File I/O in Java

Content:

a. Reading/Writing to a text file

b. The file class

What are Streams?
Stream is an object that allows flow of data between your program and I/O devices or some files.

• Two kinds of streams:

1. inputStream: if the flow is into the program from (keyboard or file), and the operation is called "Reading from".
2. outputStream: if the flow is out of the program to (screen or file), and the operation is called "Writing to".

• Two kinds of files to deal with:

1. Text files: Files contain sequence of characters when viewed in text editors or read by a program, ex: files contain java program.
   • They also called ASCII files.
   • Can be read by human.
   • Can move from one computer to other.
   • Can read or write to them using text editors.
2. Binary files: Files contain sequence of binary digits.
   • Can be read only by programs.
   • In java, binary files can move from one type of computer to another.
   • Must read or write to them using only by a program.

A. Reading From A text file:

Lord Polonius: What do you read, my lord?

Hamlet: Words, words, words.

The two main stream classes for reading from a text file:

• Scanner
• BufferedReader

a. Using Scanner
Ex:

//Opening an inputStream and convert the file name to FileInputStream object
Scanner inputStream = new Scanner(new FileInputStream("marks.txt"));

        int counter = 0;
        while (inputStream.hasNextInt()) { //Returns true if there is next int in the file
            int number = inputStream.nextInt();  //Scans the next token of the input as an int.
            System.out.println(number);
        }
        inputStream.close();

Output:

1
2
3

• The class Scanner does not care if the stream comes from the the keyboard or from a file.
• The Scanner class has no constructor takes a file name as it's argument (We need to convert the file name to object).
• A richer class to read the files.

b. Using BufferedReader
Ex:

//The program opens the text file
BufferedReader inputStream = new BufferedReader(new FileReader("quots.txt")); //FileReader to convert the file name to object
        String i= inputStream.readLine();
        while (i!=null){
             System.out.println(i);
        i= inputStream.readLine();

        }

Output:

123 hello
1 23d I am on console!

• The BufferedReader class has no constructor takes a file name as it's argument (We need to convert the file name to object).
• There are only two methods to read from the text file: readLine and read.
• The read method reads a single character, and returns an int that corresponds to the character you read.(You need to cast that int to get the character), ex:

char next = (char)inputStram.read();

• To signal the end of the file, the readLine method returns null and read method returns -1.

• Unlike class Scanner, the class BufferedReader has no
  constructor to deal with numbers. You have to read the numbers as a string then convert the string to a numeric type, ex:
  Integer.parseInt(); Double.pareDouble();

• If there are multiple numbers in a single line:

a. Read the line using readLine()
b. Use StringTokenizer to decompose the string into tokens.

B. Writing to a text file:
The class PrintWriter is the preferred class to write to a text file.
Ex:

//Opening the file
     PrintWriter outputStream = new PrintWriter(new FileOutputStream("shahed.txt")); //use FileOutputStream to convert the file name to object

        //writing to the file 
        outputStream.print("this line written from a java program");
        //close the connection
        outputStream.close();

Output:

this line written from a java program

• The program start with empty file and if the file shahed.txt already exist, the old content will be lost.
• To test if the file is already exist, we can use the File class.
• The PrintWriter class has no constructor takes a file name as it's argument (We need to convert the file name to object).
• When the program finish writing, it should close the the stream connected to that file.
• To flush the content of the buffer (temporary location for the data to be written) use the method flush()
• The method close() include an invocation of flush().
• To append new text to old text, then you have to open the file with the following manner:

PrintWriter outputStreamToAppend = new PrintWriter(new FileOutputStream("shahed.txt", true));

In this way the old content will remain and the new content will place after the old content.

• PrintWriter class throw FileNotFoundException and it indicates that file can't be created.

The File class

• The File class constructor takes a string name (Abstract name) and checks the properties if that name, ex exists methods checks if name is exists, and isDirectory() tests if the name the name of a directory. ex:

File file = new File ("Fruits.txt");
if (!file.canRead()){
System.out.println("The file can not be read!")
}

• Some methods in File class:
  • setReadOnly()
  • canWrite()
  • delete()
  • getName()
  • getPath()
  • length()