1. Reading text from Standard Input

try 
{
       BufferedReader in = new BufferedReader(new InputStreamReader(System.in));
       String str = "";
       while (str != null) 
       {
          System.out.print("> some prompt ");
          str = in.readLine();
	  dosomethingwith(str);
       }
} 
catch (IOException e) 
{
}

2. Reading text from a file

try 
{
     BufferedReader in = new BufferedReader(new FileReader("filename"));
     String str;
     while ((str = in.readLine()) != null) 
     {
	dosomethingwith(str);
     }
     in.close();
} 
catch (IOException e) 
{
}

3. Reading a file into a BityArray

    // Returns the contents of the file in a byte array.
    public static byte[] getBytesFromFile(File file) throws IOException 
    {
        InputStream is = new FileInputStream(file);

        // Get the size of the file
        long length = file.length();

        // You cannot create an array using a long type.
        // It needs to be an int type.
        // Before converting to an int type, check
        // to ensure that file is not larger than Integer.MAX_VALUE.
        if (length > Integer.MAX_VALUE) 
	{
            // File is too large
        }

        // Create the byte array to hold the data
        byte[] bytes = new byte[(int)length];

        // Read in the bytes
        int offset = 0;
        int numRead = 0;
        while (offset < bytes.length
               && (numRead=is.read(bytes, offset, bytes.length-offset)) >= 0) 
	{
            offset += numRead;
        }

        // Ensure all the bytes have been read in
        if (offset < bytes.length) 
	{
            throw new IOException("Could not completely read file "+file.getName());
        }

        // Close the input stream and return bytes
        is.close();
        return bytes;

    }

4. Writing to a file

try 
{
    BufferedWriter out = new BufferedWriter(new FileWriter("filename"));
    out.write("some string");
    out.close();
} 
catch (IOException e) 
{
}

Note: If the file does not already exist, it is automatically created.

5. Appending to a file

try 
{
     BufferedWriter out = new BufferedWriter(new FileWriter("filename", true));
     out.write("appending String");
     out.close();
} 
catch (IOException e) 
{
}

6. Using a Random Access File

try 
{
     File f = new File("filename");
     RandomAccessFile raf = new RandomAccessFile(f, "rw");

     // Read a character
     char ch = raf.readChar();

     // Seek to end of file
     raf.seek(f.length());

     // Append to the end
     raf.writeChars("aString");
     raf.close();
} 
catch (IOException e) 
{
}

reference:
http://javaalmanac.com/egs/java.io/pkg.html