Java FileReader

last modified April 16, 2021

Java FileReader tutorial 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. 

Java FileReader

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

Java FileReader example

In the following example, we use FileReader to read a text file.

package com.zetcode;

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

public class FileReaderEx {

    public static void main(String[] args) throws IOException {

        var fileName = "src/resources/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.

Java FileReader 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.

package com.zetcode;

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

public class JavaFileReaderBuffered {

    public static void main(String[] args) throws IOException {

        var fileName = "src/resources/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.

package com.zetcode;

import java.io.BufferedReader;
import java.io.FileReader;
import java.io.IOException;
import java.util.HashMap;

public class WordFrequency {

    public static void main(String[] args) throws IOException {

        var fileName = "src/resources/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.

In this tutorial, we have presented the Java FileReader class.

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