Java Compilation with Package and Jar Dependencies

In this tutorial, I will go over Java package and dependencies basics.

Let's go over the simplest java code with package declaration. Let's assume we are working on ~/java_package directory. Create Hello.java file below in the current directory:


To compile and execute this, we will first create a directory whose name is equal to the package name. In this case, the directory name should be pkgtest. So, create this directory and move Hello.java file into this. Run all of the following from ~/java_package directory:
$ mkdir pkgtest
$ mv Hello.java pkgtest/

Next, we need to compile. This is very simple.
$ javac pkgtest/Hello.java

Finally, we need to run it.
$ java pkgtest/Hello
hello

Note that you must run this from ~/java_package directory; otherwise java will complain with the error below:
$ cd pkgtest
$ java Hello
Error: Could not find or load main class Hello


Next, we will see how to import the package. Modify pkgtest/Hello.java and create ./JarTest.java as below:


To compile and run, run the following from ~/java_package directory:
$ javac JarTest.java
$ java JarTest
hello
hi


Next, we will create a jar library file and use this to compile and run. First, let's create the jar library. Run the following in ~/java_package directory
$ javac pkgtest/Hello.java
$ jar cvf pkgtest.jar pkgtest/Hello.class
added manifest
adding: pkgtest/Hello.class(in = 648) (out= 372)(deflated 42%)

This creates pkgtest.jar file, which contains Hello class.

Next, we will rename the pkgtest directory so that we don't compile from the source.
$ mv pkgtest pkgtest_bk

Let's see what happens if don't specify the jar file as we compile.
$ javac JarTest.java
JarTest.java:1: error: package pkgtest does not exist
import pkgtest.Hello;
              ^
JarTest.java:5: error: cannot find symbol
        Hello.print("hello");
        ^
  symbol:   variable Hello
  location: class JarTest
JarTest.java:6: error: cannot find symbol
        Hello h = new Hello();
        ^
  symbol:   class Hello
  location: class JarTest
JarTest.java:6: error: cannot find symbol
        Hello h = new Hello();
                      ^
  symbol:   class Hello
  location: class JarTest
4 errors

As you can see above, java compiler complains that it cannot find some symbols, since we renamed the package directory. To resolve this, let's link the jar file:
$ javac -cp ".:pkgtest.jar" JarTest.java
$ java -cp ".:pkgtest.jar" JarTest
hello
hi

Viola! Let's call it a day.