Java - Reader Class

Introduction

The Java Reader class is a abstract class for reading character streams.

Class declaration

Following is the declaration for Java.io.Reader class

public class Reader
   extends Object
      implements DataOutput, DataInput, Closeable

Field

Following are the fields for Java.io.Reader class

• protected Object lock This is the object used to synchronize operations on this stream.

Class constructors

Class methods

Methods inherited

This class inherits methods from the following classes

• Java.io.Object

Example - Using close() Manually with StringReader

The following example shows the usage of Reader close() method.

ReaderDemo.java

package com.tutorialspoint;

import java.io.IOException;
import java.io.StringReader;

public class ReaderDemo {
   public static void main(String[] args) {
      String data = "This is a sample string.";
      StringReader reader = null;

      try {
         reader = new StringReader(data);

         int ch;
         while ((ch = reader.read()) != -1) {
            System.out.print((char) ch);
         }

      } catch (IOException e) {
         e.printStackTrace();
      } finally {
         if (reader != null) {
            reader.close();
            System.out.println("\nReader closed successfully.");
         }
      }
   }
}

Output

Let us compile and run the above program, this will produce the following result

This is a sample string.
Reader closed successfully.

Explanation

• Opens a file sample.txt for reading.

• Reads characters one by one.

• close() is called in the finally block to ensure it's always executed.

Example - Mark and Reset within the Limit

The following example shows the usage of Reader mark(int readAheadLimit) method.

ReaderDemo.java

package com.tutorialspoint;

import java.io.BufferedReader;
import java.io.StringReader;
import java.io.IOException;

public class ReaderDemo {
   public static void main(String[] args) {
      String data = "ABCDEFG";
      try (BufferedReader reader = new BufferedReader(new StringReader(data))) {

         System.out.print((char) reader.read()); // A
         reader.mark(3); // Mark after reading 'A'

         System.out.print((char) reader.read()); // B
         System.out.print((char) reader.read()); // C

         reader.reset(); // Go back to after 'A'

         System.out.print((char) reader.read()); // B (again)
         System.out.print((char) reader.read()); // C (again)
         System.out.print((char) reader.read()); // D

      } catch (IOException e) {
         e.printStackTrace();
      }
   }
}

Output

Let us compile and run the above program, this will produce the following result

ABCBCD

Explanation

• We mark the position after reading 'A'.

• After reading 'B' and 'C', we reset() to the marked point.

• We re-read 'B' and 'C', and then continue.

Example - Using BufferedReader (Supports Mark)

The following example shows the usage of Reader markSupported() method.

ReaderDemo.java

package com.tutorialspoint;

import java.io.BufferedReader;
import java.io.StringReader;

public class ReaderDemo {
   public static void main(String[] args) {
      String data = "Hello World";
      BufferedReader reader = new BufferedReader(new StringReader(data));

      System.out.println("markSupported(): " + reader.markSupported());

      try {
         reader.mark(5); // This is safe
         System.out.println("Marked at start");

         System.out.print((char) reader.read()); // H
         System.out.print((char) reader.read()); // e

         reader.reset(); // Resets to 'H'
         System.out.print("\nAfter reset: ");
         System.out.print((char) reader.read()); // H again
      } catch (Exception e) {
         e.printStackTrace();
      }
   }
}

Output

Let us compile and run the above program, this will produce the following result

markSupported(): true
Marked at start
He
After reset: H

Explanation

• BufferedReader supports mark() and reset(), so markSupported() returns true.

• We use mark() safely and reset to re-read characters.