Compile time vs Run time Dependency - Java

Question

What is the difference between compile time and run time dependencies in Java? It is related to class path, but how do they differ?

Answer 1

For Java, compile time dependency is your source code's dependency. For instance, if class A calls a method from class B, then A is dependent to B at the compile time since A has to know about B (type of B) to be compiled. The trick here should be this: Compiled code is not a complete and executable code yet. It includes replaceable addresses (symbols, metadata) for the sources which are not yet compiled or existing in external jars. During linking, those addresses must be replaced by actual adresses in the memory. To do it properly, correct symbols/adresses should be created. And this can be done with the type of the class (B). I believe that's the main dependency at the compile time.

Runtime dependency is more related with the actual flow-of-control. It involes actual memory addresses. It's a dependency that you have when your program is running. You need class B details here like implementations, not only the type info. If the class not exists, then you will get RuntimeException and JVM will exit.

Both dependencies, generally and shouldn't, flow the same direction. This is a matter of OO design though.

In C++, compilation is a bit different (not just-in-time) but it has a linker too. So the process might be thought similar to Java I guess.