EJ-19 Use Interfaces only to define types

Ref: Effective Java by Joshua Bloch

In this item, Mr Bloch mention about same symptom I have faced previously and shared in this post.

The constant interface pattern is a poor use of interfaces:

	// Constant interface antipattern - do not use!
	public interface PhysicalConstants {
	// Avogadro's number (1/mol)
	static final double AVOGADROS_NUMBER = 6.02214199e23;
	// Boltzmann constant (J/K)
	static final double BOLTZMANN_CONSTANT = 1.3806503e-23;
	// Mass of the electron (kg)
	static final double ELECTRON_MASS = 9.10938188e-31;
	}

view raw ConstantInterfaceAntiPattern.java hosted with ❤ by GitHub

Instead use a utility class

	// Constant utility class
	package com.effectivejava.science;
	public class PhysicalConstants {
	private PhysicalConstants() { } // Prevents instantiation
	public static final double AVOGADROS_NUMBER = 6.02214199e23;
	public static final double BOLTZMANN_CONSTANT = 1.3806503e-23;
	public static final double ELECTRON_MASS = 9.10938188e-31;
	}

view raw ConstantUtilityClass.java hosted with ❤ by GitHub

If you make heavy use of the constants exported by a utility class, you can avoid the need for qualifying the constants with the class name by making use of the static import facility

	// Use of static import to avoid qualifying constants
	import static com.effectivejava.science.PhysicalConstants.*;
	public class Test {
	double atoms(double mols) {
	return AVOGADROS_NUMBER * mols;
	}
	}

view raw StaticImport.java hosted with ❤ by GitHub

In summary, interfaces should be used only to define types, they should not be used to export constants.