Using PrintWriter in Java

Although using System.out to write to the console is still permissible under Java, its use is recommended mostly for debugging purposes or for sample programs, such as those found in this book. For real-world programs, the recommended method of writing to the console when using Java is through a PrintWriter stream. PrintWriter is one of the character-based classes. Using a character-based class for console output makes it easier to internationalize your program.

PrintWriter defines several constructors. The one we will use is shown here:

PrintWriter(OutputStream outputStream, boolean flushOnNewline)

Here, outputStream is an object of type OutputStream, and flushOnNewline controls whether Java flushes the output stream every time a newline ('\\n') character is output. If flushOnNewline is true, flushing automatically takes place. If false, flushing is not automatic.

PrintWriter supports the print( ) and println( ) methods for all types including Object. Thus, you can use these methods in the same way as they have been used with System.out. If an argument is not a simple type, the PrintWriter methods call the object's toString( ) method and then print the result.

To write to the console by using a PrintWriter, specify System.out for the output stream and flush the stream after each newline. For example, this line of code creates a PrintWriter that is connected to console output:

PrintWriter pw = new PrintWriter(System.out, true);

The following application illustrates using a PrintWriter to handle console output:

// Demonstrate PrintWriter
import java.io.*;
public class PrintWriterDemo {
public static void main(String args[]) {
PrintWriter pw = new PrintWriter(System.out, true);
pw.println("This is a string");
int i = -7;
pw.println(i);
double d = 4.5e-7;
pw.println(d);
}
}

The output from this program is shown here:

This is a string
-7
4.5E-7

Remember, there is nothing wrong with using System.out to write simple text output to the console when you are learning Java or debugging your programs. However, using a PrintWriter will make your real-world applications easier to internationalize. Because no advantage is gained by using a PrintWriter in the sample programs shown in this book, we will continue to use System.out to write to the console.