Can Interfaces Replace Utility Classes in Java 8?

Can interfaces serve as effective replacements for utility classes in Java 8?

In Java 8, the introduction of default methods in interfaces has blurred the lines between interfaces and utility classes. This development allows developers to determine if interfaces can indeed replace utility classes, depending on the specific use case and design principles applied.

public interface MyUtilityInterface {
    static int add(int a, int b) {
        return a + b;
    }
    
    default int multiply(int a, int b) {
        return a * b;
    }
}

class UtilityClassExample {
    public static void main(String[] args) {
        System.out.println(MyUtilityInterface.add(2, 3)); // Static method
        MyUtilityInterface ui = new MyUtilityInterface() {}; // Anonymous class to use default method
        System.out.println(ui.multiply(4, 5)); // Default method
    }
}

Causes

• Interfaces are primarily designed to define behaviors (contracts) rather than implement functionality.
• Utility classes often contain static methods that perform operations independent of object state, while interfaces promote polymorphism and flexibility in design.

Solutions

• Use interfaces when you want to define a contract for behavior that can be implemented by multiple classes.
• Use utility classes for stateless operations that do not require a full object instantiation. This is common for mathematical calculations, string utilities, etc.
• Implement default methods in interfaces to provide shared functionality without forcing subclassing, thus combining some features of utility classes.

Mistake: Assuming interfaces can replace utility classes without understanding their use cases.

Solution: Evaluate if a behavior-oriented design is suitable (use interfaces) or if utility methods are more appropriate (use utility classes).

Mistake: Using interfaces just for the sake of adopting new language features without a clear benefit.

Solution: Prioritize clean, maintainable, and understandable code; choose the right abstraction for the task.