Download File input/output

Files
Advantages of files
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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
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.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
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composed of characters only
.txt
Differs from .doc file
Size of file
Can be created in notepad, wordpad
Accessing file from a Java
Program
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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 Scanner
• Construct a Scanner that reads from the console:
• Scanner console = new Scanner(System.in);//keyboard
• Construct a Scanner that reads from the file:
– File f = new File(“junk.txt”);
– Scanner inFile = new Scanner(f);
• Or
– Scanner inFile = new Scanner(new File(“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 File(“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 File(“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 File(“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 File(filename));
PrintStream outFile = new PrintStream(new File(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 File ("c:\\temp\\data.txt"));
PrintStream outFile = new PrintStream(new File("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( );
}
}
If using \ must use \\
Scanner inFile = new Scanner(new File ("c:\\temp\\data.txt"));
PrintStream outFile = new PrintStream("c:\\temp\\results.txt");
Or use /
Scanner inFile = new Scanner(new File ("c:/temp/data.txt"));
PrintStream outFile = new PrintStream("c:/temp/results.txt");
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 File (“numbers.dat")); //start
double next;
double sum = 0.0;
int count = 0;
while (inFile.hasNextDouble()) //check
{
next = inFile.nextDouble(); //get
count++;
//body or action
sum += next;
}
System.out.println(“Sum” + sum);
inFile.close();
}
Input the file name
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 File(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();
}
Line based processing
import java.util.*;
import java.io.*;
public class EchoUpper
{
public static void main(String [] args) throws FileNotFoundException
{
Scanner inFile = new Scanner(new File(“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
{
PrintStream outFile = new PrintStream(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 File (“numbers.dat”);
}
catch (FileNotFoundException e)
{
System.out.println(“File not found”);
}