Return enum array for a list of given strings contained in the enum using streams

Question

I have an Enum of Strings like this:

public enum SomeEnum {

  STRING1("some value"),
  STRING2("some other value");
  STRING3("some third value");

  ...more strings...

  private String name;

  SomeEnum(String name) {
    this.name = name;
  }

  @Override
  public String toString() {
    return name;
  }

And I have a List someStringList containing some Strings.

I want to iterate over someStringList and find the corresponding enums for it.

For example: The List contains the Strings "some value" and "some third value", then I would like to use the Java Stream-API to return me a List containing SomeEnum.STRING1 and SomeEnum.STRING3

Somehow I can't get this to work. I tried something like this:

    List<SomeEnum> enumList = Stream.of(someStringList).forEach( s -> Stream.of(SomeEnum.values()).filter(w -> w.toString().equalsIgnoreCase(s)).collect(Collectors.toList()));

but this doesn't compile because it doesn't return anything. Any ideas?

Does this answer your question? Finding enum value with Java 8 Stream API — Savior
– Savior, Commented Apr 9, 2020 at 13:19
Unfortunately no, because in all similar questions I found it is a single String instead of a List of Strings — uhmdunnolol
– uhmdunnolol, Commented Apr 9, 2020 at 13:22
Simple to adapt once you have that find method in the accepted answer. Just stream over your list of strings and map with the result of find. — Savior
– Savior, Commented Apr 9, 2020 at 13:24
Have a stream of SomeEnum values, filter it by checking if the someStringList contains the name of the enum value being filtered and collect the filtered enum values into a list — user12553383
– user12553383, Commented Apr 9, 2020 at 15:23

Ravindra Ranwala · Accepted Answer · 2020-04-09 13:28:10Z

1

In your SomeEnum you can build a Map<String, SomeEnum> from string name to enum and initialize it during class loading time. Then declare a public static method in your SomeEnum named fromString which returns an enum constant for a given string if one exists.

private static final Map<String, SomeEnum> stringToEnum = Arrays.stream(values())
    .collect(Collectors.toMap(SomeEnum::toString, e -> e));

public static SomeEnum fromString(String name) {
    return stringToEnum.get(name);
}

Then use it in your client code. Here's how it looks.

List<SomeEnum> enumList = someStringList.stream()
    .map(SomeEnum::fromString)
    .collect(Collectors.toList());

answered Apr 9, 2020 at 13:28

Ravindra Ranwala

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Savior · Accepted Answer · 2020-04-09 13:34:47Z

1

Build a map from the string to the corresponding SomeEnum value:

Map<String, SomeEnum> map = 
    Arrays.stream(SomeEnum.values()).collect(toMap(SomeEnum::toString, s -> s));

(This can be done once and stored)

Then you can look up in this:

List<SomeEnum> enumList = someStringList.stream().map(map::get).collect(toList());

(You may or may not want to handle the case where a string isn't found in the map: for example, you could throw an exception, or drop such elements).

edited Apr 9, 2020 at 13:34

Savior

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answered Apr 9, 2020 at 13:25

Andy Turner

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