Imports, jars, and heart Ache

Question

first off let me start by saying I am completely new to Java, but to give you an idea of how new; I started reading lots of books, examples and so forth and began programming Java using Eclipse about 2 months ago. However, I found a really cool bit of advise about using notepad and the terminal to program instead. Kinda crazy for a newbie to go the hard route, but I love a challenge and I'm serious about learning.

So, In Eclipse I had a really good grasp of how to import, add jars compile etc. When I started using pico and using the terminal (I'm running ubuntu) to compile all went really well, until I wanted to use packages. I've spent two days pulling my hair out because no matter what I do I can't figure it out.

I'm trying to use the acm.jar (which I have many times in Eclipse) however I'm completely lost on how to use it when compiling from the javac in terminal.

So what I'm asking for, is for someone to explain the process getting my jar file to work.

All I'm using to create my java programs is the pico (or notepad) and the javac in the terminal.

Answer 1

To compile and run a java class using external libraries, you have to add that library to the classpath. The classpath is the set of places where the java compiler and the JVM look to find any external libraries/classes that it needs during the process of compiling/executing.

Setting the classpath can be done in 2 ways:

1. Set an environment variable called CLASSPATH
2. Set it when your run javac/java

Setting the classpath when running javac/java is done like this:

javac -cp path/to/jar1:path/to/jar2:path/to/jar3:path/to/dirContainingClasses
yourMainClass.java

To run:

java -cp path/to/jar1:path/to/jar2:path/to/jar3:path/to/dirContainingClasses
yourMainClass

: is used as a separator on Linux, for windows use ;

Answer 2

Assuming your source files are in src. Assuming you want your compiled classes to be in classes. Assuming your source files reference classes that are in lib/acm.jar:

javac -cp classes:lib/acm.jar -d classes src/com/foo/bar/MyClass.java

will compile the class com.foo.bar.MyClass and put the generated MyClass.class file in classes/com/foo/bar.

You need the acm.jar file in the classpath. That's what the -cp lib/acm.jar option does. You also need classes in the classpath, because MyClass probably references other classes that you have already compiled and that are in your classes directory.

To run your class, it has to be in the classpath, and acm.jar as well:

java -cp classes:lib/acm.jar com.foo.bar.MyClass

As you see, the classpath contains jar files, and directories containing the folder hierarchy which matches the package hierarchy.

I wouldn't use javac from the command line directly, though. Try using a real build tool, that will build all your classes at once, like Gradle or Ant. Maven is also very popular, but I hate it with passion.