Java FileReader

last modified March 31, 2024

In this article we shows how to use FileReader class to read text files in Java.

The examples use this text file:

The Battle of Thermopylae was fought between an alliance of
Greek city-states, led by King Leonidas of Sparta, and the Persian Empire of
Xerxes I over the course of three days, during the second Persian invasion of
Greece. 

FileReader is a Java convenience class for reading text files. FileReader extends InputStreamReader and creates the FileInputStream.

The read method

In the following example, we use FileReader's read method to read a text file. It reads characters into an array. It will block until some input is available, an I/O error occurs, or the end of the stream is reached.

import java.io.FileReader;
import java.io.IOException;
import java.nio.charset.StandardCharsets;


void main() throws IOException {

    var fileName = "thermopylae.txt";
    char[] buf = new char[1024];

    try (var fr = new FileReader(fileName, StandardCharsets.UTF_8)) {

        int c;
        while ((c = fr.read(buf)) != -1) {

            System.out.println(buf);
        }
    }
}

The example reads a text file. It uses a character buffer to improve the performance of the operation.

try (var fr = new FileReader(fileName, StandardCharsets.UTF_8)) {

The first argument of the FileReader is the file name. The second is the encoding type. We use try-with-resources construct to clean resources after we have finished writing.

int c;
while ((c = fr.read(buf)) != -1) {

    System.out.println(buf);
}

The FileReader's read method reads characters into an array. It returns the the number of characters read, or -1 if the end of the stream has been reached.

Reading with BufferedReader

FileReader's performance can be improved with BufferedReader. BufferedReader reads text from a character-input stream, buffering characters so as to provide for the efficient reading of characters, arrays, and lines. The buffer size may be specified, or the default size may be accepted; the default is large enough for most purposes.

import java.io.BufferedReader;
import java.io.FileReader;
import java.io.IOException;
import java.nio.charset.StandardCharsets;

void main() throws IOException {

    var fileName = "thermopylae.txt";

    try (var br = new BufferedReader(new FileReader(fileName,
            StandardCharsets.UTF_8))) {

        String line;
        while ((line = br.readLine()) != null) {

            System.out.println(line);
        }
    }
}

In the example, we wrap the FileReader into the BufferedReader to improve its performance.

String line;
while ((line = br.readLine()) != null) {

    System.out.println(line);
}

The readLine reads a line of text. It returns the string containing the contents of the line, not including any line-termination characters, or null if the end of the stream has been reached without reading any characters.

Count word frequency

In the following example, we count the frequency of words in the text file.

import java.io.BufferedReader;
import java.io.FileReader;
import java.io.IOException;
import java.util.HashMap;
import java.nio.charset.StandardCharsets;


void main() throws IOException {

    var fileName = "thermopylae.txt";
    var wordFreq = new HashMap<String, Integer>();

    try (var bufferedReader = new BufferedReader(new FileReader(fileName, 
            StandardCharsets.UTF_8))) {

        String line;
        while ((line = bufferedReader.readLine()) != null) {

            var words = line.split("\s");

            for (var word : words) {

                if (word.endsWith(",") || word.endsWith("?")
                    || word.endsWith("!") || word.endsWith(".")) {

                    word = word.substring(0, word.length()-1);
                }

                if (wordFreq.containsKey(word)) {
                    wordFreq.put(word, wordFreq.get(word) + 1);
                } else {
                    wordFreq.put(word, 1);
                }

            }
        }

        wordFreq.forEach((k, v) -> {
            System.out.printf("%s -> %d%n", k, v);
        });
    }
}

We read the file line by line and split the lines into words. Then we remove any possible commans, question marks, exclamation marks, or dots from the words. The frequencies of words are kept in a hashmap.

Source

Java FileReader - language reference

In this article we have presented the Java FileReader class.

Author

My name is Jan Bodnar and I am a passionate programmer with many years of programming experience. I have been writing programming articles since 2007. So far, I have written over 1400 articles and 8 e-books. I have over eight years of experience in teaching programming.

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