More Library Puzzlers - Java Puzzlers: Traps, Pitfalls, and Corner Cases - page 248

< Day Day Up >
Puzzle 81: Charred Beyond Recognition
This program appears to do the usual thing in an unusual way. What does it print?
public class Greeter {
public static void main (String[] args) {
String greeting = "Hello world";
for (int i = 0; i < greeting.length(); i++)
System.out.write(greeting.charAt(i));
}
}
Solution 81: Charred Beyond Recognition
Although it's a bit strange, there is little reason to suspect that this program should misbehave. It
writes "Hello world" to System.out , one character at a time. You may be aware that the write
method uses only the low-order byte of its input parameter. This would cause trouble if "Hello
world" contained any exotic characters, but it doesn't: It consists entirely of ASCII characters.
Whether you print it one character at a time or all at once, the result should be the same: The
program should print Hello world . Yet, if you ran it, it almost certainly printed nothing. Where did
the greeting go? Perhaps the program just wasn't feeling all that cheerful?
The problem is that System.out is buffered. The characters in Hello world were written to the
buffer for System.out , but the buffer was never flushed. Most programmers believe that
System.out and System.err flush themselves automatically whenever output is performed. This is
almost true but not quite. They are of type PrintStream , whose documentation says, as of release
5.0 [Java-API] :
A PrintStream can be created so as to flush automatically; this means that the flush method
is automatically invoked after a byte array is written, one of the println methods is invoked,
or a newline character or byte (' \n ') is written.
The streams referenced by System.out and System.err are indeed instances of the automatically
flushing variant of PrintStream , but no mention is made of the write(int) method in the above
documentation. The documentation for write(int) says: "Write the specified byte to this stream. If
the byte is a newline and automatic flushing is enabled then the flush method will be invoked"