UserFactory {
    public static User getUser(String name) {
        if (name == null || name.isEmpty()) {
            return new NullUser();
        }
        return new RealUser(name);
    }
}

// Step 5: Client code
public class Main {
    public static void main(String[] args) {
        User user1 = UserFactory.getUser("Anubhav");
        User user2 = UserFactory.getUser(""); // returns NullUser

        user1.showDetails(); // Output: User: Anubhav
        user2.showDetails(); // Output: No user found.
    }
}