Constant Interface in Java

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. 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. It is for these reasons that implementing constants interfaces may be considered to be an anti-pattern.

However there are some downside to this pattern:

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 constants interface, the incidental run-time artifacts have little practical purpose.

3. If binary code compatibility is required in future releases, the constants 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. A variable (representing an instance) of the interface is syntactically no more useful than the interface name itself.

6. Unless a developer checks any implemented interfaces when adding a constant to a class, or does so but makes a typo in the new constant's name, the value of a constant can be silently changed.

Example 1

public interface Constants

	double PI = 3.14159;
	double PLANCK_CONSTANT = 6.62606896e-34;

public class Calculations implements Constants

	public double getReducedPlanckConstant()
		return PLANCK_CONSTANT / (2 * PI);

Example 2

public interface Constants

	public static final int	CONSTANT = 1;

public class Class1 implements Constants

	public static final int CONSTANT = 2;	// *

	public static void main(String args[]) throws Exception

References : wikipedia share on :        :  

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