Java: How to read a text file

Question

I want to read a text file containing space separated values. Values are integers. How can I read it and put it in an array list?

Here is an example of contents of the text file:

1 62 4 55 5 6 77

I want to have it in an arraylist as [1, 62, 4, 55, 5, 6, 77]. How can I do it in Java?

Answer 1

Just for fun, here's what I'd probably do in a real project, where I'm already using all my favourite libraries (in this case Guava, formerly known as Google Collections).

String text = Files.toString(new File("textfile.txt"), Charsets.UTF_8);
List<Integer> list = Lists.newArrayList();
for (String s : text.split("\\s")) {
    list.add(Integer.valueOf(s));
}

Benefit: Not much own code to maintain (contrast with e.g. this). Edit: Although it is worth noting that in this case tschaible's Scanner solution doesn't have any more code!

Drawback: you obviously may not want to add new library dependencies just for this. ^{(Then again, you'd be silly not to make use of Guava in your projects. ;-)}

Answer 2

You can use Files#readAllLines() to get all lines of a text file into a List<String>.

for (String line : Files.readAllLines(Paths.get("/path/to/file.txt"))) {
    // ...
}

Tutorial: Basic I/O > File I/O > Reading, Writing and Creating text files

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You can use String#split() to split a String in parts based on a regular expression.

for (String part : line.split("\\s+")) {
    // ...
}

Tutorial: Numbers and Strings > Strings > Manipulating Characters in a String

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You can use Integer#valueOf() to convert a String into an Integer.

Integer i = Integer.valueOf(part);

Tutorial: Numbers and Strings > Strings > Converting between Numbers and Strings

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You can use List#add() to add an element to a List.

numbers.add(i);

Tutorial: Interfaces > The List Interface

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So, in a nutshell (assuming that the file doesn't have empty lines nor trailing/leading whitespace).

List<Integer> numbers = new ArrayList<>();
for (String line : Files.readAllLines(Paths.get("/path/to/file.txt"))) {
    for (String part : line.split("\\s+")) {
        Integer i = Integer.valueOf(part);
        numbers.add(i);
    }
}

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If you happen to be at Java 8 already, then you can even use Stream API for this, starting with Files#lines().

List<Integer> numbers = Files.lines(Paths.get("/path/to/test.txt"))
    .map(line -> line.split("\\s+")).flatMap(Arrays::stream)
    .map(Integer::valueOf)
    .collect(Collectors.toList());

Tutorial: Processing data with Java 8 streams

Answer 3

Using Java 7 to read files with NIO.2

Import these packages:

import java.nio.charset.Charset;
import java.nio.file.Files;
import java.nio.file.Path;
import java.nio.file.Paths;

This is the process to read a file:

Path file = Paths.get("C:\\Java\\file.txt");

if(Files.exists(file) && Files.isReadable(file)) {

    try {
        // File reader
        BufferedReader reader = Files.newBufferedReader(file, Charset.defaultCharset());

        String line;
        // read each line
        while((line = reader.readLine()) != null) {
            System.out.println(line);
            // tokenize each number
            StringTokenizer tokenizer = new StringTokenizer(line, " ");
            while (tokenizer.hasMoreElements()) {
                // parse each integer in file
                int element = Integer.parseInt(tokenizer.nextToken());
            }
        }
        reader.close();
    } catch (Exception e) {
        e.printStackTrace();
    }
}

To read all lines of a file at once:

Path file = Paths.get("C:\\Java\\file.txt");
List<String> lines = Files.readAllLines(file, StandardCharsets.UTF_8);

Answer 4

Look at this example, and try to do your own:

import java.io.*;

public class ReadFile {

    public static void main(String[] args){
        String string = "";
        String file = "textFile.txt";

        // Reading
        try{
            InputStream ips = new FileInputStream(file);
            InputStreamReader ipsr = new InputStreamReader(ips);
            BufferedReader br = new BufferedReader(ipsr);
            String line;
            while ((line = br.readLine()) != null){
                System.out.println(line);
                string += line + "\n";
            }
            br.close();
        }
        catch (Exception e){
            System.out.println(e.toString());
        }

        // Writing
        try {
            FileWriter fw = new FileWriter (file);
            BufferedWriter bw = new BufferedWriter (fw);
            PrintWriter fileOut = new PrintWriter (bw);
                fileOut.println (string+"\n test of read and write !!");
            fileOut.close();
            System.out.println("the file " + file + " is created!");
        }
        catch (Exception e){
            System.out.println(e.toString());
        }
    }
}

Answer 5

read the file and then do whatever you want java8 Files.lines(Paths.get("c://lines.txt")).collect(Collectors.toList());

Answer 6

Java 1.5 introduced the Scanner class for handling input from file and streams.

It is used for getting integers from a file and would look something like this:

List<Integer> integers = new ArrayList<Integer>();
Scanner fileScanner = new Scanner(new File("c:\\file.txt"));
while (fileScanner.hasNextInt()){
   integers.add(fileScanner.nextInt());
}

Check the API though. There are many more options for dealing with different types of input sources, differing delimiters, and differing data types.