How to split strings from streams using Java

To make the code clearer, you could use simple foreach loop:

Stream<String> lines = File.lines(Paths.get("path-to-file");
lines.forEach(s -> s.split(" "));

/**
* Takes a stream, splits group by first part of the string:
*/
public Map<Integer, List<String>> split(Stream<String> a) {

    Map<Integer, List<String>> result = new HashMap<>();

    a.forEach(s -> {
        String[] pair = s.split(" ");

        Integer key = Integer.valueOf(pair[0]);
        String value = pair[1];

        // as 4castle suggested - to avoid unnecessary computation
        result.computeIfAbsent(key, key -> new ArrayList<>());

        result.get(key).add(value);
    });

    return result;
}

Or you can map your input directly in the stream processing:

a.map(s -> s.split(" "))
 .forEach(pair -> {
     Integer key = Integer.valueOf(pair[0]);
     String value = pair[1];

     result.putIfAbsent(key, new ArrayList<>());    
     result.get(key).add(value);
 });

Use Collectors.groupingBy with a downstream collector which gets the second part of the split line before collecting to a list.

Map<Integer, List<String> table =
    Files.lines(Paths.get("path-to-file"))
         .map(line -> line.split(" ", 2))
         .collect(Collectors.groupingBy(
             parts -> Integer.valueOf(parts[0]),
             Collectors.mapping(parts -> parts[1], Collectors.toList())
         ));

The Java streams way, I believe, is:

    Map<Integer, List<String>> parts = lines.map(s -> s.split(" "))
            .collect(Collectors.groupingBy(splitLine -> Integer.valueOf(splitLine[0]),
                    Collectors.mapping(splitLine -> splitLine[1], Collectors.toList())));

This gives you the following map:

{19=[clueweb12-1500wb-39-00002], 20=[clueweb12-1500wb-39-00003], 29=[clueweb12-1500wb-39-00001]}

Its toString method doesn’t give you the most readable output, but I believe it’s the map you asked for. For now there is only one string in each list, but if multiple lines have the same score, there will be more.

You can get in the HashMap like this : Read the file and split it using String Split function and save in into the HashMap key value pair.

public static HashMap<Integer, String>  readFile(String fileName) throws IOException {
    BufferedReader br = new BufferedReader(new FileReader(fileName));
    try {
        HashMap<Integer, String> fileData = new HashMap<>(); 
        String line = br.readLine();

        while (line != null) {
            String[] lineData = line.split(" ");
            System.out.println(lineData[0]+" "+lineData[1]);
            fileData.put(Integer.valueOf(lineData[0]), lineData[1]);
            line = br.readLine();
        }
        return fileData;
    } finally {
        br.close();
    }
}