1. Overview
In this quick tutorial, we’ll learn how to join and to split Arrays and Collections in Java, making good use of the new stream support*.*
2. Join Two Arrays
Let’s start by joining two Arrays together using Stream.concat*:*
@Test
public void whenJoiningTwoArrays_thenJoined() {
String[] animals1 = new String[] { "Dog", "Cat" };
String[] animals2 = new String[] { "Bird", "Cow" };
String[] result = Stream.concat(
Arrays.stream(animals1), Arrays.stream(animals2)).toArray(String[]::new);
assertArrayEquals(result, new String[] { "Dog", "Cat", "Bird", "Cow" });
}
3. Join Two Collections
Let’s do the same join with two Collections:
@Test
public void whenJoiningTwoCollections_thenJoined() {
Collection<String> collection1 = Arrays.asList("Dog", "Cat");
Collection<String> collection2 = Arrays.asList("Bird", "Cow", "Moose");
Collection<String> result = Stream.concat(
collection1.stream(), collection2.stream())
.collect(Collectors.toList());
assertTrue(result.equals(Arrays.asList("Dog", "Cat", "Bird", "Cow", "Moose")));
}
*4. Join Two Collections With Filter*
Now, let’s join two Collections of numbers filtering anything greater than 10:
@Test
public void whenJoiningTwoCollectionsWithFilter_thenJoined() {
Collection<String> collection1 = Arrays.asList("Dog", "Cat");
Collection<String> collection2 = Arrays.asList("Bird", "Cow", "Moose");
Collection<String> result = Stream.concat(
collection1.stream(), collection2.stream())
.filter(e -> e.length() == 3)
.collect(Collectors.toList());
assertTrue(result.equals(Arrays.asList("Dog", "Cat", "Cow")));
}
5. Join an Array Into a String
Next, let’s join an Array into a String using a Collector:
@Test
public void whenConvertArrayToString_thenConverted() {
String[] animals = new String[] { "Dog", "Cat", "Bird", "Cow" };
String result = Arrays.stream(animals).collect(Collectors.joining(", "));
assertEquals(result, "Dog, Cat, Bird, Cow");
}
6. Join a Collection Into a String
Let’s do the same but with a Collection*:*
@Test
public void whenConvertCollectionToString_thenConverted() {
Collection<String> animals = Arrays.asList("Dog", "Cat", "Bird", "Cow");
String result = animals.stream().collect(Collectors.joining(", "));
assertEquals(result, "Dog, Cat, Bird, Cow");
}
7. Join a Map Into a String
Next, let’s create a String out of a Map.
The process is very similar to previous examples, but here we have an extra step to first join each Map Entry:
@Test
public void whenConvertMapToString_thenConverted() {
Map<Integer, String> animals = new HashMap<>();
animals.put(1, "Dog");
animals.put(2, "Cat");
animals.put(3, "Cow");
String result = animals.entrySet().stream()
.map(entry -> entry.getKey() + " = " + entry.getValue())
.collect(Collectors.joining(", "));
assertEquals(result, "1 = Dog, 2 = Cat, 3 = Cow");
}
8. Join Nested Collections Into a String
Let’s do something a bit more complex. Let’s join some nested Collections into a String.
In the following example we first join within each nested Collection and then we join the result of each of them:
@Test
public void whenConvertNestedCollectionToString_thenConverted() {
Collection<List<String>> nested = new ArrayList<>();
nested.add(Arrays.asList("Dog", "Cat"));
nested.add(Arrays.asList("Cow", "Pig"));
String result = nested.stream().map(
nextList -> nextList.stream()
.collect(Collectors.joining("-")))
.collect(Collectors.joining("; "));
assertEquals(result, "Dog-Cat; Cow-Pig");
}
9. Handle Null Values When Joining
Lest’s see how we can use a Filter to skip any null values:
@Test
public void whenConvertCollectionToStringAndSkipNull_thenConverted() {
Collection<String> animals = Arrays.asList("Dog", "Cat", null, "Moose");
String result = animals.stream()
.filter(Objects::nonNull)
.collect(Collectors.joining(", "));
assertEquals(result, "Dog, Cat, Moose");
}
10. Split a Collection in Two
Let’s split a Collection of numbers into two Collections at the middle:
@Test
public void whenSplitCollectionHalf_thenConverted() {
Collection<String> animals = Arrays.asList(
"Dog", "Cat", "Cow", "Bird", "Moose", "Pig");
Collection<String> result1 = new ArrayList<>();
Collection<String> result2 = new ArrayList<>();
AtomicInteger count = new AtomicInteger();
int midpoint = Math.round(animals.size() / 2);
animals.forEach(next -> {
int index = count.getAndIncrement();
if (index < midpoint) {
result1.add(next);
} else {
result2.add(next);
}
});
assertTrue(result1.equals(Arrays.asList("Dog", "Cat", "Cow")));
assertTrue(result2.equals(Arrays.asList("Bird", "Moose", "Pig")));
}
11. Split an Array by Word Length
Next, let’s split an array by the length of the words:
@Test
public void whenSplitArrayByWordLength_thenConverted() {
String[] animals = new String[] { "Dog", "Cat", "Bird", "Cow", "Pig", "Moose"};
Map<Integer, List<String>> result = Arrays.stream(animals)
.collect(Collectors.groupingBy(String::length));
assertTrue(result.get(3).equals(Arrays.asList("Dog", "Cat", "Cow", "Pig")));
assertTrue(result.get(4).equals(Arrays.asList("Bird")));
assertTrue(result.get(5).equals(Arrays.asList("Moose")));
}
12. Split a String Into an Array
Let’s now do the opposite, let’s split a String into an Array:
@Test
public void whenConvertStringToArray_thenConverted() {
String animals = "Dog, Cat, Bird, Cow";
String[] result = animals.split(", ");
assertArrayEquals(result, new String[] { "Dog", "Cat", "Bird", "Cow" });
}
13. Split String Into a Collection
This example is similar to the previous one, there is just an extra step to convert from Array to a Collection:
@Test
public void whenConvertStringToCollection_thenConverted() {
String animals = "Dog, Cat, Bird, Cow";
Collection<String> result = Arrays.asList(animals.split(", "));
assertTrue(result.equals(Arrays.asList("Dog", "Cat", "Bird", "Cow")));
}
14. Split a String Into a Map
Now, let’s create a Map from a String. We will need to split our string twice, once for each entry, and one last time for the key and values:
@Test
public void whenConvertStringToMap_thenConverted() {
String animals = "1 = Dog, 2 = Cat, 3 = Bird";
Map<Integer, String> result = Arrays.stream(
animals.split(", ")).map(next -> next.split(" = "))
.collect(Collectors.toMap(entry -> Integer.parseInt(entry[0]), entry -> entry[1]));
assertEquals(result.get(1), "Dog");
assertEquals(result.get(2), "Cat");
assertEquals(result.get(3), "Bird");
}
*15. Split String With Multiple Separators*
Finally, let’s split a String that has multiple separators using a regular expression, we will also remove any empty results:
@Test
public void whenConvertCollectionToStringMultipleSeparators_thenConverted() {
String animals = "Dog. , Cat, Bird. Cow";
Collection<String> result = Arrays.stream(animals.split("[,|.]"))
.map(String::trim)
.filter(next -> !next.isEmpty())
.collect(Collectors.toList());
assertTrue(result.equals(Arrays.asList("Dog", "Cat", "Bird", "Cow")));
}
16. Conclusion
In this tutorial, leveraging the simple String.split function and the powerful Java 8 Stream, we illustrated how to join and split Arrays and Collections.
You can find the code for this article over on GitHub.