How do I get a platform-dependent new line character?

You can use

System.getProperty("line.separator");

to get the line separator

In addition to the line.separator property, if you are using java 1.5 or later and the String.format (or other formatting methods) you can use %n as in

Calendar c = ...;
String s = String.format("Duke's Birthday: %1$tm %1$te,%1$tY%n", c); 
//Note `%n` at end of line                                  ^^

String s2 = String.format("Use %%n as a platform independent newline.%n"); 
//         %% becomes %        ^^
//                                        and `%n` becomes newline   ^^

See the Java 1.8 API for Formatter for more details.

If you're trying to write a newline to a file, you could simply use BufferedWriter's newLine() method.

The commons-lang library has a constant field available called SystemUtils.LINE_SEPARATOR