Working on a real problem that you want to solve is the best way to learn anything. Don't go through a book -- just bookmark the JDK and make sure you skim through at least String, List, Map, Set, and Iterator.
If you can, find on github some idiomatic code for whatever language you're trying to learn (and take it with a grain of salt -- Spring Framework, for example, has good code in it, but you'd never need to make your solution so complicated because your code doesn't need to be extensible by everyone and their little sister).
If you can, find on github some idiomatic code for whatever language you're trying to learn (and take it with a grain of salt -- Spring Framework, for example, has good code in it, but you'd never need to make your solution so complicated because your code doesn't need to be extensible by everyone and their little sister).