How does it know what to compare?

Sorry i don't mean the type that the obj is but how does it know where to get the data for the second treet to compare. So how does the system know that we we say object obj it knows to compare treet one with treet 2. We cast the type so it know obj will be a treet but how does it know which treet we what to compare it to. Say i have two footballs in front of me and i want to compare football1 to football2, so i say football1.compareTo(football2), simple. But this is saying i wanna compare football1 to another object I've not told you about so, football1.compareTo(object). I know i want that object to be football2 but how does java know that if I'm not pointing it to football2?

So you are passing the object to the method. Thus the compiler now has access to that object and then can act on its data.

It's a little more complicated but you should understand how Java passes data to methods. A good post about it can be found here: http://stackoverflow.com/questions/40480/is-java-pass-by-reference-or-pass-by-value

//you are passing the method the information to access the object here
  public int compareTo(Object obj) {
    //This is where the compiler is checking to see if I'm the correct object type.
    Treet other = (Treet) obj;
    if (equals(other)) {
      return 0;
    }
   //It can compare now because it already casted the obj to the a Treet and it didn't throw an exception.
    int dateCmp = mCreationDate.compareTo(other.mCreationDate);
    if (dateCmp == 0) {
      return mDescription.compareTo(other.mDescription);
    }
    return dateCmp;
  }

Thank you for helping me with this. Ive read the stacker flow and i think i get it but i still get get this correct in my head.

//you are passing the method the information to access the object here
  public int compareTo(Object obj) {

Its this first part I'm struggling with, i get how the method is working just not how its getting the correct information to work with. The passing the method the information, how does the obj relate to say secondTreet in the example file. I see it as just saying is an Object type called obj but that doesn't relate to any information anywhere else so how does it know its talking about the trees objects set up in example??

 Treet[] treets = {secondTreet, treet};
    Arrays.sort(treets);
    for (Treet exampleTreet : treets) {
     System.out.println(exampleTreet);

I understand how the obj is working in the compareTo method, just not how java makes obj equal to secondTreet's data(or any other treet object created). I understand the Object class as the top of the hierarchy in java a superclass as such.

You just answered your own question: "Object class as the top of the hierarchy in java a superclass as such". Thus EVERY object in Java is a child of Object. That's how obj equals the secondTreet. SecondTreet is an Object since it's a child of the Object class. Thus in Java if a method accepts a superclass (or parent) then it will also accept its subclass (or child which is a Treet in this case). Since the compareTo method accepts an Object, you are free to pass in whatever you want, provided that it is not one of the primitive types. There is no way to verify, at compile time, how the class is used. That's why they do a cast.

Read this https://docs.oracle.com/javase/tutorial/java/generics/types.html Read this on Generics and inheritance: https://docs.oracle.com/javase/tutorial/java/generics/inheritance.html