The Java 2 libraries contain more than 1,500 classes!

[mcuallen]

The Java 2 libraries contain more than 1,500 classes!How many do you usually have to learn as skilled programmers?

[floaty]

you don't need to learn all of them, but you have to know what they do...

floaty

[JGRobinson]

... and mostly they are all stored in relevant packages for instance all of the file input and output stuff is in Java.io , networking stuff java.net etc.etc.

This means that if a class exists in the API you can work out roughly where to look.