The Puzzling Purpose of Interface Constants

In Java, it's possible to declare fields within an interface, making them implicitly public, static, and final. This has raised questions among developers.

The Constant Interface Trap

Contrary to expectations, defining constants within an interface is considered a "Constant Interface Antipattern." As stated in Effective Java by Joshua Bloch, this practice unnecessarily exposes implementation details to users and hinders future code modifications. Furthermore, it binds subclasses to the constant interface.

Platform Anomalies

Despite this warning, several constant interfaces can be found in the Java Standard Library, such as java.io.ObjectStreamConstants. These oddities should not serve as examples but rather as contrasts to be avoided.

Alternative Approaches

To prevent pitfalls associated with constant interfaces, consider using a dedicated utility class with private constructors and static final fields.

public final class Constants {

    private Constants() {}

    public static final double PI = 3.14159;
    public static final double PLANCK_CONSTANT = 6.62606896e-34;
}

This approach keeps constants private yet readily accessible through static imports.

import static Constants.PLANCK_CONSTANT;
import static Constants.PI;

public class Calculations {

    public double getReducedPlanckConstant() {
        return PLANCK_CONSTANT / (2 * PI);
    }
}

Thus, while interface constants may seem intriguing, they should be avoided due to potential design flaws. Use private classes with static finals instead to maintain a clean and flexible codebase.