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1. Overview

Java 8 has introduced a new Stream API that lets us process data in a declarative manner.

In this quick article, we would learn how to use the Stream API to split a comma-separated String into a list of Strings and how to join a String array into a comma-separated String.

We’ll also look at how to convert a string array to map using Stream API.

Nearly all of the time we face situations, where we need to iterate some Java Collections and filter the Collection based on some filtering logic. In a traditional approach for this type of situation, we would use lots of loops and if-else operations to get the desired result.

If you want to read more about the Stream API, check this article.

2. Joining Strings With the Stream API

Let’s use the Stream API to create a function which would join a String array into a comma-separated String:

public static String join(String[] arrayOfString){
    return Arrays.asList(arrayOfString)

Points to note here:

  • The stream() function converts any Collection into a stream of data
  • map() function is used to process the data
  • There is also another function, named filter(), where we can include filtering criteria

There can be scenarios, where we may want to join a String with some fixed prefix and postfix. With the Stream API we can do that in the following way:

public static String joinWithPrefixPostfix(String[] arrayOfString){
    return Arrays.asList(arrayOfString)

As we can see in the Collectors.joining() method, we are declaring our prefix as ‘[‘ and postfix as ‘]’; hence the generated String will be created with declared […..] format.

3. Splitting Strings With Stream API

Now, let’s create a function, which would split a comma separated String into a list of String using Stream API:

public static List<String> split(String str){
    return Stream.of(str.split(","))
      .map (elem -> new String(elem))

It’s also possible to directly convert a String to a Character list using the Stream API:

public static List<Character> splitToListOfChar(String str) {
    return str.chars()
      .mapToObj(item -> (char) item)

One interesting fact to note here is that the chars() method converts the String into a stream of Integer where each Integer value denotes the ASCII value of each and every Char sequence. That’s why we need to explicitly typecast the mapper object in the mapToObj() method.

4. String Array to Map With Stream API

We can also convert a String array to map using split and Collectors.toMap, provided each item in the array contains a key-value entity concatenated by a separator:

public static Map<String, String> arrayToMap(String[] arrayOfString) {
	return Arrays.asList(arrayOfString)
	  .map(str -> str.split(":"))
	  .collect(toMap(str -> str[0], str -> str[1]));

Here, “:” is the key-value separator for all the elements in String array.

Please remember that in order to avoid compilation error, we need to ensure that code is compiled using Java 1.8. To do this, we need to add the following plugin in the pom.xml:


5. Testing

Since we are done creating the functions, let’s create test cases to verify the outcome.

First, let’s test our simple joining method:

public void givenArray_transformedToStream_convertToString() {
    String[] programmingLanguages = {"java", "python", "nodejs", "ruby"};
    String expectation = "java,python,nodejs,ruby";

    String result  = JoinerSplitter.join(programmingLanguages);
    assertEquals(result, expectation);

Next, let’s create another one to test our simple splitting functionality:

public void givenString_transformedToStream_convertToList() {
    String programmingLanguages = "java,python,nodejs,ruby";

    List<String> expectation = new ArrayList<>();

    List<String> result  = JoinerSplitter.split(programmingLanguages);

    assertEquals(result, expectation);

Finally, let’s test our String array to map functionality:

public void givenStringArray_transformedToStream_convertToMap() {

    String[] programming_languages = new String[] {"language:java","os:linux","editor:emacs"};
    Map<String,String> expectation=new HashMap<>();
    expectation.put("language", "java");
    expectation.put("os", "linux");
    expectation.put("editor", "emacs");
    Map<String, String> result = JoinerSplitter.arrayToMap(programming_languages);
    assertEquals(result, expectation);

In the same way, we need to create the rest of the test cases.

6. Conclusion

Stream API provides us with sophisticated data processing techniques. This new way of writing code is very efficient in terms of heap memory management in a multi-threaded environment.

Like always, the full source code is available over on Github.

Course – LS – All
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Get started with Spring Boot and with core Spring, through the Learn Spring course:


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