Archive:Java File IO

File input/output is quite similar to the System.in and System.out that you already have begun using In Java, files can be represented by the class java.io.File.

Files

Files are another type of variable. You can make a file like this:

File f = new File("test.txt")

This does not return an exception if the file doesn't exist. To check if it does:

f.exists()

This will return a boolean.

To check if it is a file or directory:

f.isFile()
f.isDirectory()

For a full list of methods you can use, go to http://java.sun.com/j2se/1.4.2/docs/api/java/io/File.html

File Output

To write to a file, you can use a FileWriter or a FileOutputStream.

FileWriter

This class writes strings or char[] to a file. To create one:

FileWriter fw = new FileWriter("test.txt");//Create a filewriter writing to test.txt
FileWriter fw = new FileWriter("test.txt", true);//Do not delete contents of file - append

It is as simple to write. After you create a FileWriter just use:

String s = "Testing";
//INITIALIZE FileWriter fw
fw.write(s);

FileWriter can also write in other formats. Using char[]:

Char[] charAr=s.toCharArray();//A method in class String
fw.write(charAr);

Of course, you should only use this when you already have a character array.

File Input

Scanner

Scanner is a class that was made when it was decided by Sun that the java file IO API was too messy. Scanners make file IO ridiculously easy to say the least. To use Scanner, import the class from java.util:

import java.util.Scanner;

The scanner is a sort of wrapper class for a FileInputStream. The constructor of Scanner takes a FileInputStream object as a parameter, like so:

Scanner ourScanner = new Scanner(new FileInputStream("ourfile.txt"));

Scanner includes a number of member methods that make file IO easy. You can use a scanner to get the next item of a certain type from a stream with the methods next(), nextDouble, nextFloat, nextInt, etc. You can also determine if more of a certain type exist with the has*() methods and read and search for regexes.

Some example code:

FileInputStream istream = new FileInputStream(args[0]);
Scanner ourScanner = new Scanner(istream);

while(ourScanner.hasNext())
{ 
     Double tmp = ourScanner.nextDouble();
     System.out.println(tmp+"\n");
}