Java - Reader Class

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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.