[FIXED] Is there a method for mapping AND reduction in a stream?

Issue

Problem

I want to reduce a stream of MatchResults to a String. Is there a method in the standard-library for this or do I have to write a custom Collector?

Currently, my method looks like this:

public static String pointUnderlineRunnables(String line, String regex) {
    List<MatchResult> matches = Pattern.compile(regex).matcher(line).results() // Finds matches
            .filter(mRes -> ProgramHelper.isRunnableCode(mRes.start(), line))  // Filters out "quoted strings" and comments
            .toList();
    String underline = "";
    for (MatchResult m : matches)
        underline += " ".repeat(m.start() - underline.length()) + "^".repeat(m.end() - m.start());
    return line.strip() + "\n" + underline;
}

But with the proviced mapping methods I cannot write this:

public static String pointUnderlineRunnables(String line, String regex) {
    return line.strip() + "\n" + Pattern.compile(regex).matcher(line).results() //
            .filter(mRes -> ProgramHelper.isRunnableCode(mRes.start(), line)) //
            .reduce("", (subtotal, elem) //
                -> subtotal + " ".repeat(elem.start() - subtotal.length()) + "^".repeat(elem.end() - elem.start());
}

...because both the subtotal and the element have to share the same type. Is there another method that I just don't know, that does this?

Edit: The method underlines all occurences of the regex in the line without a few filtered exceptions.

Sample

Input:

• line: "I love food, food is cool!"
• regex: "food"

Output:

I love food, food is cool!
       ^^^^  ^^^^

Solution

You can use this version of reduce :

public static String pointUnderlineRunnables(String line, String regex) {
    String underline = Pattern.compile(regex)
                              .matcher(line)
                              .results()
                              .filter(mRes -> ProgramHelper.isRunnableCode(mRes.start(), line))
                              .reduce("",
                                      (str,mr) -> str.concat(" ".repeat(mr.start() - str.length()) + "^".repeat(mr.end() - mr.start())),
                                      String::concat);
    return line.strip() + "\n" + underline;
}

To make it somehow readable you could extract the BiFunction:

public static String pointUnderlineRunnables(String line, String regex) {

    BiFunction<String,MatchResult,String> func = (str,mr) ->
            str.concat(" ".repeat(mr.start() - str.length()) + "^".repeat(mr.end() - mr.start()));

    String underline = Pattern.compile(regex)
                              .matcher(line)
                              .results()
                              .filter(mRes -> ProgramHelper.isRunnableCode(mRes.start(), line))
                              .reduce("",func::apply, String::concat);

    return line.strip() + "\n" + underline;
}

Answered By - Eritrean
Answer Checked By - Cary Denson (JavaFixing Admin)