How can I parse List into ArrayList through lambda expression?

Question

Maybe it's quite a stupid question but I cant find any solution for it. I am new to lambda and I am trying to use it to parse a list <List> into an ArrayList <HashMap>.

Initially I parse a data set from csv file, and I turn each row of them into a List, and add them into the List <'List'> I stated above. Since I have to filter them after this, I would like to change the each row of them into a HashMap<columnName, columnValue>, but when I try to do it, it returns nothing.

Here is my code:

// import data from csv file
private static List<List> readWholeFile(BufferedReader br) throws IOException {
    List<List> rows = new ArrayList<>();
    String line = br.readLine();

    try {
        if (line != null) {
            String[] test = line.split(",");
            List<String> row = Arrays.asList((String[]) line.split(","));
            rows.add(row);
            return readWholeFile(br);
        }
    } catch (NullPointerException ex) {
        ex.printStackTrace();
    } finally {
        return rows;
    }
}

//parse List<List> into ArrayList<HashMap>
private static ArrayList<HashMap> putRowIntoMap(List<List> rows, List<String> columns) {
    ArrayList<HashMap> itemMap = new ArrayList<>();
    List<HashMap> test = new ArrayList<>();
    HashMap<String, String> eleMap = new HashMap<>();
    rows.stream().map(row -> (row.stream().map(ele -> eleMap.put(keys.get(row.indexOf(ele)), (String) ele))))
            .collect(Collectors.toCollection(ArrayList::new));
    itemMap.add(eleMap);
    System.out.println(eleMap);  //output: {}

    return itemMap;
}

Please read: [What is a raw type and why shouldn't we use it?](https://stackoverflow.com/questions/2770321/what-is-a-raw-type-and-why-shouldnt-we-use-it) — Turing85, Nov 16 '21 at 21:39
You use overcomplicated _recursive_ solution (instead of a plain loop) when reading a list which is not populated properly as a result. — Nowhere Man, Nov 16 '21 at 22:24

Nowhere Man · Accepted Answer · 2021-11-17T07:44:04.880

Read the input file into List<List<String>> using NIO Files.lines returning lines of the text file as Stream<String>

public static List<List<String>> readFile(String filename) throws IOException {
    return Files.lines(Paths.get(filename)) // Stream<String>
        .map(line -> Arrays.asList(line.split(","))) // Stream<List<String>>
        .collect(Collectors.toList());               // List<List<String>>
}

Convert the list of List<String> into map, assuming that the columnNames have the same indexes as the lists inside rows and all column names are different.

public static List<Map<String, String>> intoMap(List<List<String>> rows, List<String> columnNames) {
    return rows.stream() // Stream<List<String>>
        .map(row -> IntStream.range(0, row.size()) // IntStream
            .boxed() // Stream<Integer>
            .collect(Collectors.toMap(
                i -> columnNames.get(i), // key: columnName
                i -> row.get(i)          // value
                // ,(r1, r2) -> r1,       // placeholder for merge function
                // LinkedHashMap::new    // supplier of LinkedHashMap
            )) // Map<String, String>
        ) // List<Map<String, String>>
        .collect(Collectors.toList());
}

By default, Collectors.toMap returns a HashMap implementation which is not ordered, so it could make sense to use LinkedHashMap maintaining insertion order (if needed, two commented lines in the toMap should be uncommented).

not working for the intoMap, it shows "method collect in interface IntStream cannot be applied to given types; required: Supplier,ObjIntConsumer,BiConsumer found: Collector — AuGuStInE, Nov 17 '21 at 05:16
Added missing `.boxed()`, to convert `IntStream` to `Stream`. it works now — Nowhere Man, Nov 17 '21 at 07:01

How can I parse List into ArrayList through lambda expression?

1 Answers1