println format : Java Glossary

This essay covers both JDK 1.0.2 PrintStream.print and JDK 1.1.1 PrintWriter.print

print and println are methods in PrintStream. They are useful for creating 8-bit ASCII output directly readable by humans. Use a DataOutputStreaminstead if you want compact binary format. JDK 1.1 has classes InputStreamReader and OutputStreamWriter for reading and creating streams of 16-bit Unicode characters.

Java out-the-box contains only the most primitive tools for displaying character output. There are no formatting tools. There are no data input tools other than a stream tokenizer. You have to read characters from the input stream and convert them to binary yourself. In JDK 1.0 you can’t even print out the full precision of a double.

PrintStream.print and PrintStream.println have variants for the various possible primitive types: bytes, char, int etc. They just use an appropriate toString method.

PrintStream versus PrintWriter — UNIX-style Single-Char Newline

Warning! PrintStream.println inserts a single \n, 0x0a, ^J, LineFeed character — even when running on Windows-95. Windows-95 normally marks line ends in text files with \r \n, 0x0d 0x0a, Cr Lf, ^M ^J. If you want the ^M, you must add it yourself!!. The CMP TabIn or TabOut utility will insert the needed \r’s.

In JDK 1.1, PrintStream is deprecated. Use PrintWriter instead. PrintWriter.println is smart enough to emit CrLf (under Windows-95, Windows-98 and NT) and plain Lf (under Unix), however embedded \n characters are never expanded! In JDK 1.2 PrintStream was undeprecated again. I have not yet experimented to find out how it handles line separators.

The expansion is controlled by the lineSeparator system property. See File for how this is defined on various platforms.

static String lineSeparator = System.getProperty ("line.separator" );

Output Formats

print inserts no padding space characters before or after. It just squirts out exactly what toString provides. There are no quotes, commas, trailing nulls or other delimiters inserted.

The output is 8-bit ASCII, not 16-bit Unicode.

Concatentation Gotcha

What do you think this little code snippet produces?

System.out.println( 'A' );
System.out.println( 'A' + 'B' );

You might naively expect: A AB, or perhaps 65 131, however, the answer is: A 131.

The problem is Java’s design blunder of using + to mean both addition and concatentation. Addition also promotes to int, which println displays differently from char.

JDK 1.0 Sample println Output

There are quite a few differences in the output of println between JDK 1.0 and 1.1. Make sure you are looking at the appropriate table.

JDK 1.1 Sample println Output

JDK 1.1 has gives extra precision for float and double, 8 or 9 digits for float and 16 or 17 for double. I suspect the cases where you get an extra significant digit is a bug because the final digit appears to be random. Note also the change in output format for bytes and Unicode chars.