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Efficient way to get line number of bunch of string from a file

I want to search a list of strings from the file and get the line number. I am looking for an efficient way to do it. I don’t want to search one string at a time and open and close file.

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Answer

Store the line numbers where each String appears in the file using a HashMap<String, List<Integer>>.

This will allow you to find all lines in which a given String appears in a few microseconds.

Here’s one way, if each line is one String:

    Map<String, List<Integer>> index = new HashMap<>();
    LineNumberReader lines = new LineNumberReader(new FileReader("myfile.txt"));
    for (String line = lines.readLine(); line != null; line = lines.readLine()){
        index.computeIfAbsent(line, x -> new ArrayList<>()).add(lines.getLineNumber());
    }

If each line is multiple Strings, change this line

index.computeIfAbsent(line, x -> new ArrayList<>()).add(lines.getLineNumber());

to

Arrays.stream(line.split(" ")).forEach(word ->
   index.computeIfAbsent(word, x -> new ArrayList<>())
       .add(lines.getLineNumber())
);
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