Java: Convert file to byte array

Approach 1: Read entire input stream to ByteArrayOutputStream.

public static byte[] getByteArray(InputStream inputStream) throws IOException {

         ByteArrayOutputStream byteArrayOutputStream = new ByteArrayOutputStream();

         int nRead;

         byte[] data = new byte[16384];

         try {

                  while ((nRead = inputStream.read(data, 0, data.length)) != -1) {

                           byteArrayOutputStream.write(data, 0, nRead);

                  }

         } catch (IOException e) {

                  e.printStackTrace();

                  if (inputStream != null) {

                           inputStream.close();

                  }

         }

         return byteArrayOutputStream.toByteArray();

}

App.java

package com.sample.app;

import java.io.ByteArrayOutputStream;
import java.io.File;
import java.io.FileInputStream;
import java.io.IOException;
import java.io.InputStream;

public class App {

 public static byte[] getByteArray(InputStream inputStream) throws IOException {
  ByteArrayOutputStream byteArrayOutputStream = new ByteArrayOutputStream();

  int nRead;
  byte[] data = new byte[16384];

  try {
   while ((nRead = inputStream.read(data, 0, data.length)) != -1) {
    byteArrayOutputStream.write(data, 0, nRead);
   }
  } catch (IOException e) {
   e.printStackTrace();
   if (inputStream != null) {
    inputStream.close();
   }
  }

  return byteArrayOutputStream.toByteArray();
 }

 public static void main(String args[]) throws IOException {
  FileInputStream fis = new FileInputStream(new File("/Users/krishna/Desktop/TODO.txt"));
  byte[] data = getByteArray(fis);
  
  System.out.println(new String(data));
 }
}

Approach 2: From Java9 onwards, inputstream class provides ‘readAllBytes’ method to get byte array from input stream.

public byte[] readAllBytes​()

public int readNBytes​(byte[] b, int off, int len)

Approach 3: Using DataInputStream.

byte[] bytes = new byte[(int) file.length()];

try (DataInputStream dataInputStream = new DataInputStream(new FileInputStream(file));) {

         dataInputStream.readFully(bytes);

         return bytes;

}

App.java

package com.sample.app;

import java.io.DataInputStream;
import java.io.File;
import java.io.FileInputStream;
import java.io.IOException;

public class App {

 public static byte[] getByteArray(File file) throws IOException {

  byte[] bytes = new byte[(int) file.length()];
  
  try (DataInputStream dataInputStream = new DataInputStream(new FileInputStream(file));) {
   dataInputStream.readFully(bytes);
   return bytes;
  }

 }

 public static void main(String args[]) throws IOException {
  byte[] data = getByteArray(new File("/Users/krishna/Desktop/TODO.txt"));

  System.out.println(new String(data));
 }
}

Approach 4: Using ‘readFully’ method of RandomAccessFile.

RandomAccessFile raf = new RandomAccessFile(file, "r");

byte[] bytes = new byte[(int) raf.length()];

raf.readFully(bytes);

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