Ok, You've been writing Java code for months, maybe even a couple of years. Objects aren't anything special... they're just the natural way to do things. You don't even need to LOOK at the Servlet API anymore. You might even have a SCJP or SCWCD under your belt.
Then, for the first time in years, it happens: you need to interact with a real, honest-to-god file sitting on the hard drive. Or parse a String into a Date object. And this time, you can't just throw the job at Tomcat or JDBC and let it do the dirty work for you. And to your absolute horror, you realize that you don't have the slightest clue in hell how to do it in Java.
That's right... simple, trivial things like file i/o. Something stupid, like reading a text file into a String. After cursing Gosling and Sun for a half hour for not giving String a constructor that takes a File object as its argument and making things that should be trivially easy to do needlessly complicated [ok, all in unison... 'if ((foo != null) && (foo.equals("whatever")))', vs. 'if (foo == "whatever")' ...], it sinks in: You don't know how to do it. Well, OK, that's not quite fair. You have a general idea. Hell, you did it all the time in Perl and C++. You know it probably has something to do with java.io.File, and following the deprecation chain from java.util.Date will lead you to java.util.Calendar. But the devil's in the details, and trying to figure out how to do it from the javadocs alone isn't exactly the most efficient way to burn an afternoon. Especially since all the nice, convenient methods that let you ignore ugly things like character encoding were deprecated LONG ago. Ditto for date parsing.
OK, so you dig out the old books you haven't touched in months, maybe years, on introductory Java. They ignore the topic completely. File I/O? Date parsing? Ewwwww[...] That's *so* 20th century. Objects, Swing, and j2ee are SO much sexier and profitable to write about. What? You really DO need to soil your hands and do it? Well, you'll have to look elsewhere.
That's where this book comes in. It covers all the non-glamourous stuff that 99% of the books on Java more or less ignore or gloss over. Things like I/O. Text handling. You get the idea. The stuff that everyone wants to just delegate to the servlet container or database, but occasionally you really DO need to deal with directly. There's not really anything in this book that you can't find online. But that's not the point... you can blow an hour or two scavenging the info and experimenting to make it work, or you can get the answer in 2 minutes with this book.
Buy it. BEFORE you need it. You'll be glad you did.