How to write text file in Java? java.io have a number of useful classes for working with files, one of them is FileWriter that we’re going to show in this post. Read on!

In the following program we’re going to generate some data and write them into a CSV file. To write text file in Java use FileWriter. For better performance always wrap it in BufferedWriter like in the example.

The program is using FileWriter in two ways:

1. To create a “highscores.csv” file with generated data
2. To reopen the file for appending to add some more data.

package com.farenda.java.io;

import java.io.BufferedWriter;
import java.io.FileWriter;
import java.io.IOException;
import java.io.Writer;
import java.util.AbstractMap.SimpleImmutableEntry;
import java.util.LinkedHashMap;
import java.util.Map;
import java.util.Map.Entry;
import java.util.Random;

public class WriteTextFileExample {

    private static final Random rand = new Random();

    public static void main(String[] args) throws IOException {
        Map<String,Integer> playersPoints = generateSampleData();

        String scoreFile = "highscores.csv";

        // Create a new file and override when already exists:
        try (Writer output = openWriter(scoreFile)) {
            writeHeader(output);
            for (Entry<String, Integer> player : playersPoints.entrySet()) {
                output.write(formatRow(player));
            }
        }

        Entry<String, Integer> newUser =
                new SimpleImmutableEntry<>("newUser", generatePoints());
        // Reopen the file but for appending:
        try (Writer output = openWriter(scoreFile, true)) {
            output.write(formatRow(newUser));
        }
    }

    private static Map<String,Integer> generateSampleData() {
        Map<String,Integer> playersPoints = new LinkedHashMap<>();
        for (int i = 1; i <= 5; ++i) {
            playersPoints.put("user" + i, generatePoints());
        }
        return playersPoints;
    }

    private static int generatePoints() {
        return rand.nextInt(1000) + 1;
    }

    private static void writeHeader(Writer output) throws IOException {
        output.append("#name,points").append(System.lineSeparator());
    }

    private static BufferedWriter openWriter(String fileName)
            throws IOException {
        return openWriter(fileName, false);
    }

    private static BufferedWriter openWriter(String fileName, boolean append)
            throws IOException {
        // Don't forget to add buffering to have better performance!
        return new BufferedWriter(new FileWriter(fileName, append));
    }

    private static String formatRow(Entry<String, Integer> player) {
        String row = String.format("%s,%s%n",
                player.getKey(), player.getValue());
        System.out.print("Adding row: " + row);
        return row;
    }
}

The result of running the above program:

Adding row: user1,757
Adding row: user2,482
Adding row: user3,155
Adding row: user4,269
Adding row: user5,31
Adding row: newUser,680

And the contents of the created file:

$> cat highscores.csv
#name,points
user1,757
user2,482
user3,155
user4,269
user5,31
newUser,680