• it's almost always a good idea to use buffering (default buffer size is 8K)
• it's often possible to use references to abstract base classes, instead of references to specific concrete classes
• there's always a need to pay attention to exceptions (in particular, IOException and FileNotFoundException)

The close method :

• usually needs to be called, or else resources will leak.
• for streams that don't use the disk or network, such as the ByteArrayXXX streams, the close operation is a no-operation. In these cases, you don't need to call close.
• close will automatically flush the stream, if necessary.
• calling close on a "wrapper" stream will automatically call close on its underlying stream.
• closing a stream a second time is a no-operation.

Commonly used classes when reading and writing binary files :

When dealing with structured data persisted in raw binary form, you should use DataInput and DataOutput streams.

Example

This example reads and write binary data, moving it from disk to memory, and then back again.

import java.io.BufferedInputStream;
import java.io.BufferedOutputStream;
import java.io.ByteArrayOutputStream;
import java.io.File;
import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;

/**  
 Converting binary data into different forms.
 
 <P>Reads binary data into memory, and writes it back out.
 (If your're actually copying a file, there are better ways to do this.)
 
 <P>Buffering is used when reading and writing files, to minimize the number 
 of interactions with the disk.
*/
public final class BytesStreamsAndFiles {

  /** Change these settings before running this class. */
  private static final String INPUT_FILE_NAME = "C:\\TEMP\\cottage.jpg";
  private static final String OUTPUT_FILE_NAME = "C:\\TEMP\\cottage_copy.jpg";

  /** Run the example. */
  public static void main(String... aArgs) {
    BytesStreamsAndFiles test = new BytesStreamsAndFiles();
    //read in the bytes
    byte[] fileContents = test.read(INPUT_FILE_NAME);
    //test.readAlternateImpl(INPUT_FILE_NAME);
    //write it back out to a different file name
    test.write(fileContents, OUTPUT_FILE_NAME);
  }
  
  /** Read the given binary file, and return its contents as a byte array.*/ 
  byte[] read(String aInputFileName){
    log("Reading in binary file named : " + aInputFileName);
    File file = new File(aInputFileName);
    log("File size: " + file.length());
    byte[] result = new byte[(int)file.length()];
    try {
      InputStream input = null;
      try {
        int totalBytesRead = 0;
        input = new BufferedInputStream(new FileInputStream(file));
        while(totalBytesRead < result.length){
          int bytesRemaining = result.length - totalBytesRead;
          //input.read() returns -1, 0, or more :
          int bytesRead = input.read(result, totalBytesRead, bytesRemaining); 
          if (bytesRead > 0){
            totalBytesRead = totalBytesRead + bytesRead;
          }
        }
        /*
         the above style is a bit tricky: it places bytes into the 'result' array; 
         'result' is an output parameter;
         the while loop usually has a single iteration only.
        */
        log("Num bytes read: " + totalBytesRead);
      }
      finally {
        log("Closing input stream.");
        input.close();
      }
    }
    catch (FileNotFoundException ex) {
      log("File not found.");
    }
    catch (IOException ex) {
      log(ex);
    }
    return result;
  }
  
  /**
   Write a byte array to the given file. 
   Writing binary data is significantly simpler than reading it. 
  */
  void write(byte[] aInput, String aOutputFileName){
    log("Writing binary file...");
    try {
      OutputStream output = null;
      try {
        output = new BufferedOutputStream(new FileOutputStream(aOutputFileName));
        output.write(aInput);
      }
      finally {
        output.close();
      }
    }
    catch(FileNotFoundException ex){
      log("File not found.");
    }
    catch(IOException ex){
      log(ex);
    }
  }
  
  /** Read the given binary file, and return its contents as a byte array.*/ 
  byte[] readAlternateImpl(String aInputFileName){
    log("Reading in binary file named : " + aInputFileName);
    File file = new File(aInputFileName);
    log("File size: " + file.length());
    byte[] result = null;
    try {
      InputStream input =  new BufferedInputStream(new FileInputStream(file));
      result = readAndClose(input);
    }
    catch (FileNotFoundException ex){
      log(ex);
    }
    return result;
  }
  
  /**
   Read an input stream, and return it as a byte array.  
   Sometimes the source of bytes is an input stream instead of a file. 
   This implementation closes aInput after it's read.
  */
  byte[] readAndClose(InputStream aInput){
    //carries the data from input to output :    
    byte[] bucket = new byte[32*1024]; 
    ByteArrayOutputStream result = null; 
    try  {
      try {
        //Use buffering? No. Buffering avoids costly access to disk or network;
        //buffering to an in-memory stream makes no sense.
        result = new ByteArrayOutputStream(bucket.length);
        int bytesRead = 0;
        while(bytesRead != -1){
          //aInput.read() returns -1, 0, or more :
          bytesRead = aInput.read(bucket);
          if(bytesRead > 0){
            result.write(bucket, 0, bytesRead);
          }
        }
      }
      finally {
        aInput.close();
        //result.close(); this is a no-operation for ByteArrayOutputStream
      }
    }
    catch (IOException ex){
      log(ex);
    }
    return result.toByteArray();
  }
  
  private static void log(Object aThing){
    System.out.println(String.valueOf(aThing));
  }
}