What is the use of interface constants?

Question

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?

Answer 1

They are useful if you have common constants that will be used in classes that implement the interface.

Here's an example: http://www.javapractices.com/topic/TopicAction.do?Id=32

But note that the recommended practice is to use static imports instead of constants in interfaces. Here's a reference: http://www.javapractices.com/topic/TopicAction.do?Id=195

Answer 2

The javax.swing.SwingConstants interface is an example which got static fields which are used among the swing classes. This allows you to easily use something like

• this.add(LINE_START, swingcomponent);
• this.add(this.LINE_START, swingcomponent); or
• this.add(SwingComponents.LINE_START, swingcomponent);

However this interface doesn't have methods...

Answer 3

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);
    }
}

Answer 4

Joshua Bloch, "Effective Java - Programming Language Guide":

"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."