Item 22: Use interfaces only to define types

When a class implements an interface, the interface serves as a type that can be used to refer to instances of the class. That a class implements an interface should therefore say something about what a client can do with instances of the class. It is inappropriate to define an interface for any other purpose.

One kind of interface that fails this test is the so-called constant interface.

// Constant interface antipattern - do not use!
public interface PhysicalConstants {
    // Avogadro's number (1/mol)
    static final double AVOGADROS_NUMBER   = 6.022_140_857e23;

    // Boltzmann constant (J/K)
    static final double BOLTZMANN_CONSTANT = 1.380_648_52e-23;

    // Mass of the electron (kg)
    static final double ELECTRON_MASS      = 9.109_383_56e-31;
}

There are several constant interfaces in the java platform libraries, such as java.io.ObjectStreamConstants. These interfaces should be regarded as anomalies and should not be emulated.

The constant interface pattern is a poor use of interfaces. 

// Constant utility class
package com.effectivejava.science;

public class PhysicalConstants {
  private PhysicalConstants() { }  // Prevents instantiation

  public static final double AVOGADROS_NUMBER = 6.022_140_857e23;
  public static final double BOLTZMANN_CONST  = 1.380_648_52e-23;
  public static final double ELECTRON_MASS    = 9.109_383_56e-31;
}

// 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;
    }
    ...
    // Many more uses of PhysicalConstants justify static import
}

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