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Java Program to download file from URL.

Java Code Example — Utility Programs

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Java Program to download file from URL.

Objective

Write a Java program to download a file from a direct URL using NIO.

Algorithm / Approach

  1. Prompt the user for a File URL.
  2. Extract the filename from the URL string using substring().
  3. Create a URL object and open a ReadableByteChannel from its stream.
  4. Open a FileOutputStream using the extracted filename.
  5. Use fos.getChannel().transferFrom(rbc, 0, Long.MAX_VALUE) to directly stream the data from the internet to the hard drive.
  6. Close the channels.
Download.java 
import java.net.*;
import java.io.*;
import java.nio.*;
import java.nio.channels.*;
import java.util.*;
class Download{
 public static void main(String [] ar){
  System.out.println("DOWNLOAD MANAGER:");
  Scanner sc = new Scanner(System.in);
  System.out.print("FileURL: ");
  String url = sc.nextLine();
  System.out.println("downloading...");
  try {
   int fnsi = url.lastIndexOf('/');
   int end = url.length();
   String fname=url.substring(fnsi+1, end);
   URL ws = new URL(url);
   ReadableByteChannel rbc;
   rbc=Channels.newChannel(ws.openStream());
   FileOutputStream fos;
   fos = new FileOutputStream(fname);
   fos.getChannel().transferFrom(rbc,0,Long.MAX_VALUE);
   fos.close();
   rbc.close();
   System.out.print("Download Completed");
  } 
  catch (IOException e){
   e.printStackTrace();
  }
 }
}

Expected Output

FileURL: http://www.prowessapps.in/java.pdf
downloading...
Download Completed

Explanation of the Program

  • Downloading files via standard Byte Streams requires creating a custom loop and an intermediate byte buffer array.
  • By using Java NIO Channels, specifically the transferFrom() method, you instruct the operating system to handle the stream transfer internally. This is drastically faster and requires much less code than legacy stream downloading.

Complexity

Time Complexity O(n) - Where n is the file size.
Space Complexity O(1) - Handled internally by NIO.
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