creating test classes or units for classes in eclipse

I have a very large project application that I do not know much about. I think I have identified the classes I need to work with but not sure. I cannot just click and run application as java application as there are so many components. I would like to create a test in order to test the class I think is the one I need to reference but don't know much about unit testing. How Would I write a test method for example for the following method;

public AudioFetcher(NetworkConnctor eRid,String eventID) throws 
  ServerException{
    eKid = eRid;
    this.eventID = eventID; 
    eof = false;
    calculateSize();
}

Will this even work? If not how do I identify the functionality of the different classes with no documentation or previous developers?