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.
Such an interface contains no methods; it consists solely of static final
fields, each exporting a constant. Here is an example:
// 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;
}
The constant
interface pattern is a poor use of interfaces. There are
several constant interfaces in Java platform libraries, such as java.io.ObjectStreamConstants.
If you want to
export constants, there are several reasonable choices. Integer and Double,
export
MIN_VALUE and MAX_VALUE constants. If the constants are best viewed
as members of an enumerated type, you should export them with an enum type (Item 30). Otherwise, you should export the constants with a
noninstantiable utility class (Item 4). Here
is a utility class version of the PhysicalConstants
example
above:
// 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;
}
Normally a
utility class requires clients to qualify constant names with a class name, you
can avoid the need for qualifying the constants with the class name by making
use of the static import facility,
introduced in release 1.5:
// 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 to
export constants.
Reference: Effective Java 2nd Edition by Joshua Bloch
