You could also try and think about some real world possibilities for writing code and then start writing. Why is it called writing code when we type it? Anyhow, practice makes perfect so practice a lot. I just perused the outline for Java in 21 days and there is so much more to learn but at least you have the basics. Other Java areas you should look into are Generics, Lists & Linked Lists, Mapping, Trees, GUIs, and many more but my brain just sto.... :)
it is hard to learn a programming language like Java in just 21 days or even a month that 31 days. Like Stuugie said, there is a lot more than just getting the basics down.
rather correct: it is impossible to learn a programming language like Java in just 21 days.
what you can learn is: syntaxis, some basic rules and the very (very) core of Java. but in which depth? so, which topics were treated in that 21 days course, and how well do you know/understand them?
Java is way too big to completely learn, no matter how long you study (one of the characteristics of OO, everyone can create his own libraries and frameworks) so you can study for years and still only have "scratched the surface". but if you get enough experience, and know how to use documentation, your life 'll become a lot easier.