Using `enum` to Represent Complex Immutable States

This is a very cool design pattern I learned on the job.

Say you have a few related states that you want to represent. These states are static and immutable. Think of a collection of states where what actions can be taken depends on the state of object.

A more concrete example is US coin denominations. A penny is always 1c, a nickle is 5c, a dime is 10c and a quarter is 25c.

A naive (but correct) way to represent this would be with a class containing final and static fields:

But, a much cleaner and understandable implementation would use an enum.

An enum is implicitly static and final and is immutable.

enums can have constructors, variables, methods. Moreover, you can use == to compare enum constants.

Further reading

• Enum Types (Java documentation)
• Enum (Python documentation)
• Beginner’s Guide to Java eNum – Why and for What should I use Enum? Java Enum Examples