Struggling to understand various concepts in this lecture

1st question: Why do we have to cast obj to a Tweet?

public int compareTo (Object obj) {
    Treet other = (Treet) obj;

Answer: because, at compile time, obj is an Object, not a Tweet. In the method Craig uses two properties with other: mCreationDate and mDescription. other, since it is a Tweet, has these properties. obj, since it is an Object, does not have these properties. So without the cast you'd get a compiler error.

2nd question: What is being compared here?

if (equals(other)) {
    return 0;
}

Answer: this is short hand for this.equals(other), where this means "me", the current object (as you surmised). As lamda points out, you can avoid this if your class implements Comparable<E> rather than Comparable, i.e., uses generics. Because then the compiler will require that the parameter for compareTo() be E, rather than Object. E.g., if the Tweet class implements Comparable<Tweet> then compareTo(Tweet t) would be required, with no casting. One nice thing about doing it with generics is that Craig's method will throw an exception if it's passed anything other than a Tweet, while compareTo(Tweet t) won't because it can't be given anything other than a Tweet.

3rd question: what's going on here:

int dateCmp = mDate.compareTo(other.mDate);
if (dateCmp == 0) {
    return mDescription.compareTo(other.mDescription);
}

Answer: the first line assigns an int named dateCmp a value, the result of comparing this.mDate to other.mDate. Then, in the second line, it says that if the int is 0, return the result of another comparison. (As you may remember, any compareTo() method has to return either a positive integer, a 0, or a negative integer.) So, what's happening here is that if the dates are equal, then the decision which is "before" is made based on the descriptions. For a different example, say you were comparing date objects that have just month and day properties. Then, for 4/4 and 4/11, since the months are equal, a compareTo() would return 0, and then you'd look at the days, and return a -1 since 4 is "before" 11.

4th question: "I understand that having a toString() method will print out a string of the specified format if you sysout the object, but don't understand how this piece of code above has automatically implemented the compareTo() method without us actually referring to it."

for (Treet exampleTreet : treets) {
    System.out.println(exampleTreet);
}

There's nothing about compareTo() here. System.out.print(), if given an object, will call the toString() method on that object. If the method has been overridden, then it will print whatever the method tells it to (as Craig does at 12:15, by adding mCreationDate). If not, then it will use the method inherited from Object and print something like Tweet@FE26B45..., i.e., the class and memory address.

Happy coding!

public int compareTo (Object obj) {
    Treet other = (Treet) obj;

Why are we casting an 'Object' object to a 'Treet' object?
if the compareTo method accepts an object as a parameter couldn't we just use (Treet other) in the method signature?

I was wondering the same thing and it gets clarified in a later video (topic: Generics). I believe you have to use "Object" and can't use "Treet" because when you are overriding the inherited compareTo() method, it always expects "Object" type. If you try to replace it, you'll get a compiler error. Try changing "Object" to "Treet" and removing the cast.

Regarding your question about equals. By default, all Objects have an equals() that checks to see if they are the same object and it is using that. On a later video, you will write your own equals() method in Treet.java.

Regarding your question about for( ... ){ ... }

for (Treet exampleTreet : treets) {
  //System.out.println(exampleTreet);
  // the above line is equivalent to:
  System.out.println(exampleTreet.toString());
  // All Objects (including Treet) inherits toString() from the main Object class
}

Still struggling with this one, particularly understanding the recursive nature of the method. I have looked at other similar answers but I didn't understand them.

• Does an implementation of the compareTo() method have to include all of -1, 0 and 1 as return statements?
• How is if(equals(other)) performed? What part of the objects are being compared?
• Once we return our value from the compareTo() method, what is done with that value that then organizes the treets? I understand the value refers to less than, equal to or greater than, but how do we use that value to move the treets in the array to the ordered position.
• Looking at the API is it this sort method we are implementing from Arrays? public static void sort(Object[] a)
• Is the compareTo method used in mCreationDate.compareTo(other.mCreationDate) referencing this.compareTo() or date.compareTo?

I don't suppose anyone is available to step through the code with me in real time outside the forums? I can leave a Skype if someone is willing to help a brother out!

Thanks,