1. Overview
In this tutorial, we will learn how to use the Joiner and Splitter in the Guava library. We’ll convert collections into a String with the Joiner and we’ll split a String into a collection with the Splitter.
2. Convert List into String Using Joiner
Let’s start with a simple example to join a List into a String using Joiner. In the following example, we join a List of names into one String using the comma “,” as a separator:
@Test
public void whenConvertListToString_thenConverted() {
List<String> names = Lists.newArrayList("John", "Jane", "Adam", "Tom");
String result = Joiner.on(",").join(names);
assertEquals(result, "John,Jane,Adam,Tom");
}
3. Convert Map to String Using Joiner
Next – let’s see how to use Joiner to convert a Map to a String. In the following example, we use withKeyValueSeparator() to join the key with its value:
@Test
public void whenConvertMapToString_thenConverted() {
Map<String, Integer> salary = Maps.newHashMap();
salary.put("John", 1000);
salary.put("Jane", 1500);
String result = Joiner.on(" , ").withKeyValueSeparator(" = ")
.join(salary);
assertThat(result, containsString("John = 1000"));
assertThat(result, containsString("Jane = 1500"));
}
4. Join Nested Collections
Now – let’s see how to join nested collections into a String. In the following example, we join the result of transforming each List to a String:
@Test
public void whenJoinNestedCollections_thenJoined() {
List<ArrayList<String>> nested = Lists.newArrayList(
Lists.newArrayList("apple", "banana", "orange"),
Lists.newArrayList("cat", "dog", "bird"),
Lists.newArrayList("John", "Jane", "Adam"));
String result = Joiner.on(";").join(Iterables.transform(nested,
new Function<List<String>, String>() {
@Override
public String apply(List<String> input) {
return Joiner.on("-").join(input);
}
}));
assertThat(result, containsString("apple-banana-orange"));
assertThat(result, containsString("cat-dog-bird"));
assertThat(result, containsString("apple-banana-orange"));
}
5. Handle null values while using Joiner
Now – let’s see different ways to handle null values while using Joiner.
To skip null values while joining collection use skipNulls() as in the following example:
@Test
public void whenConvertListToStringAndSkipNull_thenConverted() {
List<String> names = Lists.newArrayList("John", null, "Jane", "Adam", "Tom");
String result = Joiner.on(",").skipNulls().join(names);
assertEquals(result, "John,Jane,Adam,Tom");
}
If you don’t want to skip null values and want to replace them instead, use useForNull() as in the following example:
@Test
public void whenUseForNull_thenUsed() {
List<String> names = Lists.newArrayList("John", null, "Jane", "Adam", "Tom");
String result = Joiner.on(",").useForNull("nameless").join(names);
assertEquals(result, "John,nameless,Jane,Adam,Tom");
}
Note that useForNull() doesn’t change the original list, it only affect the output of the join.
6. Create List from String using Splitter
Now – let’s see how to split a String into a List. In the following example, we use “-” separator to split the input String to List:
@Test
public void whenCreateListFromString_thenCreated() {
String input = "apple - banana - orange";
List<String> result = Splitter.on("-").trimResults()
.splitToList(input);
assertThat(result, contains("apple", "banana", "orange"));
}
Note that trimResults() removes the leading and trailing whitespace from the resulting substrings.
7. Create Map from String using Splitter
Next – let’s see how create Map from String using Splitter. In the following example, we use withKeyValueSeparator() to split a String into a Map:
@Test
public void whenCreateMapFromString_thenCreated() {
String input = "John=first,Adam=second";
Map<String, String> result = Splitter.on(",")
.withKeyValueSeparator("=")
.split(input);
assertEquals("first", result.get("John"));
assertEquals("second", result.get("Adam"));
}
8. Split String with multiple separators
Now – let’s see how to split a String with multiple separators. In the following example, we use both “.” and “,” to split our String:
@Test
public void whenSplitStringOnMultipleSeparator_thenSplit() {
String input = "apple.banana,,orange,,.";
List<String> result = Splitter.onPattern("[.|,]")
.omitEmptyStrings()
.splitToList(input);
assertThat(result, contains("apple", "banana", "orange"));
}
Note that omitEmptyStrings() ignores empty strings and doesn’t add them to the resulting List.
9. Split a String at specific length
Next – let’s take a look on splitting a String at specific length. In the following example, we split our String every 3 characters:
@Test
public void whenSplitStringOnSpecificLength_thenSplit() {
String input = "Hello world";
List<String> result = Splitter.fixedLength(3).splitToList(input);
assertThat(result, contains("Hel", "lo ", "wor", "ld"));
}
10. Limit the split result
Finally – let’s see how to limit the split result. If you want the Splitter to stop splitting after specific number of items – use limit() as in the following example:
@Test
public void whenLimitSplitting_thenLimited() {
String input = "a,b,c,d,e";
List<String> result = Splitter.on(",")
.limit(4)
.splitToList(input);
assertEquals(4, result.size());
assertThat(result, contains("a", "b", "c", "d,e"));
}
11. Conclusion
In this tutorial we illustrated how to use both the Joiner and Splitter in Guava to do a variety of transformations between collections and Strings.
The implementation of all these examples and code snippets can be found in my Guava github project – this is an Eclipse based project, so it should be easy to import and run as it is.
Java 8 is out and about now - to keep track of everything, I will keep updating the Gran Kahuna of Java 8 Resources.