Nowadays nobody programs any application from scratch. In my opinion, almost everything one needs has been done before, you just have to find it and use it. By "it" I mean classes, which are typically distributed in form of JAR files. Any JAR file can be used in two ways. As an executable Java application or as a library.
Where can you find these libraries? Just google it. But there's one site, that will appear in your search results very often. Apache. If you go to Apache homepage and scroll lower, you will find many Apache projects. One of them is Apache http server, but many of the others are Java libraries. Most of them are web frameworks, but one of the projects is interesting for Java SE programmer. Project commons.
In this Apache project are many small subprojects such as project lang. It is a project, that provides useful extensions to basic Java classes. Here is Javadoc and there you can see many classes. For example SystemUtils. There you can find many fields and some methods, which are useful if you want to know if client runs on Windows or Linux, which Java version etc.
Every library from each vendor on the internet is specific, but here are some common things to search for: jar files, quick start guide and javadoc. Many open source vendors such as Apache also provide source.
In next part we'll use this library in Eclipse.