I can't find the source code for your book XXX, or some part of the code

The code is included with the HTML versions of downloads, which you can find here. More specifically, the main download page for MindView is here, or you can search for a mirror site.

All the packages that begin with "com.sun.java" are part of the JDK that you must first download and install from http://java.sun.com. The JDK is what you use to build and execute Java programs - this is described in the early chapters of "Thinking in Java," so keep reading.

The code packaged with the HTML downloads includes all the code from the respective book. Some people have difficulty finding things like the com.bruceeckel.util package. If this is your question, it's because you haven't read enough in the book to understand how packages are structured as directories in Java. Once you unpack the code, look in the code's root directory: you'll see a subdirectory called com, and below that bruceeckel, and below that util where the file will be found.