This week's book giveaway is in the OCAJP 8 forum. We're giving away four copies of OCA Java SE 8 Programmer I Study Guide and have Edward Finegan & Robert Liguori on-line! See this thread for details.
You saved the .java file, the source file. However it's possible that the .class file did not get updated. Depending on how you're compiling, this can happen easily when using Java. Unfortunately. I recommend you look for a file named FamilyTree.class, and look at its timestamp. Compare it to FamilyTree.java. Is it newer, or older? Should be newer, but if it's older, you've found the problem. Now even if it's newer - is it possible that somewhere on your system you've got another FamilyTree.class file, an old one that you've forgotten about?
Check your class path to see all the places the JVM could be looking for class files. I recommend you don't even use the CLASSPATH system variable; instead just use the java -cp option to specify the classpath when you compile. Don't let the classpath accumulate a bunch of stuff - keep it short, limited to places that have code that you actually need for the current program. That will make it less likely in the fiture that you might have old, forgotten class files lying around someplace that will cause problems.
Do you have another copy of the file FamilyTree.class somewhere in another directory that's also in your CLASSPATH, or do you maybe have a JAR file in your CLASSPATH that contains an old version of FamilyTree.class?
To be extra sure, carefully check your CLASSPATH, delete all *.class files and recompile everything.
This is really the only possible cause for the error you get - so check and check again.