What are enums and why are they useful?

Question

Today I was browsing through some questions on this site and I found a mention of an enum being used in singleton pattern about purported thread safety benefits

Answer 1

Something none of the other answers have covered that make enums particularly powerful are the ability to have template methods. Methods can be part of the base enum and overridden by each type. And, with the behavior attached to the enum, it often eliminates the need for if-else constructs or switch statements as this blog post demonstrates - where enum.method() does what originally would be executed inside the conditional. The same example also shows the use of static imports with enums as well producing much cleaner DSL like code.

Some other interesting qualities include the fact that enums provide implementation for equals(), toString() and hashCode() and implement Serializable and Comparable.

For a complete rundown of all that enums have to offer I highly recommend Bruce Eckel's Thinking in Java 4th edition which devotes an entire chapter to the topic. Particularly illuminating are the examples involving a Rock, Paper, Scissors (i.e. RoShamBo) game as enums.