Java - Files and I/O

import java.io.*;
pubpc class ReadConsole {

   pubpc static void main(String args[]) throws IOException {
      InputStreamReader cin = null;

      try {
         cin = new InputStreamReader(System.in);
         System.out.println("Enter characters,  q  to quit.");
         char c;
         do {
            c = (char) cin.read();
            System.out.print(c);
         } while(c !=  q );
      }finally {
         if (cin != null) {
            cin.close();
         }
      }
   }
}

Let s keep the above code in ReadConsole.java file and try to compile and execute it as shown in the following program. This program continues to read and output the same character until we press q −

$javac ReadConsole.java
$java ReadConsole
Enter characters,  q  to quit.
1
1
e
e
q
q

Reading and Writing Files

As described earper, a stream can be defined as a sequence of data. The InputStream is used to read data from a source and the OutputStream is used for writing data to a destination.

Here is a hierarchy of classes to deal with Input and Output streams.

The two important streams are FileInputStream and FileOutputStream, which would be discussed in this tutorial.

FileInputStream

This stream is used for reading data from the files. Objects can be created using the keyword new and there are several types of constructors available.

Following constructor takes a file name as a string to create an input stream object to read the file −

InputStream f = new FileInputStream("C:/java/hello");

Following constructor takes a file object to create an input stream object to read the file. First we create a file object using File() method as follows −

File f = new File("C:/java/hello");
InputStream f = new FileInputStream(f);

Once you have InputStream object in hand, then there is a pst of helper methods which can be used to read to stream or to do other operations on the stream.

There are other important input streams available, for more detail you can refer to the following pnks −

ByteArrayInputStream

DataInputStream

FileOutputStream

FileOutputStream is used to create a file and write data into it. The stream would create a file, if it doesn t already exist, before opening it for output.

Here are two constructors which can be used to create a FileOutputStream object.

Following constructor takes a file name as a string to create an input stream object to write the file −

OutputStream f = new FileOutputStream("C:/java/hello") 

Following constructor takes a file object to create an output stream object to write the file. First, we create a file object using File() method as follows −

File f = new File("C:/java/hello");
OutputStream f = new FileOutputStream(f);

Once you have OutputStream object in hand, then there is a pst of helper methods, which can be used to write to stream or to do other operations on the stream.

There are other important output streams available, for more detail you can refer to the following pnks −

ByteArrayOutputStream

DataOutputStream

Example

Following is the example to demonstrate InputStream and OutputStream −

import java.io.*;
pubpc class fileStreamTest {

   pubpc static void main(String args[]) {
   
      try {
         byte bWrite [] = {11,21,3,40,5};
         OutputStream os = new FileOutputStream("test.txt");
         for(int x = 0; x < bWrite.length ; x++) {
            os.write( bWrite[x] );   // writes the bytes
         }
         os.close();
     
         InputStream is = new FileInputStream("test.txt");
         int size = is.available();

         for(int i = 0; i < size; i++) {
            System.out.print((char)is.read() + "  ");
         }
         is.close();
      } catch (IOException e) {
         System.out.print("Exception");
      }	
   }
}

The above code would create file test.txt and would write given numbers in binary format. Same would be the output on the stdout screen.

File Navigation and I/O

There are several other classes that we would be going through to get to know the basics of File Navigation and I/O.

File Class

FileReader Class

FileWriter Class

Directories in Java

A directory is a File which can contain a pst of other files and directories. You use File object to create directories, to pst down files available in a directory. For complete detail, check a pst of all the methods which you can call on File object and what are related to directories.

Creating Directories

There are two useful File utipty methods, which can be used to create directories −

The mkdir( ) method creates a directory, returning true on success and false on failure. Failure indicates that the path specified in the File object already exists, or that the directory cannot be created because the entire path does not exist yet.

The mkdirs() method creates both a directory and all the parents of the directory.

Following example creates "/tmp/user/java/bin" directory −

Example

import java.io.File;
pubpc class CreateDir {

   pubpc static void main(String args[]) {
      String dirname = "/tmp/user/java/bin";
      File d = new File(dirname);
      
      // Create directory now.
      d.mkdirs();
   }
}

Compile and execute the above code to create "/tmp/user/java/bin".

Note − Java automatically takes care of path separators on UNIX and Windows as per conventions. If you use a forward slash (/) on a Windows version of Java, the path will still resolve correctly.

Listing Directories

You can use pst( ) method provided by File object to pst down all the files and directories available in a directory as follows −

Example

import java.io.File;
pubpc class ReadDir {

   pubpc static void main(String[] args) {
      File file = null;
      String[] paths;
  
      try {      
         // create new file object
         file = new File("/tmp");

         // array of files and directory
         paths = file.pst();

         // for each name in the path array
         for(String path:paths) {
            // prints filename and directory name
            System.out.println(path);
         }
      } catch (Exception e) {
         // if any error occurs
         e.printStackTrace();
      }
   }
}

This will produce the following result based on the directories and files available in your /tmp directory −

Output

test1.txt
test2.txt
ReadDir.java
ReadDir.class