Learning Java - Chapter 9 : Java

The program PrintWriterApp, shown below, demonstrates string output, primitive type data output, and the wrapping of streams to gain greater capabilities. We first wrap the System.out object, which is an 8-bit stream, with an OutputStreamWriter to obtain 16-bit streaming.

However, the OutputStreamWriter class (see specifications) provides only a few methods. So we wrap our OutputStreamWriter object with PrintWriter, which offers a full set of print() and println() methods for printing strings and primitive types.

PrintWriterApp.java

import java.io.*;

/**
  * Demonstrate stream wrapping by writing text to the
  * console using the standard output from System.out
  * wrapped with OutputStreamWriter and PrintWriter.
**/
public class PrintWriterApp
{
  public static void main (String arg[]) {

    // The System.out standard output stream is already opened by default.
    // Wrap in a new writer stream to obtain 16 bit capability.
    OutputStreamWriter writer = new OutputStreamWriter (System.out);

    // In turn, wrap the PrintWriter stream around this output stream
    // and use the second argument in the constructor to turn
    // on the autoflush.
    PrintWriter print_writer = new PrintWriter (writer,true);

    // PrintWriter does autoflushing and offers a println
    // method that includes line return.
    print_writer.println ("Text output with PrintWriter.");
    print_writer.println ("Primitives converted to strings:");

    boolean a_boolean =  false;
    byte    a_byte    =  114;
    short   a_short   =  1211;
    int     an_int    =  1234567;
    long    a_long    =  987654321;
    float   a_float   =  983.6f;
    double  a_double  = -4.297e-15;

    print_writer.println (a_boolean);
    print_writer.println (a_byte);
    print_writer.println (a_short);
    print_writer.println (an_int);
    print_writer.println (a_long);
    print_writer.println (a_float);
    print_writer.println (a_double);

    // PrintWriter doesn't throw IOExceptions but instead
    // offers the catchError() method
    if( print_writer.checkError ()) {
        System.out.println ("An output error occurred!" );
    }
  } // main

}// class PrintWriterApp

The output from this class goes as

Note that some wrapper stream classes include a buffer that collects data until it is full and then the whole buffer data set is sent to its destination in one operation. This is more efficient than sending each byte one at a time because of the overhead involved in any transfer.

To insure that data does not remain in a partially filled buffer, you may need to invoke a flush method . With both PrintStream and PrintWriter, you have the option of turning on an auto-flushing switch.

References & Web Resources