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About Nikos Maravitsas

Nikos Maravitsas
Nikos has graduated from the Department of Informatics and Telecommunications of The National and Kapodistrian University of Athens. During his studies he discovered his interests about software development and he has successfully completed numerous assignments in a variety of fields. Currently, his main interests are system’s security, parallel systems, artificial intelligence, operating systems, system programming, telecommunications, web applications, human – machine interaction and mobile development.

FileReader Java Example

In this example, we are going to see how to use FileReader Java class in order to read characters from a file. As you might remember from a previous FileInpustream Example it is easy to wrap an InputStream, that creates an input byte stream, to a Reader class that bridges a byte stream to a character stream. To be more convenient, Java offers FileReader that directly connects a file to an input character stream, so you can directly read characters from it.

Let’s see some FileReader Java examples in the next chapters.

1. Introduction

The first thing that we must know is what FileReader does. The  FileReader class of the java.io package can be used to read data (in characters) from files. The FileReader extends the InputStreamReader class which extends Reader.

2. Technologies Used

The example code in this article was built and run using:

  • Java 1.8.231(1.8.x will do fine)
  • Eclipse IDE for Enterprise Java Developers-Photon

3. FileReader constructors

There three different FileReader constructors:

  • FileReader(File f): Creates a new FileReader given the File to read from.
  • FileReader(String f): Creates a new FileReader, given the name of the file to read from.
  • FileReader(FileDescriptor f): Creates a new FileReader, given the FileDescriptor to read from.
FileReader Java

4. FileReader methods in Java

There are two FileReader methods which are:

  • public int read(): This method reads a single character and returns an int, which represents the character read.
  • public int read(char[] a, int b, int c): This method reads characters int an array and returns the number of the characters read.

5. FileReader alternatives

Some alternative ways to read text files in Java are:

  • Files.readAllLines(): This method reads all lines from a file and ensures that the file is closed when all bytes have been read or an exception is thrown. The bytes from the file are decoded into characters using the specified charset.
  • Reading text file with Java 8 streaming API:  The  Files.lines() reads all lines from a file as a stream. The bytes from the file are decoded into characters using the StandardCharsets.UTF-8 charset.
  • InputStreamReader: The InputStreamsReader reads bytes and decodes them into characters using a specified charset.
  • FileChannel: The FileChannel is a channel for reading, writing, mapping, and manipulating a file.
  • Scanner: This way can parse primitive types and strings using regular expressions.

6. Simple FileReader Java Examples

Let’s see the first example:

FileReaderExample.java

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package com.javacodegeeks.core.io.outputstream;
 
import java.io.File;
import java.io.FileReader;
import java.io.IOException;
import java.util.Arrays;
 
public class FileReaderExample {
 
    private static final String OUTPUT_FILE = "C:\\Users\\nikos\\Desktop\\TestFiles\\testFile.txt";
    public static void main(String[] args) {
 
        char[] chars = new char[100];
        char[] chars2 = new char[100];
 
        try (FileReader fileReader = new FileReader(new File(OUTPUT_FILE))) {
 
            // read a single bytes
            int r =  fileReader.read();
            System.out.println("Read byte :" +r);
 
            // read a sequence of bytes and store it to a char[] array
            int charsRead = fileReader.read(chars);
            System.out.println("Read bytes : "+charsRead);
            System.out.println(Arrays.toString(chars));
 
            // read a sequence of bytes and store it to arbitrary position to a char[] array
            charsRead = fileReader.read(chars2,5,30);
            System.out.println("Read bytes : "+charsRead);
            System.out.println(Arrays.toString(chars2));
 
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

This is the output:

Read byte :105
Read bytes : 100
[a, s, i, d, a, i, o, s, d, h, i, o, h, a, d, h, a, i, s, h, f, a, i, s, h, f, o, i, a, h, f, o, i, a, s, f, a, i, s, h, f, o, i, h, a, o, s, f, i, h, o, a, s, f, a, s, i, f, h, o, a, h, s, f, o, i, h, a, s, i, o, f, h, a, i, s, h, f, o, a, i, s, h, f, i, o, a, h, f, i, o, a, s, h, f, o, i, a, s, h]
Read bytes : 30
[, , , , , f, o, i, a, h, s, o, i, f, i, a, j, s, f, j, a, i, o, j, f, o, a, j, s, f, o, i, a, j, s,  , , , , , , , , ,  , , , , , , , , , ]

As you can see, it’s very easy to read characters from the file and store it into a char[] array. You can either choose to read a single character, or a sequence of characters, which you can either use to fill up a char array or store it in an arbitrary offset in the char array.

7. Buffering a FileReader Java Example

Of course you can also buffer an FileReader using an BufferedReader. If your application is very I/O intensive and it intends to read large amounts of data from large files, then it’s highly advised to buffer the FileReader. For that, you can use a BufferedReader. This will automatically create an internal buffer and perform as less I/O operations as possible. You can also choose the internal buffer size.

BufferedReader offers a very convenient readLine method that enables to read character streams line by line. I

Let’s see how you can read a text file line by line:

FileReaderExample.java

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package com.javacodegeeks.core.io.outputstream;
 
import java.io.BufferedReader;
import java.io.File;
import java.io.FileReader;
import java.io.IOException;
 
public class FileReaderExample {
 
    private static final String OUTPUT_FILE = "C:\\Users\\nikos\\Desktop\\TestFiles\\testFile.txt";
    public static void main(String[] args) {
 
        String str = "";
        try (BufferedReader bufReader = new BufferedReader(new FileReader(new File(OUTPUT_FILE)),1024)) {
 
            while ( (  str = bufReader.readLine() ) != null )
                System.out.println(str);
 
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

This is the output:

aoifjaiofjaoisfjaoisjaosijfaoisjfoiasjfioasjinoasjcniajna]aifja]fa]sfafa
asfajsifjaoisjfoiiasidaiosdhiohadhaishfaishfoiahfoiasfaishfoihaosfihoasf
asifhoahsfoihasiofhaishfoaishfioahfioashfoiashfoiahsoif
iajsfjaiojfoajsfoiajsoifajsoifjasfjsajfoasjfoiasjfoisajfoia
oiasjfojaoifjaiofjaoisfjaoisjaosijfaoisjfoiasjfioasjinoasjcniajna]aifja]fa]sfafa
iasidaiosdhiohadhaishfaishfoiahfoiasfaishfoihaosfihoasf
asifhoahsfoihasiofhaishfoaishfioahfioashfoiashfoiahsoif
iajsfjaiojfoajsfoiajsoifajsoifjasfjsajfoasjfoiasjfoisajfoia
oiasjfojaoifjaiofjaoisfjaoisjaosijfaoisjfoiasjfioasjinoasjcniajna]aifja]fa]sfafa
asfajsifjaoisjfoiiasidaiosdhiohadhaishfaishfoiahfoiasfaishfoihaosfihoasf
...

8. Download the source code

This was a FileReader Java Example.

Download
You can download the source code of this example here: FileReader Java Example

Last updated on Mar. 16th, 2020

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