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

What is an enum

• enum is a keyword defined for Enumeration a new data type. Typesafe enumerations should be used liberally. In particular, they are a robust alternative to the simple String or int constants used in much older APIs to represent sets of related items.

Why to use enum

• enums are implicitly final subclasses of java.lang.Enum
• if an enum is a member of a class, it's implicitly static
• new can never be used with an enum, even within the enum type itself
• name and valueOf simply use the text of the enum constants, while toString may be overridden to provide any content, if desired
• for enum constants, equals and == amount to the same thing, and can be used interchangeably
• enum constants are implicitly public static final

Note

• enums cannot extend any class.
• An enum cannot be a superclass.
• the order of appearance of enum constants is called their "natural order", and defines the order used by other items as well: compareTo, iteration order of values, EnumSet, EnumSet.range.
• An enumeration can have constructors, static and instance blocks, variables, and methods but cannot have abstract methods.