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File I/O in Java

Shahed Abdulwahab
・3 min read
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. Alt Text

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();
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Output:

1
2
3
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  • 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();

        }

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Output:

123 hello
1 23d I am on console!
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  • 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();
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  • 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();
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Output:

this line written from a java program
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  • 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));
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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!")
}
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  • Some methods in File class:
    • setReadOnly()
    • canWrite()
    • delete()
    • getName()
    • getPath()
    • length()

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