How to convert a CSV file to List<Map<String,String>>

Question

I have a CSV file which has a header in the first line. I want to convert it to List<Map<String, String>>, where each Map<String, String> in the list represents a record in the file. The key of the map is the header and the value is the actual value of the field. What I have so far:

BufferedReader br = <handle to file>;
// Get the headers to build the map.
String[] headers = br.lines().limit(1).collect(Collectors.toArray(size -> new String[size]));
Stream<String> recordStream = br.lines().skip(1);

What further operations can I perform on recordStream so that I can transform it to List<Map<String, String>>?

Sample CSV file is:

header1,header2,header3   ---- Header line
field11,field12,field13   ----> need to transform to Map where entry would be like header1:field11 header2:field12 and so on.
field21,field22,field23
field31,field32,field33

Finally all these Maps need to be collected to a List.

Could you post a sample CSV file with a sample output? I'm not sure I understand what should happen. — Tunaki
– Tunaki, Commented Oct 13, 2015 at 15:58
You can't skip the first line like that. You need the names of the columns. Don't use streams for this. — Sotirios Delimanolis
– Sotirios Delimanolis, Commented Oct 13, 2015 at 16:05
You can retrieve the header line with an ordinary br.readLine() invocation, followed by processing the remaining lines via br.lines(). stream ops. — Holger
– Holger, Commented Oct 13, 2015 at 16:14

Tunaki · Accepted Answer · 2015-10-13 16:28:00Z

9

The following will work. The header line is retrieved by calling readLine directly on the BufferedReader and by splitting around ,. Then, the rest of the file is read: each line is split around , and mapped to a Map with the corresponding header.

try (BufferedReader br = new BufferedReader(...)) {
    String[] headers = br.readLine().split(",");
    List<Map<String, String>> records = 
            br.lines().map(s -> s.split(","))
                      .map(t -> IntStream.range(0, t.length)
                                         .boxed()
                                         .collect(toMap(i -> headers[i], i -> t[i])))
                      .collect(toList());
    System.out.println(headers);
    System.out.println(records);
};

A very important note here is that BufferedReader.lines() does not return a fresh Stream when it is called: we must not skip 1 line after we read the header since the reader will already have advanced to the next line.

As a side note, I used a try-with-resources construct so that the BufferedReader can be properly closed.

edited Oct 13, 2015 at 16:28

answered Oct 13, 2015 at 16:18

Tunaki

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4 Comments

Vivek Kothari Over a year ago

the i in toMap function needs explicit typecast to Integer. This worked for me .collect(toMap((Integer i) -> headers[i], i -> t[i])))

Tagir Valeev Over a year ago

As a side note I would still recommend to use libraries like Commons CSV to read CSV. CSV format supports quoting which allows having a comma inside the field. The provided implementation would read such files incorrectly.

Tunaki Over a year ago

@VivekKothari Maybe this is a bug of the IDE you are using. The code given in the answer compiles fine with Eclipse Mars and javac 1.8.0_51

Vivek Kothari Over a year ago

I am using eclipse luna and java version "1.8.0_40" Java(TM) SE Runtime Environment (build 1.8.0_40-b25) Java HotSpot(TM) 64-Bit Server VM (build 25.40-b25, mixed mode). But I am getting the error.

Leo · Accepted Answer · 2021-06-05 16:06:22Z

3

I know this is a bit of an old question, but I ran into the same problem, and created a quick sample of the Commons CSV solution mentioned by Tagir Valeev:

            Reader in = new FileReader("path/to/file.csv");
            Iterable<CSVRecord> records = CSVFormat.RFC4180.withFirstRecordAsHeader().parse(in);

            List<Map> listOfMaps = new ArrayList<>();
            for (CSVRecord record : records) {
                listOfMaps.add(record.toMap());
            }

answered Jun 5, 2021 at 16:06

Leo

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Comments

Kel · Accepted Answer · 2025-03-18 09:07:42Z

1

You can use CsvMapper to convert CSV data to List<Map<String,String>>

import com.fasterxml.jackson.databind.MappingIterator;
import com.fasterxml.jackson.dataformat.csv.CsvMapper;
import com.fasterxml.jackson.dataformat.csv.CsvSchema;

public List<Map<String,String>> getCsvData() throws IOException {
    BufferedReader br = new BufferedReader(new InputStreamReader(new FileInputStream("your_path"),"SHIFT-JIS"));
    CsvMapper csvMapper = new CsvMapper();
    CsvSchema csvSchema = csvMapper.typedSchemaFor(Map.class).withHeader();
    MappingIterator<Map<String, String>> it = csvMapper.readerFor(Map.class).with(csvSchema.withColumnSeparator(CsvSchema.DEFAULT_COLUMN_SEPARATOR)).readValues(br);
    return it.readAll();
}

edited Mar 18 at 9:07

answered Mar 18 at 7:26

Kel

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2 Comments

julaine Mar 18 at 8:49

Is it this class? javadoc.io/doc/com.fasterxml.jackson.dataformat/… Your answer could be improved by clarifying/linking the specific class (the java ecosystem often has multiple things with equal or similar names doing similar things in the style of a different decade)

Kel Mar 18 at 9:07

That's right, it's from com.fasterxml.jackson.dataformat.csv

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