Operations on Files

Structure of (Computer) Files

Logically a file behaves like an array of characters along with a file pointer

+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+ | H | e | l | l | | W | o | r | l | d | . | . | . | ....... +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+ ^ | file pointer

The file pointer gives the location of the file that file operations will operate on.
The file pointer is set to the start of the file (first character) when the file is opened.

A simple File Access Java class

I have written a Java class called DataFile that allow you to open a file and perform simple operations
The source code is here: click here

Functionality provided by the DataFile class:

public DataFile(String fname) Constructor. Opens the file `fname' so you can read/write the file
public void WriteInt(int x) throws IOException Write the integer x to the file (at the file pointer location) public void WriteDouble(double x) throws IOException Write the double x to the file (at the file pointer location) public void WriteString(String x, int len) throws IOException Write the String x to the file (at the file pointer location) - the string is stored using `len' characters
public int ReadInt() throws IOException Read an integer data from the file (at the file pointer location) public double ReadDouble() throws IOException Read a double data from the file (at the file pointer location) public String ReadString(int maxLen) throws IOException Read a String data from the file (at the file pointer location) - `len' characters will be read to find the string
public void rewind() Reset the file pointer to the start of the file

Opening a file

Opening a computer file:

Example:

DataFile myFile; myFile = new DataFile( "student-file" ); // Open the file named "student-file"

Operations on an opened file

Computer files support two basic operations:
NOTE:

I have tried to made thing easier to understand, and wrote a Java class to read/write 3 types of data:

Integer typed data:

public void WriteInt(int x) throws IOException Write the integer x to the file (at the file pointer location) public int ReadInt() throws IOException Read an integer data from the file (at the file pointer location)

Example:

int x; DataFile myFile = new DataFile( "student-file" );
To write the integer x to the file: myFile.WriteInt( x ); To read an integer from the file into x: x = myFile.ReadInt();

Double typed data:

public void WriteDouble(double x) throws IOException Write the double x to the file (at the file pointer location) public double ReadDouble() throws IOException Read a double data from the file (at the file pointer location)

Example:

double x; DataFile myFile = new DataFile( "student-file" );
To write the double x to the file: myFile.WriteDouble( x ); To read an double from the file into x: x = myFile.ReadDouble();

String typed data:

public void WriteString(String x, int len) throws IOException Write the String x to the file (at the file pointer location) - the string is stored using `len' characters public String ReadString(int maxLen) throws IOException Read a String data from the file (at the file pointer location) - `len' characters will be read to find the string

Example:

String x; DataFile myFile = new DataFile( "student-file" );
To write the String x to the file: myFile.WriteString( x, len ); To read an String from the file into x: x = myFile.ReadDouble( len );

Note:

Because a String can contain any number of characters, you must specify the number of bytes used to store the string using the len parameter

Demo (shows you how to use the methods in the "DataFile" class)

Directory:

cd /home/cs377001/demo/Phys-Data-Dependance/file0

Demo 1: write/read integers to/from a file

writeIntFile.java Writes 4 integers to file "datafile" readIntFile.java Read 4 integers to file "datafile" and print the values as decimal numbers readIntFile2.java Read 4 integers to file "datafile" and print the values as hexa-decimal numbers
Run: java writeIntFile // write 4 ints to "datafile" java readIntFile // read 4 ints from "datafile" and print values dump-bin < datafile // Show content of "datafile" as binary numbers dump-hex < datafile // Show content of "datafile" as hexadec numbers

Demo 2: write/read strings to/from a file

writeCharFile.java Writes 4 strings to file "datafile" readCharFile.java Read 4 strings to file "datafile" and print them out Run: dump-bin < datafile // Show content of "datafile" as binary numbers dump-hex < datafile // Show content of "datafile" as hexadec numbers

Experiment:

Write 4 strings to "datafile" and read (and print) them as integers
Write 4 integers to "datafile" and read (and print) them as strings