Java Cookbook

Writing Standard Output

Problem

You want your program to write to the standard output.

Solution

Use System.out.

Discussion

Again despite Rusty’s quote, there are circumstances (such as a server program with no connection back to the user’s terminal) in which System.out can become a very important debugging tool (assuming that you can find out what file the server program has redirected standard output into; see Section 9.7).

System.out is a PrintStream, so in every introductory text you see a program containing this line, or one like it:^[24]

System.out.println("Hello World of Java");

The println method is polymorphic; there are forms of it for Object (which obviously calls the given object’s toString( ) method), for String, and for each of the base types (int , float, boolean, etc.). Each takes only one argument, so it is common to use string concatenation:

System.out.println("The answer is " + myAnswer + " at this time.");

Remember that string concatenation is also polymorphic: you can “add” anything at all to a string, and the result is a string.

Up to here I have been using a Stream, System.out. What if you want to use a Writer? The PrintWriter class has all the same methods as PrintStream and a constructor that takes a Stream, so you can just say:

PrintWriter pw = new PrintWriter(System.out);
pw.println("The answer is " + myAnswer + " at this time.");

One caveat with this string concatenation is that if you are appending a bunch of things, and a number and a character come togetherat ...