Java: Packages - Installing

As extensive as Java's library is, there are sometimes other packages that you want to use (eg for look and feel, data structures, GUI, ...). These are almost universally distributed as .jar files.

Where to put .jar file library packages

A .jar file is basically a .zip file plus a little extra information. This file may contain one or more packages, and all the classes in them. If it knows where to find the .jar file, Java will search it for the imports or references that you have in your program, both so the compilier can find the imports, and so the runtime system can find package. There are two alternatives.

Modify the CLASSPATH variable to include the jar file.
Put the jar file in the JRE lib/ext directory, which is automatically searched but the compiler and runtime systems for needed packages. In my Windows installation of Java, the JRE is installed twice, so I put it in both. I don't know that both are actually used, but I didn't want to take any chances.
```
    C:\Program Files\Java\jre1.4.2\lib\ext\
    C:\j2sdk1.4.2\jre\lib\ext\
    
```