Following is a quick demonstration to import .jar files via Eclipse IDE
See Also: TRIE ADT Tutorial
You will need to understand why you are doing this. Any java program, for it to execute, first needs to be compiled without errors. The compilation process (using "javac") is going to convert your source code into bytecode (.class) file which you can then use the "java" executable to run. An IDE like Eclipse will do the compilation automatically in the background. That is why when you have any sort of error in the program (syntax/logical etc.,), you see it getting highlighted immediately.
So, when you are referencing an external library, javac executable needs to know which library you are referencing. You can do this by setting the CLASSPATH variable before executing the program if it is via command line. Eclipse makes it easier, instead of specifying the Java library file's location through the command line, you can do it through the editor by Right clicking on the project and selecting Build Path --> Configure Build Path. With the Libraries tab selected, click on "Add External Archives" and give your .jar files. Your program should now compile smoothly without errors.
Using an IDE introduces insane efficiency for designing and development. However, always have an understanding of what is going behind the scenes. Hope you learned something useful today!