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?

Solution 1

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);
    }
}

---

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

Solution 2

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.

Solution 3

This example code shows you how to read file in Java.

import java.io.*;

/**
 * This example code shows you how to read file in Java
 *
 * IN MY CASE RAILWAY IS MY TEXT FILE WHICH I WANT TO DISPLAY YOU CHANGE WITH YOUR   OWN      
 */

 public class ReadFileExample 
 {
    public static void main(String[] args) 
    {
       System.out.println("Reading File from Java code");
       //Name of the file
       String fileName="RAILWAY.txt";
       try{

          //Create object of FileReader
          FileReader inputFile = new FileReader(fileName);

          //Instantiate the BufferedReader Class
          BufferedReader bufferReader = new BufferedReader(inputFile);

          //Variable to hold the one line data
          String line;

          // Read file line by line and print on the console
          while ((line = bufferReader.readLine()) != null)   {
            System.out.println(line);
          }
          //Close the buffer reader
          bufferReader.close();
       }catch(Exception e){
          System.out.println("Error while reading file line by line:" + e.getMessage());                      
       }

     }
  }

Solution 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());
        }
    }
}

Solution 5

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. ;-)}

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Comments

[Jonik]

Of course one could also use transform() & a Function from Google Collections instead of the loop, but IMHO that would be less readable and not even shorter.

[avanderw]

this is by far easier to remember than the buffered, io, reader combination

[Alex Beardsley]

Note that there are better ways to do this in Java 7 and 8: stackoverflow.com/questions/4716503/…