I'm not really a Java 2D person, but I contributed a small bugfix to OpenJDK for the miter line join rendering in the Java 2D Rendering Engine:
http://hg.openjdk.java.net/jdk7/2d/jdk/rev/9318628e8eee
What's miter line join? For a quick tutorial on the basics of Java 2D rendering, see this page from the Java tutorial:
http://java.sun.com/docs/books/tutorial/2d/geometry/strokeandfill.html
