Java：如何将字符串 ArrayList 分配给列表<Enum>

Question

I have an Enum class, where each Enum has a String value.

I want to create a List<> with my Enum type from a List. Here the strings are the value of the enums.

Is it possible to assign them directly during initialization? If yes, what is the best way to do that?

here is an example code:

public class SomeController {

    public enum MyEnum {
        A("a"),
        B("b"),
        C("c");

        private final String value;

        MyEnum(String value) {
            this.value = value;
        }
    }

    public String handler(
            @RequestParam(name = "enumList") List<MyEnum> myEnumList ) {

        //do something with myEnumList

        return "something";
    }

}

PS I need to directly assign the String-list to MyEnum-list as above. I cannot do a loop on the String-list and add one by one.

Answer 1

First create a map of all the enum constants inside your enum:

private static final Map<String, MyEnum> CONSTANTS = Arrays.stream(values())
    .collect(Collectors.toMap(e -> e.value, e -> e));

And then create a lookup method with @JsonCreator in your enum:

@JsonCreator
public static MyEnum fromValue(String value) {
   MyEnum myEnum = CONSTANTS.get(value);
   if(myEnum == null) {
       throw new NoSuchElementException(value);
   }
   return myEnum;
}

Jackson will detect the json creator method and uses it to convert your list of strings into a list of enums (It all does this before even entering your handler method)

Answer 2

In case you use Java 8 and if stringList contains the names of the enums;

List<MyEnum> myenums = stringList
        .stream()
        .map(MyEnum::valueOf)
        .collect(Collectors.toList());

Answer 3

You can do something like this:

public static <T extends Enum<T>> List<T> toEnumList(
        Iterable<String> strings, Class<T> clazz) {
    List<T> result = new ArrayList<>();
    for (String s : strings) {
        result.add(Enum.valueOf(clazz, s));
    }
    return result;
}

It works with any enum type.