Could java reflection add method into class file that are not in java source file?

I'm digging into the source code of the deeplearning for java recently. There is such a class NeuralNetConfiguration in which there are tons of fields that all requires getters and setters. The NeuralNetConfiguration.java source code does not provide any, however.

When I open this project in IntelliJ, ctrl click on the usage of this class, which are methods mostly like, NeuralNetConfiguration.getNInput() or NeuralNetConfiguration.getKernelSize(), the IDE direct me to the compiled class file in which all the getters are defined for each of the field in this class.

Just wonder how this is done since I'm a new bee to java. Posts I found about java reflect suggest that reflect can not add method to a method to a class unless you wrote your own classloader. I check the deep learning for java project and I don't think they have done that.

What bothers me too from time to time is, IntelliJ starts to report errors that those getFields methods could not be resolved since they are not in the source file at all, especially after my building the project using IntelliJ instead of using mvn command line.