How do I convert a JsonArray of JsonString to a List of String

Question

This question is scoped to The Java API for JSON Processing (JSR 353).

Given a JsonArray whose elements are JsonString, how can I easily convert this to a List<String> or a Set<String>? For Set<String>, let's assume that the JsonArray contains a unique collection of string values.

Assume I have a JSON object like this:

{
  "name" : "James Johns",
  "street" : "22 Nancy St.",
  "emails" : [
               "james@a.com",
               "james@b.net",
               null,
               ""
             ]
}

I want my resulting collection to ignore any null or empty string entries in the emails array.

Let's assume my JsonObject instance has already been created.

JsonObject person = <parse the JSON input>;
JsonArray emails = person.get("emails");

I keep getting stumped on JsonValue and JsonString and trying to get the actual String value. If I use the JsonValue.toString() and JsonString.toString() methods, I get string values that include the double quotes, as if the original JSON was "\"james@a.com\"". I need the string values to be the equivalent JSON value "james@a.com".

Possible answer to your question: [Convert Json Array to normal Java list](https://stackoverflow.com/questions/3395729/convert-json-array-to-normal-java-list). — LHCHIN, Dec 16 '19 at 04:42
Wouldn't you agree that the Java8 technique I described is cleaner and easier to understand that having to null check the array and then use a brute force indexing for loop? It turns out there is an answer supplied that is similar to mine, but it is pretty far down the list. I created this post in the hopes it will help new inquiries. I searched a long time looking for a solution and never found the one you linked. Thank you for your comment @LHCHIN — John Czukkermann, Dec 17 '19 at 15:00
Hi, I just provided a possible solution I found to your QUESTION not to your ANSWER as a reply. And I also agreed that Java 8's solution is better! : ) — LHCHIN, Dec 18 '19 at 00:29

score 0 · Answer 1 · answered Dec 05 '19 at 17:05

Using Java 8, one can easily convert a JsonArray of JsonString to a List like this:

JsonObject person = <parse the JSON input>;

List<String> emailList = 
    Optional.ofNullable(person.getJsonArray("emails"))
            .orElse(JsonValue.EMPTY_JSON_ARRAY) // Use an empty list if the property's value was null
            .getValuesAs(JsonString::getString) // Get each JsonString entry as String
            .stream()                           // Turn the collection into a stream for filter/collect
            .filter(email -> email!=null && !email.isEmpty())  // Exclude any null/empty entries
            .collect(Collectors.toList());                     // Create the List

The ofNullable/orElsecombination guarantees I get a JsonArray back, even if that array is empty, thus avoiding any Null Pointer Exception considerations.
The getValuesAs(JsonString::getString) is a JsonArray method that returns a List by applying the JsonString.getString() method to each returned JsonString. The JsonString.getString() method returns the string value "james@a.com" instead of the "\"james@a.com\"" value that the JsonString.toString() method returns.
The stream() method turns the collection into a sequential stream that we can use to filter and collect members of the original collection.
The filter() method applies the Predicate email -> email!=null && !email.isEmpty() to each entry in the stream, throwing out any null entries or entries that are empty strings.
And finally, the collect() method collects the filtered entries into the specified collection type.

To obtain a Set<String>, change the .collect() to use:

.collect(Collectors.toSet())

Once you get accustomed to the Java 8 mechanics, this approach provides a concise, easy to read and understand technique for manipulating collections without the more complicated approach of null/empty checking and using for loops.

How do I convert a JsonArray of JsonString to a List of String

1 Answers1