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5.9. Input/Output with java.io

The java.io package defines a large number of classes for reading and writing streaming, or sequential, data. The InputStream and OutputStream classes are for reading and writing streams of bytes while the Reader and Writer classes are for reading and writing streams of characters. Streams can be nested, meaning you might read characters from a FilterReader object that reads and processes characters from an underlying Reader stream. This underlying Reader stream might read bytes from an InputStream and convert them to characters.

5.9.1. Reading Console Input

You can perform a number of common operations with streams. One is to read lines of input the user types at the console:

import java.io.*;

BufferedReader console = new BufferedReader(new InputStreamReader(System.in));
System.out.print("What is your name: ");
String name = null;
try { 
  name = console.readLine(); 
} 
catch (IOException e) { name = "<" + e + ">"; }  // This should never happen
System.out.println("Hello " + name);

5.9.2. Reading Lines from a Text File

Reading lines of text from a file is a similar operation. The following code reads an entire text file and quits when it reaches the end:

String filename = System.getProperty("user.home") + File.separator + ".cshrc";
try {
  BufferedReader in = new BufferedReader(new FileReader(filename));
  String line;
  while((line = in.readLine()) != null) {  // Read line, check for end-of-file
    System.out.println(line);              // Print the line
  }
  in.close();    // Always close a stream when you are done with it
}
catch (IOException e) {
  // Handle FileNotFoundException, etc. here
}

5.9.3. Writing Text to a File

Throughout this book, you've seen the use of the System.out.println() method to display text on the console. System.out simply refers to an output stream. You can print text to any output stream using similar techniques. The following code shows how to output text to a file:

try {
  File f = new File(homedir, ".config");
  PrintWriter out = new PrintWriter(new FileWriter(f));
  out.println("## Automatically generated config file. DO NOT EDIT!");
  out.close();  // We're done writing
}
catch (IOException e) { /* Handle exceptions */ }

5.9.4. Reading a Binary File

Not all files contain text, however. The following lines of code treat a file as a stream of bytes and read the bytes into a large array:

try {
  File f;                             // File to read; initialized elsewhere
  int filesize = (int) f.length();    // Figure out the file size
  byte[] data = new byte[filesize];   // Create an array that is big enough
  // Create a stream to read the file
  DataInputStream in = new DataInputStream(new FileInputStream(f));
  in.readFully(data);  // Read file contents into array
  in.close();
}
catch (IOException e) { /* Handle exceptions */ }

5.9.5. Compressing Data

Various other packages of the Java platform define specialized stream classes that operate on streaming data in some useful way. The following code shows how to use stream classes from java.util.zip to compute a checksum of data and then compress the data while writing it to a file:

import java.io.*;
import java.util.zip.*;

try {
  File f;                         // File to write to; initialized elsewhere
  byte[] data;                    // Data to write; initialized elsewhere
  Checksum check = new Adler32(); // An object to compute a simple checksum

  // Create a stream that writes bytes to the file f
  FileOutputStream fos = new FileOutputStream(f);
  // Create a stream that compresses bytes and writes them to fos
  GZIPOutputStream gzos = new GZIPOutputStream(fos);
  // Create a stream that computes a checksum on the bytes it writes to gzos
  CheckedOutputStream cos = new CheckedOutputStream(gzos, check);  

  cos.write(data);             // Now write the data to the nested streams
  cos.close();                 // Close down the nested chain of streams
  long sum = check.getValue(); // Obtain the computed checksum
}
catch (IOException e) { /* Handle exceptions */ }

5.9.6. Reading ZIP Files

The java.util.zip package also contains a ZipFile class that gives you random access to the entries of a ZIP archive and allows you to read those entries through a stream:

import java.io.*;
import java.util.zip.*;

String filename;   // File to read; initialized elsewhere
String entryname;  // Entry to read from the ZIP file; initialized elsewhere
ZipFile zipfile = new ZipFile(filename);        // Open the ZIP file
ZipEntry entry = zipfile.getEntry(entryname);   // Get one entry
InputStream in = zipfile.getInputStream(entry); // A stream to read the entry
BufferedInputStream bis = new BufferedInputStream(in); // Improves efficiency
// Now read bytes from bis... 
// Print out contents of the ZIP file
for(java.util.Enumeration e = zipfile.entries(); e.hasMoreElements();) {
  ZipEntry zipentry = (ZipEntry) e.nextElement();
  System.out.println(zipentry.getName());
}

5.9.7. Computing Message Digests

If you need to compute a cryptographic-strength checksum (also known as a message digest), use one of the stream classes of the java.security package. For example:

import java.io.*;
import java.security.*;
import java.util.*;


File f;          // File to read and compute digest on; initialized elsewhere
List text = new ArrayList();  // We'll store the lines of text here

// Get an object that can compute an SHA message digest
MessageDigest digester = MessageDigest.getInstance("SHA");
// A stream to read bytes from the file f
FileInputStream fis = new FileInputStream(f);
// A stream that reads bytes from fis and computes an SHA message digest
DigestInputStream dis = new DigestInputStream(fis, digester);
// A stream that reads bytes from dis and converts them to characters
InputStreamReader isr = new InputStreamReader(dis);
// A stream that can read a line at a time
BufferedReader br = new BufferedReader(isr);
// Now read lines from the stream
for(String line; (line = br.readLine()) != null; text.add(line)) ;
// Close the streams
br.close();
// Get the message digest
byte[] digest = digester.digest();

5.9.8. Streaming Data to and from Arrays

So far, we've used a variety of stream classes to manipulate streaming data, but the data itself ultimately comes from a file or is written to the console. The java.io package defines other stream classes that can read data from and write data to arrays of bytes or strings of text:

import java.io.*;

// Set up a stream that uses a byte array as its destination
ByteArrayOutputStream baos = new ByteArrayOutputStream();
DataOutputStream out = new DataOutputStream(baos);
out.writeUTF("hello");          // Write some string data out as bytes
out.writeDouble(Math.PI);       // Write a floating-point value out as bytes
byte[] data = baos.toByteArray(); // Get the array of bytes we've written
out.close();                      // Close the streams

// Set up a stream to read characters from a string
Reader in = new StringReader("Now is the time!");
// Read characters from it until we reach the end
int c;
while((c = in.read()) != -1) System.out.print((char) c);

Other classes that operate this way include ByteArrayInputStream, StringWriter, CharArrayReader, and CharArrayWriter.

5.9.9. Thread Communication with Pipes

PipedInputStream and PipedOutputStream and their character-based counterparts, PipedReader and PipedWriter, are another interesting set of streams defined by java.io. These streams are used in pairs by two threads that want to communicate. One thread writes bytes to a PipedOutputStream or characters to a PipedWriter, and another thread reads bytes or characters from the corresponding PipedInputStream or PipedReader:

// A pair of connected piped I/O streams forms a pipe. One thread writes
// bytes to the PipedOutputStream, and another thread reads them from the
// corresponding PipedInputStream. Or use PipedWriter/PipedReader for chars. 
final PipedOutputStream writeEndOfPipe = new PipedOutputStream();
final PipedInputStream readEndOfPipe = new PipedInputStream(writeEndOfPipe);

// This thread reads bytes from the pipe and discards them
Thread devnull = new Thread(new Runnable() {
  public void run() { 
    try { while(readEndOfPipe.read() != -1); }
    catch (IOException e) {}  // ignore it
  }
});
devnull.start();

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