Welcome to Geoff Hayward's Weblog

Java Stream API: ArrayList to JsonArray
03 January 2017

You can convert an ArrayList to a Java EE JsonArray using the Java Stream API in the following way.

// set up example
ArrayList<Pet> pets = new ArrayList<>();
pets.add(new Pet("Goldie", "Fish"));
pets.add(new Pet("Daisy", "Cow"));
pets.add(new Pet("Snowball", "Cat"));

// the work
pets.stream()
	.map((a) -> { 
		return Json.createObjectBuilder()
			.add("id", a.getName())
			.add("type", a.getGroup())
			.build();
	})
	.collect(
		Json::createArrayBuilder,
		JsonArrayBuilder::add,
		JsonArrayBuilder::add
	)
	.build();

The .map operation of the stream API takes a Function<T,R>. The function converts each item to a JsonObject. Then the .collect operation creates the JsonArray using each of the JsonObjects.

I hope you find this useful.

The Java 8 Lambda way: ArrayList to an Array
15 July 2016

In an earlier post this year, 'Convert an ArrayList to an Array in Java', I commented on a way to convert an ArrayList to an Array in Java. Here is the example code from the earlier post.

List<String> results = new ArrayList<>();
...
results.toArray(new String[results.size()]);

This earlier way works fine, however it's old school. Here is the Java 8 Lambda way to convert an ArrayList to an Array:

Long[] longArray = longArrayList.stream()
    .map(Long::new)
    .toArray(Long[]::new);

This example works by mapping a new object for each item in the ArrayList's stream to the new stream, and then collecting that stream as an array.