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APSU Notes


Streams, File I/O, and Networking

Savitch 10.4-10.6

Overview

Objectives

  • Describe the concept of an I/O stream
  • Explain the difference between text and binary files
  • Save data, including objects, in a file
  • Read data, including objects, in a file

10.4 – Basic Binary-File I/O

Creating a Binary File

  • Stream class ObjectOutputStream allows files which can store
    • Values of primitive types
    • Strings
    • Other objects
  • View program which writes integers, listing 10.5 class BinaryOutputDemo

CreatingABinaryFile

  • Note the line to open the file
    • Constructor for ObjectOutputStream cannot take a String parameter
    • Constructor for FileOutputStream can

Writing Primitive Values to a Binary File

  • Method println not available
    • Instead use writeInt Method
    • View in listing 10.5
  • Binary file stores numbers in binary form
    • A sequence of bytes
    • One immediately after another

WritingPrimitiveValuesToABinaryFile

Figure 10.5a

Figure 10.5b

Writing Strings to a Binary File

  • Use method writeUTF
  • Example:


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outputStream.writeUTF("Hi Mom");
  • UTF stands for Unicode Text Format
  • Uses a varying number of bytes to store different strings
    • Depends on length of string
    • Constrast to writeInt which uses same for each

Reading from a Binary File

  • File must be opened as an ObjectInputStream
  • Read from binary file using methods which correspond to write methods
    • Integer written with writeInt will be read with readInt
    • Be careful to read same type as was written

Figure 10.6.a

Figure 10.6.b

ReadingFromABinaryFileOutput

  • View program to read, listing 10.6 class BinaryInputDemo

The Class EOFException

  • Many methods that read form a binary file will throw an EOFException
    • Can be used to test for end of file
    • This can end a reading loop
  • View example program, listing 10.7 class EOFExceptionDemo

EOFExceptionOutput

  • Note the -1 formerly needed as a sentinel value is now also read
  • Always a good idea to check for end of file even if you have a sentinel value

Programming Example: Processing a File of Binary Data

  • Processing a file of binary data
    • Asks user for 2 file names
    • Reads numbers in input file
    • Doubles them
    • Writes them to output file
  • View processing program, listing 10.8 class Doubler

10.5 – Binary-File I/O, Objects, & Arrays

Binary-File I/O with Class Objects

  • Consider the need to write/read objects other than Strings
    • Possible to write the individual instances variables values
    • Then reconstruct the object when file is read
  • A better way is provided by Java
    • Object serialization - represent an object as a sequence of bytes to be written/read
    • Possible for any class implementing Serializable
  • Interface Serializable is an empty interface
    • No need to implement additional methods
    • Tells Java to make the class serializable (class objects convertible to sequence of bytes)
  • View sample class, listing 10.9 class Species
  • Once we have a class that is specified as Serializable we can write objects to a binary file
    • Use method writeObject
  • Read objects with method readObject();
    • Also required to use typecast of the object
  • View sample program, listing 10.10 class ObjectIODemo

ClassObjectsFileOutput

Some Details of Serialization

  • Requirement for a class to be serializable
    • Implements interface Serializable
    • Any instance variables of a class type are also objects of a serializable class
    • Class’s direct superclass (if any) is either serializable or defines a default constructor.
  • Effects of making a class serializable
    • Affects how Java performs I/O with class objects
    • Java assigns a serial number to each object of the class it writes to the ObjectOutputStream
    • If same object written to stream multiple times, only the serial number written after first time.

Array Objects in Binary Files

  • Since an array is an object, possible to use writeObjectwith entire array
    • Similarly use readObjectto read entire array
  • View array I/O program, listing 10.11 class ArrayIODemo

ArrayInBinaryOutput

10.6 – Introduction to Sockets and Networking

  • Classes such as Scanner and PrintWriter can be used with any data system - such as communicating over a network using streams.
  • Java uses sockets
    • A socket consist of the address that identifies the remote computer and a port ranging from 0 to 65535
    • The process of communicating between a client and server is shown in the following figure

Figure 10.7

  • Server Program
    • Listen for a connection on a specified port; when one is made:
      • Create a Scanner with an InputStreamReaderbased on the socket that the server will listen on; use this for input from a client
      • Create a PrintWriter with the socket to send data to the client
    • See listing 10.12
  • Client Program
    • Initiate a connection to the server on a specified port
    • Create a Scanner to read from the socket
    • Create a PrintWriter to send to the socket
    • See Listing 10.13

The URL Class

  • The URL class gives us a simple way to read from a webpage
    • Thanks to polymorphism we can create a Scanner that is linked to a website
    • The example outputs the text from wikipedia 


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URL website = new URL("https://www.wikipedia.org");
Scanner inputStream = new Scanner(new InputStreamReader(website.openStream)));

while (inputStream.hasNextLine())
{
    String s = inputStream.nextLine();
    System.out.println(s);
}
inputStream.close();

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