Java Reference
In-Depth Information
calls the
toString()
method for the object to produce the string to be written to the stream. The
print()
methods just write the string representation of the argument, whereas the
println()
method appends
\n
to
the output. You can create a
PrintWriter
object from a stream or from another
Writer
object.
An important point note when using a
PrintWriter
object is that its methods do not throw I/O excep-
tions. To determine whether any I/O errors have occurred, you have to call the
checkError()
method for
the
PrintWriter
object. This method returns
true
if an error occurred and
false
otherwise.
The
StringWriter
and
CharArrayWriter
classes are for writing character data to a
StringBuffer
ob-
ject, or an array of type
char[]
. You typically use these to perform data conversions so that the results are
available to you from the underlying array or string. For example, you could combine the capabilities of a
PrintWriter
with a
StringWriter
to obtain a
String
object containing binary data converted to charac-
ters:
StringWriter strWriter = new StringWriter();
PrintWriter writer = new PrintWriter(strWriter);
Now you can use the methods for the
writer
object to write to the
StringBuffer
object underlying the
StringWriter
object:
double value = 2.71828;
writer.println(value);
You can get the result back as a
StringBuffer
object from the original
StringWriter
object:
StringBuffer str = strWriter.getBuffer();
Of course, the formatting done by a
PrintWriter
object does not help make the output line up in
neat columns. If you want that to happen, you have to do it yourself. You see how you might do this for
command-line output a little later in this chapter.
Let's now turn to keyboard input and command-line output.
THE STANDARD STREAMS
Your operating system typically defines three standard streams that are accessible through members of the
System
class in Java:
• A
standard input stream
that usually corresponds to the keyboard by default. This is encapsulated
by the
in
member of the
System
class and is of type
InputStream
.
• A
standard output stream
that corresponds to output on the command line or in the console win-
dow of an IDE. This is encapsulated by the
out
member of the
System
class and is of type
PrintStream
.
• A
standard error output stream
for error messages that usually maps to the command-line output
by default. This is encapsulated by the
err
member of the
System
class and is also of type
PrintStream
.
You can reassign any of these to another stream within a Java application. The
System
class provides the
static methods
setIn()
,
setOut()
, and
setErr()
for this purpose. The
setIn()
method requires an argu-
ment of type
InputStream
that specifies the new source of standard input. The other two methods expect
an argument of type
PrintStream
.
Because the standard input stream is of type
InputStream
, you are not exactly overwhelmed by the cap-
abilities for reading data from the keyboard in Java. Basically, you can read a byte or an array of bytes using
