From Static to Smart: Replace Utility Classes with Functional Interfaces

Absolutely! Let’s walk through “Stop Writing Utility Classes the Old Way: Use Functional Interfaces Instead” with a clear, real-world example that compares the old static utility class approach to the modern functional interface-based approach.

❌ The Old Way: Static Utility Classes

Static utility classes are often used for operations like validation, string manipulation, or math operations.

Example: Static Validation Utility

// Static utility class
public class ValidationUtils {
    public static boolean isValidEmail(String email) {
        return email != null && email.contains("@");
    }
}

// Usage
String email = "[email protected]";
if (ValidationUtils.isValidEmail(email)) {
    System.out.println("Valid email!");
}

Problems with this approach:

• Can’t change behavior at runtime
• Hard to test in isolation (no mocking)
• Not extensible (no dependency injection)
• Breaks OOP and functional design principles

✅ The Modern Way: Use Functional Interfaces

Instead of static utility methods, define behavior using functional interfaces, such as Predicate<T>, Function<T, R>, or custom interfaces.

Example: Using Predicate<String> for…