As Java has just 49 keywords in it, but to really program in Java, one needs to have the ability to effectively use the classes provided in the API.
The API contains thousands of ready-made classes already present to make the job pretty easier.
Definetly the above line sounds more good than it is, as its really not a cakewalk to remember all the classes.
So whats the best way??
I guess and infact i believe that no one can remember all the classes, so the best way is to start with the basic classes which will help us start coding in Java. Afterwards as we require something which we dont know, search for it in the API.
When you get it, use it and yes, dont forget it!!
After some time in development, one realises that one will remember what is useful to us, and in this way, the very large API becomes a bit smaller for us!!