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Java Program to read data from File using NIO.

Java Code Example — Input/Output Programs

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Java Program to read data from File using NIO.

Objective

Write a Java program to read data from a file using Java NIO Channels.

Algorithm / Approach

  1. Ask the user for the name of the file to read.
  2. Open a FileInputStream and extract its ReadableByteChannel.
  3. Allocate a fixed-size ByteBuffer (e.g., 1024 bytes).
  4. Use a do-while loop to read chunks of data from the channel into the buffer: count = chan.read(buff).
  5. If bytes were read, call buff.rewind(), loop through the buffer, cast each byte to a char, and print it.
  6. Continue looping until read() returns -1 (End of File).
ReadFile.java 
import java.io.*;
import java.nio.*;
import java.util.*;
import java.nio.channels.*;
class ReadFile {
 public static void main(String[] a)
 {
  FileInputStream fis = null;
  int count = 0;
  ReadableByteChannel chan =null;
  Scanner s=new Scanner(System.in);
  System.out.print("Enter File: ");
  String file = s.nextLine();
  ByteBuffer buff = null;
  try{
   fis=new FileInputStream(file);
   chan = fis.getChannel();
   buff = ByteBuffer.allocate(1024);  
   do {    
    count = chan.read(buff);
    if(count != -1) {  
     buff.rewind();
     for(int i=0; i < count; i++)
      System.out.print((char)buff.get());
    }
   } while(count != -1);
  }
  catch(Exception e) {
   System.out.print(e);
  }
  finally {
   try {
    fis.close();
    chan.close();
   }
   catch(Exception e) { 
    System.out.print(e);
   }
  }
 }
}

Expected Output

Enter File: Alok.txt
Java Prowess is one of the largest application for java

Explanation of the Program

  • Reading with NIO is the exact reverse of writing.
  • We use a loop because we don't know how large the file is. The 1024-byte Buffer acts as a bucket. We scoop up to 1024 bytes from the file channel, process them, and then scoop the next batch.
  • The method chan.read() returns the number of bytes successfully scooped. Once the file is empty, it returns -1, signaling us to stop the loop and close our resources in the finally block.

Complexity

Time Complexity O(n) - Where n is the number of bytes in the file.
Space Complexity O(1) - Constant buffer size (1024 bytes) regardless of file size.
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