Requirement

Given a sentence comprising of one or multiple words separated by space (e.g. java, Hello World, aadsbbaba), return a sorted list of distinct characters excluding space

Solve this problem using Java Stream

Hint: Use String.split(“”)to split a string into a String[], then use String.charAt(0) to convert each single-char string to char

Discussion

This problem tests candidate’s knowledge of the following Stream methods:

• Stream.of(...) or Array.stream(...) to convert an array of single-char string (e.g. from str.split("")) to Stream<String>
• map(str -> str.charAt(0)) to convert String to Character. Note: while String.toCharArray can readily convert a String to an char[], turning the resulting array to a Character stream isn’t straightforward due to lack of built-in support for char type in Stream and Collection API. The Arrays.asList(str.toCharArray() or List.of(...) results in a List<char[]> instead
• distinct() to keep only 1 instance of each encountered character
• sorted() to sort the encountered characters in their natural order
• collect(Collectors.toList()) to collect stream elements into a list

Sample Solution

A sample solution is shown below. In addition to above operations, we only utilize an extra filter(...) operation to discard spaces from getting into our response

public static List<Character> distinctCharsStream2(String sentence) {  
    return Arrays.stream(sentence.split(""))  
        .map(s -> s.charAt(0))  
        .filter(i -> i != ' ')  
        .distinct()  
        .sorted()  
        .collect(Collectors.toList());  
}

Recall again the use of String.split("") and String.charAt(0) instead of String.toCharArray(). Due to the lack of built-in support for char type in Stream and Collection API, Arrays.asList(sentence.toCharArray()) or List.of(sentence.toCharArray()) result in a List<char[]> instead of List<Character> that we need

Extra

Instead of a sentence string, candidate shall be given an list of words.

This modification results in mostly the same solution as the original problem, with 2 twists:

• Spaces are omitted
• Candidate may use flatMap to merge characters from each word into a single Stream<Character>

words.stream()  
    .flatMap(word -> Stream.of(word.split("")))  
    .map(s -> s.charAt(0))  
    ...

A small modification to the sample solution can lead us to the same flatMap(...) usage

return Arrays.stream(sentence.split(" "))  
        .flatMap(word -> Stream.of(word.split("")))  
        ...

Imperative Solution

If we omit the Stream requirement, some candidate may derive an imperative solution using loop and Set

Set<Character> characters = new HashSet<>();  
for (char c : sentence.toCharArray()) {  
    if (c == ' ') continue;  
    characters.add(c);  
}  
List<Character> result = new ArrayList<>(characters);  
result.sort((c1, c2) -> c1 - c2);  
return result;

There is nothing wrong with this approach. To some extend, I think it is even more understandable than the Stream-based one.

But as functional and reactive programming become more and more popular, so does the need to master Stream operations. If there are still time, I would politely ask the candidate to convert his/her codes to Stream.

Sample Codes

Checkout sample solutions and tests in my github