o7planning

Java FileReader Tutorial with Examples

  1. FileReader
  2. Examples
  3. UTF-8 BOM Problem!

1. FileReader

FileReader is a subclass of InputStreamReader, which is used to read text files.
FileReader has no more methods than methods inherited from InputStreamReader, you can actually use InputStreamReader to read characters from any source. However, FileReader is specifically designed to read characters from file system.
FileReader​ constructors
FileReader​(File file)    

FileReader​(FileDescriptor fd)    

FileReader​(File file, Charset charset)    

FileReader​(String fileName)    

FileReader​(String fileName, Charset charset)
Note: Constructors with Charset parameter were added to FileReader since Java 11. So if you are using older Java and want to read a file with specified encoding, let use InputStreamReader class instead.

2. Examples

For example, read a text file:
file-test.txt
File Content
FileReaderEx1.java
package org.o7planning.filereader.ex;

import java.io.File;
import java.io.FileReader;
import java.io.IOException;
import java.net.MalformedURLException;

public class FileReaderEx1 {

    public static void main(String[] args) throws MalformedURLException, IOException {
        File file = new File("file-test.txt");
 
        FileReader fis = new FileReader(file);
        
        int charCode;
        while((charCode = fis.read()) != -1) {
            System.out.println((char)charCode + "  " + charCode);
        }
        fis.close();
    }
}
Output:
F  70
i  105
l  108
e  101
   32
C  67
o  111
n  110
t  116
e  101
n  110
t  116
When reading a text file, you should use a combination of BufferedReader and FileReader for best performance:
students.txt
# Students:

John P
Sarah M
# Sarah B
Charles B
Mary T
Sophia B
FileReaderEx2.java
package org.o7planning.filereader.ex;

import java.io.BufferedReader;
import java.io.File;
import java.io.FileReader;
import java.io.IOException;
import java.io.Reader;
import java.net.MalformedURLException;

public class FileReaderEx2 {

    public static void main(String[] args) throws MalformedURLException, IOException {
        File file = new File("students.txt");
 
        Reader reader = new FileReader(file);
        BufferedReader br = new BufferedReader(reader);
        
        String line;
        while((line = br.readLine()) != null) {
            System.out.println(line);
        }
        br.close();
    }
}
Output:
# Students:

John P
Sarah M
# Sarah B
Charles B
Mary T
Sophia B
Example: Read a text file and print out text lines does not start with character '#' (comment line):
FileReaderEx3.java
package org.o7planning.filereader.ex;

import java.io.BufferedReader;
import java.io.File;
import java.io.FileReader;
import java.io.IOException;
import java.io.Reader;
import java.net.MalformedURLException;

public class FileReaderEx3 {

    public static void main(String[] args) throws MalformedURLException, IOException {
        File file = new File("students.txt");
 
        Reader reader = new FileReader(file);
        BufferedReader br = new BufferedReader(reader);
        
        br.lines() // java.util.stream.Stream
           .filter(line -> !line.startsWith("#")) // Not starts with "#".
           .forEach(System.out::println);
        
        br.close();
    }
}
Output:
John P
Sarah M
Charles B
Mary T
Sophia B

3. UTF-8 BOM Problem!

Before UTF-8 became popular, UTF-8 file creators always added the first 3 bytes to mark that this file was UTF-8 encoded, they are called BOM (Byte Order Mark). While UTF-8 files created by Java do not include BOM.
FileReader does not automatically remove BOM when reading UTF-8 files. Java design team understood this. However, no update was made as it would break previous Java-based third-party libraries such as XML Parser,...
Example, below is a UTF-8 (BOM) file created by an old tool, you can download it to test problem being discussed:
utf8-file-with-bom-test.txt
Hello
Use FileReader to read above file:
FileReader_Utf8_BOM.java
package org.o7planning.filereader.ex;

import java.io.File;
import java.io.FileInputStream;
import java.io.FileReader;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.net.MalformedURLException;
import java.nio.charset.StandardCharsets;

public class FileReader_Utf8_BOM {

    public static void main(String[] args) throws MalformedURLException, IOException {
        File file = new File("utf8-file-with-bom-test.txt");  
        
        System.out.println("--- Read by FileReader ---");
        readByFileReader(file);
        
        System.out.println("--- Read by InputStreamReader ---");
        readByInputStreamReader(file);
    }

    private static void readByFileReader(File file) throws IOException {
        FileReader fr = new FileReader(file, StandardCharsets.UTF_8);

        int charCode;
        while ((charCode = fr.read()) != -1) {
            System.out.println((char) charCode + "  " + charCode);
        }
        fr.close();
    }
    
    private static void readByInputStreamReader(File file) throws IOException {
        InputStream is = new FileInputStream(file);

        InputStreamReader isr = new InputStreamReader(is, StandardCharsets.UTF_8);

        int charCode;
        while ((charCode = isr.read()) != -1) {
            System.out.println((char) charCode + "  " + charCode);
        }
        isr.close();
    }
}
Output:
--- Read by FileReader ---
  65279
H  72
e  101
l  108
l  108
o  111
--- Read by InputStreamReader ---
  65279
H  72
e  101
l  108
l  108
o  111
Character with code 65279 appears in the result, which is an unwanted character.
Some of the following classes support BOM elimination you might consider using:
BOMInputStream
BOMInputStream is a class in Apache Commons IO library that supports BOM removal.
Maven dependency
<!-- https://mvnrepository.com/artifact/commons-io/commons-io -->
<dependency>
    <groupId>commons-io</groupId>
    <artifactId>commons-io</artifactId>
    <version>2.8.0</version>
</dependency>
BOMInputStreamEx1.java
package org.o7planning.filereader.ex;

import java.io.File;
import java.io.FileInputStream;
import java.io.IOException;
import java.io.InputStreamReader;
import java.nio.charset.StandardCharsets;

import org.apache.commons.io.input.BOMInputStream;

public class BOMInputStreamEx1 {

    public static void main(String[] args) throws IOException {
        File file = new File("utf8-file-with-bom-test.txt");
        FileInputStream fis = new FileInputStream(file);

        BOMInputStream bis = new BOMInputStream(fis);

        InputStreamReader isr = new InputStreamReader(bis, StandardCharsets.UTF_8);

        int charCode;
        while ((charCode = isr.read()) != -1) {
            System.out.println((char) charCode + "  " + charCode);
        }
        isr.close();
    }
}
Output:
H  72
e  101
l  108
l  108
o  111
UnicodeReader
UnicodeReader is a class in the "Google Data Java Client Library" that supports BOM removal.
Maven dependency
<!-- https://mvnrepository.com/artifact/com.google.gdata/core -->
<dependency>
    <groupId>com.google.gdata</groupId>
    <artifactId>core</artifactId>
    <version>1.47.1</version>
</dependency>
UnicodeReaderEx1.java
package org.o7planning.filereader.ex;

import java.io.File;
import java.io.FileInputStream;
import java.io.IOException;

import com.google.gdata.util.io.base.UnicodeReader;

public class UnicodeReaderEx1 {

    public static void main(String[] args) throws IOException {
        File file = new File("utf8-file-with-bom-test.txt");
        FileInputStream fis = new FileInputStream(file);
        
        UnicodeReader isr = new UnicodeReader(fis, "UTF-8");

        int charCode;
        while ((charCode = isr.read()) != -1) {
            System.out.println((char) charCode + "  " + charCode);
        }
        isr.close();
    }  
}
Output:
H  72
e  101
l  108
l  108
o  111

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