What is the use of interface constants?
constants, interface, java
Solution
Putting static members into an interface (and implementing that interface) is a bad practice and there is even a name for it, the Constant Interface Antipattern, see Effective Java, Item 17:
The constant interface pattern is a poor use of interfaces. That a class uses some constants internally is an implementation detail. Implementing a constant interface causes this implementation detail to leak into the class's exported API. It is of no consequence to the users of a class that the class implements a constant interface. In fact, it may even confuse them. Worse, it represents a commitment: if in a future release the class is modified so that it no longer needs to use the constants, it still must implement the interface to ensure binary compatibility. If a nonfinal class implements a constant interface, all of its subclasses will have their namespaces polluted by the constants in the interface.
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.
To avoid some pitfalls of the constant interface (because you can't prevent people from implementing it), a proper class with a private constructor should be preferred (example borrowed from Wikipedia):
public final class Constants {
private Constants() {
// restrict instantiation
}
public static final double PI = 3.14159;
public static final double PLANCK_CONSTANT = 6.62606896e-34;
}
And to access the constants without having to fully qualify them (i.e. without having to prefix them with the class name), use a static import (since Java 5):
import static Constants.PLANCK_CONSTANT;
import static Constants.PI;
public class Calculations {
public double getReducedPlanckConstant() {
return PLANCK_CONSTANT / (2 * PI);
}
}
Problem
I am learning Java and just found that the Interface can have fields, which are public static and final. I haven't seen any examples of these so far. What are some of the use cases of these Interface Constants and can I see some in the Java Standard Library?