BOOK NOTES
CHAPTER 16
FILES AND STREAMS

16.1 READING AND WRITING TEXT FILES

- text file
  - created w/ a simple text editor
  - e.g. Notepad .txt files, Java source, HTML files
- read text
  - construct a java.io.FileReader object w/ name of input file (can throw 
    java.io.FileNotFoundException if file with that name doesn't exist)
    FileReader reader = new FileReader("input.txt");
  - use FileReader to construct a Scanner object
    Scanner in = new Scanner(reader);
  - use Scanner methods to read text from input file
- write text
  - construct a java.io.PrintWriter object with the given file name
    PrintWriter out = new PrintWriter("output.txt");
    - if file already exists, old contents are deleted before new data is added
    - if file doesn't exist, empty file is created
  - use print and println methods to write data
    - convert numbers to their decimal string representations
    - use the toString method to convert objects to strings
  - must close PrintWriter when done; if program exits without closing, 
    output may not make it to file
    out.close();
- backslashes in file names
  - backslash is the escape character in strings
  - when creating a string in code, use the character '\\' to represent a literal 
    backslash

AT 16.1

- java.io.File class
  - object to represent a file
  - constructor
    File(String pathname)
    File inputFile = new File("input.txt");
  - contains methods to delete and rename a file
  - does not have to exist
    - test for existance
      boolean File::exists()
    - pass File object to output stream or writer to create
  - cannot use directly for reading or writing
  - read a File object
    - use the File to construct a FileReader
      FileReader(File file)
      FileReader in = new FileReader(inputFile);

AT 16.2 COMMAND LINE ARGUMENTS

- invoking program from command line: starting program by typing its name in a 
  terminal/shell window
- command line arguments
  - specify additonal input to program on invocation
  - type each one, whitespace delimited, after the program name
  - placed in the args parameter of the main method
- program determines how to interpret input
  - customary
    - strings starting with a hyphen as options
    - other strings as file names
- GUI better for casual and infrequent user
- GUI hard to automate: can use shell scripts to automate command line input

16.2 TEXT AND BINARY FORMATS ***ELIMINATED***

- ways to store data
- text format
  - data items represented as sequence of characters
  - human-readable
  - requires Reader and Writer classes to process IO
  - more convenient for people
    - create with text editor
    - verify by reading
  - read from disk file
    FileReader reader = new FileReader("input.txt");
  - write data to disk
    FileWriter writer = new FileWriter("output.txt");
- binary format
  - data items represented in bytes
    - 8 bits
    - 256 possible values
  - usually represented as 2 hexadecimal digits
  - process IO with InputStream and OutputStream classes (and subclasses)
  - more compact and efficient
  - read from disk
    FileInputStream inputStream = new FileInputStream("input.bin");
  - write data to disk
    FileOutputStream outputStream = new FileOutputStream("output.bin");
- Reader class
  - read method
    - read a single character at a time
    - FileReader overrides to obtain characters from a disk file
    - actually returns an int: signal
      - character has been read: returns integer between 0 and 65, 535
      - end of input has been reached: returns -1
    - test return value, and if not -1, cast to a char
      Reader reader = ...;
      int next = reader.read();
      char c;
      if (next != 1) c = (char) next;
- InputStream class
  - read method
    - read a single byte
    - returns an int
      - byte that was input: 0-255
      - end of input stream: -1
      - test, and if not -1, convert to byte
        InputStream in = ...;
        int next = in.read();
        byte b;
        if (next != -1)
            b = (byte) next;
- Writer class write method writes single char
- FileOutputStream class write method writes single byte
- read and write are only io provided by file io classes
- Java stream package
  - each class has a very focused responsibility
  - must combine with other classes that know how to interpret bytes as Java 
    primitives/objects

CE 16.2 NEGATIVE BYTE VALUES

- cast int to byte: lowest byte of int is taken
- bytes are signed
  - first bit determines if positive or negative
  - 0 is positive, 1 is negative
- means when cast int to byte, sign might not stay the same
- if convert back to int, get byte value (not necessarily the same as original int)

16.3 AN ENCRYPTION PROGRAM

- encryption: scrambles a file so it is unreadable except to those who know the 
  decryption method and secret keyword
- Caeser Cipher
  - shift letters by some amount between 1 and 25
  - decrypt by shifting by -(key)

16.4 RANDOM ACCESS *** ELIMINATED ***

- sequential access
  - starting at the beginning,read all contents of file through the end
  - can be inefficient
- random access
  - read and modify any byte at any location in any order
  - only supported by disk files
- file pointer
  - one per disk file
  - specifies where to access the file
    - reads occur where it points
    - if at end and write, adds data
    - if in middle and write, overwrites data
- java.io.RandomAccessFile
  - access a file
  - move a file pointer
  - RandomAccessFile(String name, String mode)
    - mode
      - "r": reading only 
      - "rw": reading and writing
  - void 	seek(long pos)
    - move file pointer to byte n 
    - counted from beginning of file
  - long 	getFilePointer()
    - get current position of file pointer
    - counted in bytes from beginning of file
- pay special attention to data formatting
  - give each value a fixed size that is sufficiently large
    - since updates are overwrites
    - easily skip to a specific record by multiplying by size of record
  - easier to store numbers in binary format than as text
    - int RandomAccessFile::readInt(): read 4B starting at file pointer
    - void RandomAccessFile::writeInt(int v): overwrite 4B starting at file pointer
    - double RandomAccessFile::readDouble(): read 8B from fp
    - void RandomAccessFile::writeDouble(double v): overwrite 8B at fp

16.5 OBJECT STREAMS *** OPTIONAL ***