Streams

Chapter Goals

• To be able to read and write files
• To become familiar with the concepts of text and binary files
• To be able to read and write objects using serialization
• To be able to process the command line
• To learn about encryption
• To understand when to use sequential and random access files

Two Ways to Store Data

• Text format
  
• Binary format

  note: Classes discussed in this chapter are located in java.io.

Text Format

• Human-readable form
• Sequence of characters
  -Integer 12345 stored as characters "1" "2" "3" "4" "5"
• Use Reader and Writer and their subclasses

Binary Format

• More compact and efficient
• Integer 12345 stored as 00 00 48 57
• Use InputStream and OutputStream and their subclasses

To Read Text Data From a Disk File

• Create a FileReader
• Use its read method to read a single character
  -returns the next char as an int or the integer -1 at end of input
• Test for -1
• If not -1, cast to char
• Close the file when done

Code to Read a Character From Disk

FileReader reader = new FileReader ("input.txt");
int next = reader.read()
char c;
if (next != -1)
   c = (char)next();

   ...

reader.close()

Code to Write a Character to Disk

FileWriter writer = new FileWriter("output.txt");
  ...
char c='';
  ...
writer.write(c);
  ...
write.close();

To Read Binary Data From a Disk File

• Create a FileInputStream
• Use its read method to read a single byte
  -returns the next byte as an int or the integer -1 at end of input
• Test for -1
• If not -1, cast to byte
• Close the file when done

Code to Read a Byte From Disk

FileInputStream inputStream = new FileInputStream("input.dat");
int next = inputStream.read();
byte b;
if (next != -1)
   b = (byte)next;

      ...

inputStream.close();

Code to Write a Byte to Disk

FileOutputStream output = new FileOutputStream("output.txt");
  ...
byte b = 0;
  ...
output.write(b);
  ...
write.close();

Writing Text Files

• Use a PrintWriter
  -breaks up numbers and strings into individual characters
  -sends them one at a time to a FileWriter
• Code to create a PrintWriter

  FileWriter writer = new FileWriter("output.txt");
  PrintWriter out = new PrintWriter(writer);

• Use print and println to print numbers, objects, or strings

  out.println(29.95);
  out.println(new Rectangle(5,10,15,25));
  out.println("Hello, World!");
  

Reading Text Files

• Use a BufferedReader
  -Reads a character at a time from a FileReader
  -Assembles the characters into a line and returns it
• Use Integer.parseInt or Integer.parseDouble to convert the strings to numbers
• Code

  FileReader reader = new FileReader ("input.txt");
  BufferedReader in = new BufferedReader(reader);
  String inputLine = in.ReadLine();
  double x = Double.parseDouble(inputLine);
  

File Class

• File class describes disk files and directories
• Create a File object
  

  File inputFile = new File("input.txt");

• Some file methods

  delete
  renameTo
  exists

• Constructing a FileReader from a File object

  FileReader reader = new FileReader(inputFile) ;

File Dialogs

• Construct a file chooser object
• Call its showOpenDialog or showSaveDialog method
• Specify null or the user interface component over which to pop up the dialog
• If the user chooses a file, these methods return JFileChooser.APPROVE_OPTION
• If the user cancels the selection, these methods return JFileChooser.CANCEL_OPTION
• If a file is chosen, use GetSelectedFile method to obtain a File object describing the file

A JFileChooser Dialog

Code to Use a JFileChooser

JFileChooser chooser new JFileChooser();
FileReader in = null;
if (chooser.showOpenDialog(null) == JFileChooser.APPROVE_OPTION)
{
    File selectedFile = chooser.getSelectedFile();
    in = new FileReader(selectedFile);
}

  note: JFileChooser is in java.io

Encryption

To encrypt a file means to scramble it so that it is readable only to those who know the encryption method and the secret keyword.

• Choose an encryption key - a number between 1 and 25
• If the key is 3, replace A with D, B with E
• To decrypt, use the negative of the encryption key

Caesar Cipher

To Encrypt Binary Data

int next - in.read();
if (next == -1)
   done = true;
else
{
   byte b = (byte)next;
   byte c = encrypt(b);    //call the method to encrypt the byte
   out.write(c)
}

File Encryptor.java

File EncrytorTest.java

Command Line Arguments

• Launch a program from the command line
• Specify arguments after the program name
• The program accesses these strings by processing the args parameter of the main method

Example of Command Line Arguments

class MyProgram
{
   public static void main(String[] args)
   {
      ...
   }
}

File Crypt.java

Object Streams

• ObjectOutputStream class can save entire objects to disk
• ObjectInputStream class can read objects back in from disk
• Objects are saved in binary format; hence, you use streams

Writing a Coin Object to a File

Coin c = ...;
ObjectOutputStream out = new ObjectOutputStream(new FileOutputStream("coins.dat"));
out.writeObject(c);

Reading a Coin Object from a File

ObjectInputStream in = new ObjectInputStream(new FilInputStream("coins.dat"));
Coin c = (Coin)in.readObject();

• readObject method can throw a ClassCastException
• It is a checked exception
• You must catch or declare it

Write and Read an ArrayList to a File

ArrayList a = new ArrayList();
//add many objects to the array list
out.writeObject(a);

ArrayList a = (ArrayList)in.readObject();

Serializable

• Objects that are written to an object stream must belong to a class that implements the Serializable interface.
• Serializable interface has no methods.

class Coin implements Serializable
{
   ...
}

File PurseTest.java

Random and Sequential Access

• Sequential access

In sequential file access, a file is processed a byte at a time.

• Random access
  

Random access allows access at arbitrary locations in the file

RandomAccessFile

• To open a random-access file for reading and writing

RandomAccessFile f = new RandomAcessFile("bank.dat","rw");

• To move the file pointer to a specific byte

f.seek(n);

• To get the current position of the file pointer.

long n = f.getFilePointer();

• To find the number of bytes in a file

long fileLength = f.length();

File BankDataTest.java

File BankData.java