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

Java lets you restrict variable to having one of only a few predefined values - in other words, one value from an enumerated list. Using enums can help to reduce bug's in your code. Here is an example of enums outside a class:

enums coffeesize{BIG , HUGE , OVERWHELMING }; 
//This semicolon is optional.

This restricts coffeesize to having either: BIG , HUGE , or OVERWHELMING as a variable.