I have recently read the api vs implementaion configuration of gradle here. Then I thought that it will be very useful to create some simple java programs which can easily explain the difference between compile time depencies and run time depencies. For that purpose, I have created some dummy classes just to intoroduce the concepts.

Create a folder b and create two classes B.java and C.java inside it

# source code of B.java #
package b;
  
public class B {
  public String sayHello() {
  C c = new C();
  return "Hello world!"+c.inc(10);
  }
}

# source code of C.java #
package b;
public class C {
 public String inc(int i) {
    return i+1+"";
   }

Create a folder hello and create class Greeter.java inside it.

# source code of Greeter.java #
package hello;
import b.B;
public class Greeter {
  public static void main (String []args) {
   B b = new B();
   System.out.println( b.sayHello());
  }
}

You can see that B.java depends on C.java and Greeter.java.

Now if we compile B.java by javac b/B.java then you can see that two class files B.class and C.class were created inside b. Then if we compile Greeter.java by javac hello/Greeter.java then you can see that Greeter.class was created inside hello.

Then if we run by java hello.Greeter then it will print Hello world!11

Now we will create a new folder f and will move C.class from b to f. Then if we recompile javac hello/Greeter.java then we can see that it’s compiling suceessfully but if we run Greeter java hello.Greeter then we can see some run time error.

Exception in thread "main" java.lang.NoClassDefFoundError: b/C
	at b.B.sayHello(B.java:5)
	at hello.Greeter.main(Greeter.java:6)
Caused by: java.lang.ClassNotFoundException: b.C
	at java.net.URLClassLoader.findClass(URLClassLoader.java:382)
	at java.lang.ClassLoader.loadClass(ClassLoader.java:418)
	at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:355)
	at java.lang.ClassLoader.loadClass(ClassLoader.java:351)

This happened because B.class needs C.class to run but at the time of compiling Greeter.java, it wont throw compilation error because Greeter.java depends only on B.class and that’s avaialble in b. But at run time Greeter.class instance invokes B.class but B.class needs C.class for running but it can not find C.class in the directory ( in the provided class path). That’s why it’s failing. Therefore to make it working, we will add f/C.class in the runtime class path as java -cp f:. hello.Greeter and you can see that it’s printing * Hello world!11* as expected