Java lookup enum from enum

Question

I have two enums in two different classes (shown below):

enum State {
 NOCAR, SOLD, TO_BUY, TOYOTA, HONDA, NISSAN, BMW, MERCEDES, NO_MONEY
}

enum Car {
 TOYOTA, HONDA, NISSAN, BMW, MERCEDES
}

as you can see that the State enum contains all the possiblities of the Car enum.

So I want to do something like this:

public void sayHello(Car brand) {
  State s = ... // how to convert the brand parameter into the corresponding State enum?
  ...
  ...
}

So I want to be able to convert a given Car enum into a State enum in minimal code that is easy to understand. I know I can use a switch but that is a lot of code wastage. So please help me here.

Just for the record: the accepted answer works, but has some drawbacks. I don't mind the accept on it, but please read my comment there, and the content I added to my answer to get a complete picture. — GhostCat
– GhostCat, Commented Mar 23, 2019 at 5:34

GhostCat · Accepted Answer · 2019-03-23 05:28:39Z

4

You can give the State enum a private field of type Car. Next, you add a private constructor that takes a Car object, and stores that in said field. Then you can declare your State constants like

SOLD(null), BMW(Car.BMW),...

But: conceptually, this doesn't make sense. A Car is a Car, and maybe a transaction has two properties state and car (when the state is SOLD), but the state should not reflect a car type. Meaning: whatever you do to get a Car enum from a State instance, your real problem is your design. You are duplicating things by having the "same" constants in two places already.

Your question asks how to best deal with a symptom of bad design, and the real answer is: you don't. You fix the design instead of duct taping the implementation.

edited Mar 23, 2019 at 5:28

answered Mar 23, 2019 at 5:16

GhostCat

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1 Comment

Jofbr Over a year ago

Thanks GhostCat. Your solution is a valid one too and your comments above do make sense and I am aware of the potential disadvantages of @ernest_k's solution but that solution suits better what I was looking for.

ernest_k · Accepted Answer · 2019-03-23 05:14:59Z

3

Having a Car value, you can just use a name-based lookup:

public void sayHello(Car brand) {
  State s = State.valueOf(brand.name());
  ...
}

As long as all Car value names are in the State enum, this should always work.

answered Mar 23, 2019 at 5:14

ernest_k

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4 Comments

GhostCat Over a year ago

This for sure works, but it has two disadvantages: as you said yourself : it forces you to manually keep the two enums to be in sync. If you add a new Car, but forget about the State, this code throws at runtime. This approach kills compile time checking. You could as well resort to flat strings then! Plus: this solution further manifests the underlying bad design. It makes things worse, and over time, more bad Band aids will come in.

GhostCat Over a year ago

So the minimum that you should do here is to have a unit test in the State class that iterates all Car instances and ensures the mapping to State works. Otherwise one typo is enough to crash your code later!

ernest_k Over a year ago

@GhostCat That's right. This is a String to enum conversion solution. Of course, I counted on the OP's assumption that "the State enum contains all the possiblities of the Car enum" and can only suppose this is tested. I see the point you're making. But we all know that there are cases where the kind of coupling you're suggesting is a bad idea (or plain infeasible). Not saying your solution is wrong, but that it has its place, just as the OP's original design has its place too.

GhostCat Over a year ago

As I told the op: just wanted to make him aware of these things. I don't care where accepts go, as long as they go somewhere reasonable :-)

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