Files

Advantages of files

• Can edit data, prepare ahead of time• Can rerun file without reentering data• Can examine output at leisure • Can display output to screen or print• Can use output as input into another

program– means of communicating between

programs

files

• naming rules(dos) - 1 to 8 character name, optionally followed by a period and up to a 3 character extension

• extension indicates the type of file by convention–  .java – java source code file– .cpp – c++ program file– .doc,.docx – Word file– .exe – executable file– .pdf – Adobe Portable Document File– .txt - Text files(ASCII form, printable)– .dat - data files – attackers can easily change the extension of a file to bypass a security check

Text file

• composed of characters only• .txt• Differs from .doc file• Size of file• Can be created in notepad, wordpad

Accessing file from a Java Program

• Create an object• File class• Pass the name of the file, including path• File f = new File(name);• File f = new File (“c:\\temp\\junk.txt”);

import java.io.*; // for File

public class FileInfo

{

public static void main (String[] args)

{

File f = new File ("hamlet.txt");

System.out.println("exists returns " + f.exists());

System.out.println("can read returns " + f.canRead());

System.out.println("length returns " + f.length());

System.out.println("getAbsolutePath returns " + f.getAbsolutePath());

}

}

Sample Output

No file

exists returns false

can read returns false

length returns 0

getAbsolutePath returns C:\Documents and Settings\csuser\junk.txt

junk.txt: This is a test.

exists returns true

can read returns true

length returns 15

getAbsolutePath returns C:\Documents and Settings\csuser\junk.txt

Method Description

delete() Deletes the given file

exists() Returns whether or not this file exists on the system

getAbsolutePath() Returns the full path where this file is located

getName() Returns the name of this file as a string

isDirectory() Returns whether this file is a directory/folder

isFile() Returns whether this file is a file

length() Returns the number of characters in the file

mkdirs() Creates the directory represented by this file, if it doesn’t exist

renameTo(file) Changes current file name to arg

Reading a file with ScannerScanner <name> = new Scanner(new File("<file name>"));

• Construct a Scanner that reads from the console:• Scanner console = new Scanner(System.in);//keyboard• Construct a Scanner that reads from the file:

– 1. create a File object (or stream)• File f = new FileReader(“junk.txt”);

– 2. create a Scanner object and pass the File object as a parameter

• Scanner inFile = new Scanner(f);

• Or– Scanner inFile = new Scanner(new FileReader(“junk.txt”));

File not found

• Checked exception – exception that must be caught or specifically declared in the header of the method that might generate it

• throws Clause – A declaration that a method will not attempt to handle a particular type of exception

public static void main(String[] args) throws FileNotFoundException {

Scanner inFile = new Scanner(new FileReader(“junk.txt”));

import java.io.*; // for File

import java.util.*; // for Scanner

public class CountWords

{

public static void main (String[] args) throws FileNotFoundException

{

Scanner inFile = new Scanner(new FileReader(“junk.txt”)); //open

String word;

int count = 0;

while (inFile.hasNext()) //check

{

word = inFile.next(); //get

count++; //body

}

System.out.println("total words = " + count);

}

while (inFile.hasNext())

{

word = inFile.next();

count++;

}

extra = inFile.next();

// causes NoSuchElementException

• Error– Scanner inFile = new Scanner(“junk.txt”);

• Must create new file object and Scanner object– File f = new FileReader(“junk.txt”);– Scanner inFile = new Scanner(f);

• Or– Scanner inFile = new Scanner(new File(“junk.txt”));

File i/o:

1. Import the necessary classes from java.util and java.io.

    import java.util.*;import java.io.*;

2. Create and associate the appropriate objects with the input/output sources.Scanner inFile = new Scanner(new FileReader(filename));PrintWriter outFile = new PrintWriter(filename);  

3. Use the appropriate methods associated with the variables created in Step 2 to input/output the data.

some_input_var = inFile.next();    outFile.println(some_output_var);

4. Close the files. Security Rule: Ensure all resources are properly closed when they are no longer needed

inFile.close();     outFile.close();

Example: import java.util.*;import java.io.*;public class FileClass{ public static void main(String [] args) throws FileNotFoundException { Scanner inFile = new Scanner(new FileReader ("c:\\temp\\data.txt")); PrintWriter outFile = new PrintWriter("c:\\temp\\results.txt"); int num1; int num2; int sum; num1 = inFile.nextInt(); num2 = inFile.nextInt(); sum = num1 + num2; outFile.println("The sum is " + sum); inFile.close(); outFile.close( ); }}

Token-based processing

• nextInt• nextDouble• next

Example: import java.util.*;import java.io.*;public class ShowSum{ public static void main(String [] args) throws FileNotFoundException { Scanner inFile = new Scanner(new FileReader (“numbers.dat")); double next; double sum = 0.0; int count = 0; while (inFile.hasNextDouble()) { next = inFile.nextDouble(); count++; sum += next; } System.out.println(“Sum” + sum); inFile.close();}

import java.util.*;import java.io.*;public class ShowSum{ public static void main(String [] args) throws FileNotFoundException { Scanner console = new Scanner(System.in); System.out.println(“What is the file name?”); String fileName = console.nextLine; Scanner inFile = new Scanner(new FileReader(fileName)); double next; double sum = 0.0; int count = 0; while (input.hasNextDouble()) { next = inFile.nextDouble(); count++; sum += next; } System.out.println(“Sum” + sum); inFile.close();}

Input the file name

Line based processingimport java.util.*;import java.io.*;public class EchoUpper{ public static void main(String [] args) throws FileNotFoundException { Scanner inFile = new Scanner(new FileReader(“poem.txt")); String text; while (inFile.hasNextLine()) { text = inFile.nextLine(); System.out.println(text.toUpperCase()); } inFile.close();}

Opening files

• open the files before using any input or output

• Creates object for physical file • Input file

– If the input file does not exist, open is not successful

• Output file– If the output file does not exist, a new file with

that name is created – If the output file already exists, it is erased

import java.io.*;public class HelloF{ public static void main(String [] args) throws

FileNotFoundException { PrintWriter outFile = new PrintWriter(new File (“hello.txt”)); outFile.println(“Hello world”); outFile.println(); outFile.println(“This program produces”); outFile.println(“four lines of output”); outfile.close(); }}

try/ catch

try

{

statements

}

catch (exceptiontype name)

{

statements

}

Replacing throws FileNotFoundException in main

Scanner input;

try

{

input = new Scanner (new FileReader (“numbers.dat”);

}

catch (FileNotFoundException e)

{

System.out.println(“File not found”);

}

Exceptions

• Events that occur during the execution of a program that disrupts the normal flow of instructions.– Exception - an object that represents a program

error. Programs that contain invalid logic cause ("throw") exceptions. • example, trying to read a file that does not exist

– Checked exception - an error that Java forces to be handled in a program; otherwise the program will not compile.

• example (file I/O)

Options: • 1. (catch) Declare that the program will

handle ("catch") the exception try {

inputFile = new Scanner(new FileReader(someFile)); } catch (FileNotFoundException e) {

System.out.println("--- File Not Found! ---"); fileOpened = false;

}

• 2. (throw) states that we choose not to handle the exception, and we accept that our program will crash if an exception occurs

• public static void main(String[] args)throws FileNotFoundException {