For what purpose can you declare constants in a Java interface?

Question

Specification states that interface is intended to define the contract for what a class can do and contains a set of methods required to implement. But at the same time, interface may have constants.

For what purpose it is allowed in Java?

What is the point of existence of constants in interface and how can they be used in it? As I understand they can only be taken as arguments by methods. But I do not see much point because interface doesn't say anything about how a class will implement its methods.

Answer 1

A constant is part of an interface, too. Constant values are used in design to avoid Magic Numbers, i.e. numbers which have a certain meaning to the implementation, but seem to pop out of nowhere.

There are many cases where numerical values influence the behavior of the code. Consider, for example, an interface for a GUI button. How this button is actually drawn is up to the implementation; but what kind of button it is is part of the contract which forms the interface: Is it a normal push button, has it an image, or is it a checkbox? This behavior can be modified using constants, commonly used by OR'ing values: For example, int buttonType = PUSHBUTTON|IMAGEBUTTON.

Answer 2

The constants can be used outside the interface.

For example, they can be used by the classes that implement the interface. This may be useful if a single definition is required across implementation classes.

For example, consider an interface that defines some bitwise-OR style constants that may be used consistently across all the subclasses. Only a single definition of these is required, and only a single set need be learned.