Java IO: PrintWriter
Jakob Jenkov |
The Java PrintWriter class (java.io.PrintWriter) enables you to write formatted data to an
underlying Writer. For instance, writing int, long and other primitive data
formatted as text, rather than as their byte values.
The Java PrintWriter is useful if you are generating reports (or similar) where you have to mix text
and numbers. The PrintWriter class has all the same methods as the
PrintStream except for the methods to write raw bytes. Being a
Writer subclass the PrintWriter is intended to write text.
PrintWriter Example
Here is a simple Java PrintWriter example:
FileWriter writer = new FileWriter("d:\\data\\report.txt");
PrintWriter printWriter = new PrintWriter(writer);
printWriter.print(true);
printWriter.print((int) 123);
printWriter.print((float) 123.456);
printWriter.printf(Locale.UK, "Text + data: %1$", 123);
printWriter.close();
This example first creates a PrintWriter instance which is connected to a FileWriter.
Second, the example writes a boolean, an int and a float to the
PrintWriter. Third, the example calls the advanced printf() method of the
PrintWriter which can insert formatted numbers into a text string. Finally the PrintWriter
is closed.
PrintWriter Constructors
The PrintWriter has a wide selection of contructors that enable you to connect it to
a File, an OutputStream, or a Writer. In that way the PrintWriter
is a bit different from other Writer subclasses which tend to have mostly constructors that can
take other Writer instances as parameters (except for a few, like OutputStreamWriter).
print() and format()
The Java PrintWriter class contains the powerful format() and printf()
methods. The two methods do exactly the same, but the name "printf" is more familiar to C-programmers.
The format() and printf() methods allow you to mix text and data in very advanced ways,
using a formatting string. For more information about format() and printf() see
this page:
https://docs.oracle.com/javase/tutorial/java/data/numberformat.html
Closing a PrintWriter
When you are finished writing characters to the Java PrintWriter you should remember to close it.
Closing a PrintWriter will also close the Writer instance to which the
PrintWriter is writing.
Closing a PrintWriter is done by calling its close() method. Here is how
closing a PrintWriter looks:
printWriter.close();
You can also use the try-with-resources construct
introduced in Java 7. Here is how to use and close a PrintWriter looks with the try-with-resources
construct:
FileWriter writer = new FileWriter("data/report.txt");
try(PrintWriter printWriter =
new PrintWriter(writer)){
printWriter.write("Hello World ");
printWriter.write((int)123);
}
Notice how there is no longer any explicit close() method call. The try-with-resources construct
takes care of that.
Notice also that the first FileWriter instance is not created inside
the try-with-resources block. That means that the try-with-resources block will not automatically close this
FileWriter instance. However, when the PrintWriter is closed
it will also close the OutputStream instance it writes to, so the FileWriter
instance will get closed when the PrintWriter is closed.
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Jakob Jenkov | |
