Reading a text file in Java

Question

I want to read a text file containing space sepearted 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

You can use java.io.BufferedReader to read a file line by line.

BufferedReader reader = new BufferedReader(new FileReader("/path/to/file.txt"));
String line = null;
while ((line = reader.readLine()) != null) {
    // ...
}

Tutorial: http://java.sun.com/docs/books/tutorial/essential/io/charstreams.html

---

You can use String#split() to split a String in parts.

String[] parts = line.split("\\s");

Tutorial: http://java.sun.com/docs/books/tutorial/java/data/manipstrings.html

---

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

Integer i = Integer.valueOf(part);

Tutorial: http://java.sun.com/docs/books/tutorial/collections/interfaces/list.html

---

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

list.add(i);

Tutorial: http://java.sun.com/docs/books/tutorial/collections/index.html

Answer 2

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

Its use for getting integers from a file would look something like this.

ArrayList<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 delimeters, and differing data types.

Answer 3

look at this example an 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(fichier); 
            InputStreamReader ipsr=new InputStreamReader(ips);
            BufferedReader br=new BufferedReader(ipsr);
            String line;
            while ((ligne=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 4

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 Commons IO and Google Collections).

String text = FileUtils.readFileToString(new File("textfile.txt"));
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 the library dependencies just for this. ^{(Especially Google Collections just for Lists! Then again, you'd be silly not to make use of Google Collections in your projects. ;-)}