Constant interface

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In the Java programming language, the constant interface pattern describes the use of an interface solely to define constants, and having classes implement that interface in order to achieve convenient syntactic access to those constants. However, since constants are very often merely an implementation detail, and the interfaces implemented by a class are part of its exported API, this practice amounts to putting implementations details into the API, which was considered inappropriate by, e.g., Java designer Joshua Bloch.^[1] In general, collecting system constants into classes independent of behaviour might create a poor object-oriented design because it is often a sign of low cohesion. For these reasons, constant interfaces may be considered an anti-pattern.

Use of this pattern has a few other downsides:^{[original research?]}

1. It pollutes the class namespace with read-only variables that may not be of use.
2. Contrary to the compile-time tactical utility of implementing a constant interface, the incidental run-time artifacts have little practical purpose (cf. marker interfaces which also have no methods but are useful at run-time).
3. If binary code compatibility is required in future releases, the constant interface must remain forever an interface (it cannot be converted into a class), even though it has not been used as an interface in the conventional sense.
4. Without an IDE that resolves where the constant are coming from, tracking it back to its containing class or interface can be time consuming.
5. An instance of the interface is syntactically no more useful than the interface name itself (since it has no methods).
6. Unless a developer checks any implemented interfaces when adding a constant to a class, or does so but makes a typo in the name of the added constant, the value of a constant can be silently changed. Consider Example 2 below.

Note that the Java libraries use constant interface pattern themselves. For example, the SwingConstants interface^[2] was released in 1998,^[3] and then it was a reasonable choice.