什么时候在 Java 中使用 Enum 类？

Question

It may be very obvious questions, but is it good to use Enum class if you know that the list of values will keep increasing?

Let's say you define an Event Enum first it contains only [Poo, Too] then as we know we always have some new requirement it becomes [Poo, Too, App, Laa] and that keep changing again and again,

So what is the best approach in this case?

Answer 1

tl;dr

• If the entire set of possible values is known at compile-time, use enum .
• If values can be added or dropped while your system is in use (at runtime), then you cannot use an enum . Use a Set , List , or Map instead.

Known at compile-time

An enum is appropriate when the domain (set of all possible values) is known at compile-time.

If this year your company is offering two products ( Poo & Too ), then make an enum for those two elements.

public enum Product { POO , TOO }

Next year, your company decides to grow their product offerings by adding App & Laa. As part of a planned deployment, add two more objects to your enum.

public enum Product { POO , TOO , APP , LAA }

By the way, notice the naming conventions. The enum has a regular class name (initial cap). The objects being automatically instantiated are constants, and so are named in all-uppercase.

Also, be aware that the enum facility in Java is quite flexible and powerful, much more so than the usual naming-a-number enum scheme seen in most languages. You can have member variables and methods and constructors on a Java enum. For example, you can add a getDisplayName method to provide text more appropriate to a user-interface than the all-caps object name, as seen in DayOfWeek::getDisplayName . You can add quite a bit of functionality, such as ChronoUnit.between .

What you cannot do at runtime with an enum in Java is add or remove objects. Thus the requirement that you know your domain at compile-time. However, when working with a group of enum objects, you can use the highly-optimized EnumSet and EnumMap classes.

Known at runtime

If you cannot determine the domain at compile-time, if users can add or remove elements at runtime, then use a collection such as a List , Set , or Map rather than an enum.

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Singleton

Though not originally intended as a purpose of Enum in Java, an enum happens to be the safest (and simplest) way to implement the Singleton design pattern.

This approach to a singleton is explained in the famous book Effective Java by Dr. Joshua Bloch, et al. Using an enum solves multiple obscure technical problems with other approaches to a singleton.

Answer 2

Your question is pretty generic and I'm pretty sure there is no single right answer. But judging based on spring* tags, I suppose you might be asking about enums in DTOs that being sent over your system in serialized form. If that's the case, I would recommend to choose String in DTO, while inside single app it's ok to use enum . Then you would just care about deserialization/conversion in a factory manner, having ability to handle unknown/missing constant gracefully by logging/providing fallback or meaningful error.

Answer 3

It depends on a case-by-case situation and your question doesn't have much context. However, I do recommend using ENUMs for many cases, including if you expect the list of ENUMs to increase.

Some reasons to use them are:

1. It creates a definite guide of ENUM elements that can be used throughout your code. It eliminates uncertainty over what something is named or what it is. For example ENUM that contains list of animals, or enum of "something".
2. Its easy to refactor later if you need to change anything.

I'm sure there are many more reasons, I find it like a table of contents sometimes. For many cases, you can completely avoid it and be fine but I think its better to use it in general if you're on the fence.