Readers and Writers in Java

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Readers and Writers and their respective subclasses provide simple I/O for text / character-based data.

BufferedReader

Introduction

The BufferedReader class is a wrapper for other Reader classes that serves two main purposes:

  1. A BufferedReader provides buffering for the wrapped Reader. This allows an application to read charactersmone at a time without undue I/O overheads.
  2. A BufferedReader provides functionality for reading text a line at a time.

Basics of using a BufferedReader

The normal pattern for using a BufferedReader is as follows. First, you obtain the Reader that you want to readMcharacters from. Next you instantiate a BufferedReader that wraps the Reader. Then you read character data. Finally you close the BufferedReader which close the wrapped `Reader. For example:

File someFile = new File(…);
int aCount = 0;
try (FileReader fr = new FileReader(someFile);
     BufferedReader br = new BufferedReader(fr)) {
     // Count the number of 'a' characters.
     int ch;
     while ((ch = br.read()) != -1) {
          if (ch == 'a') {
              aCount++;
          }
     }
     System.out.println("There are " + aCount + " 'a' characters in " + someFile);
}

You can apply this pattern to any Reader

Notes:

  1. We have used Java 7 (or later) try-with-resources to ensure that the underlying reader is always closed. This avoids a potential resource leak. In earlier versions of Java, you would explicitly close the BufferedReader in a finally block.
  2. The code inside the try block is virtually identical to what we would use if we read directly from the FileReader. In fact, a BufferedReader functions exactly like the Reader that it wraps would behave. The difference is that this version is a lot more efficient.
The BufferedReader buffer size
The BufferedReader.readLine() method
Example: reading all lines of a File into a List

This is done by getting each line in a file, and adding it into a List. The list is then returned:

public List getAllLines(String filename) throws IOException {
      List lines = new ArrayList();
      try (BufferedReader br = new BufferedReader(new FileReader(filename))) {
          String line = null;
          while ((line = reader.readLine) != null) {
               lines.add(line);
         }
     }
     return lines;
}

Java 8 provides a more concise way to do this using the lines() method:

public List getAllLines(String filename) throws IOException {
      try (BufferedReader br = new BufferedReader(new FileReader(filename))) {
          return br.lines().collect(Collectors.toList());
          }
          return Collections.empty();
}

StringWriter Example

Java StringWriter class is a character stream that collects output from string buffer, which can be used to construct a string.
The StringWriter class extends the Writer class.

In StringWriter class, system resources like network sockets and files are not used, therefore closing the StringWriter is not necessary. 

import java.io.*;
public class StringWriterDemo {
     public static void main(String[] args) throws IOException {
          char[] ary = new char[1024];
          StringWriter writer = new StringWriter();
          FileInputStream input = null;
          BufferedReader buffer = null;
          input = new FileInputStream("c://stringwriter.txt");
          buffer = new BufferedReader(new  InputStreamReader(input, "UTF-8"));
          int x;
          while ((x = buffer.read(ary)) != -1) {
          writer.write(ary, 0, x);
          }
          System.out.println(writer.toString());
          writer.close();
          buffer.close();
    }
}

The above example helps us to know simple example of StringWriter using BufferedReader to read file data from the stream.

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