I am so confuse to the point where my head is about to explode.

Hi! equals() is a method of the Object class, and every type inherits it, including our Treet class (we can override the default implementation of course). So equals() is readily usable by our Treet instances (without having to be defined in the Treet class), and equals(other) is equivalent to this.equals(other). Inside the class definition, we do not need to use this explicitely if no confusion arises without it.

The equals() method compares the object on which it was called with the object passed in as argument. So, we are comparing this particular instance (a Treet object) to another object (other). The variable other was originally the parameter in our compareTo() method (in which we have this conditional block you've highlighted), so anything we pass in as argument to compareTo() will become the argument of our equals() method (and it will compare this Treet instance with that particular object).

We implemented the compareTo() method (and by doing so, the Comparable interface) in the Treet class so that later we could use the Arrays.sort() method on treets (which is an array of Treet objects containing - at the moment - Treet and secondTreet). Arrays.sort() orders the elements of the array passed in by calling the elements' compareTo() method (which in turn - at least in our implementation in the Treet class - calls the equals() method to check first if the two objects are equal before checking the date, description and author), so it will eventually call equals() on Treet to compare it to secondTreet.

I put the whole code here for others reading this post for reference:

public class Treet implements Comparable<Treet> {
...    

    @Override
    public int compareTo(Treet other) {
        if (equals(other)) {  // same as this.equals(other)
            return 0;
        }

        int dateCmp = creationDate.compareTo(other.creationDate);
        int descriptionCmp = description.compareTo(other.description);
        int authorCmp = author.compareTo(other.author);

        if (dateCmp == 0) {
            if (descriptionCmp == 0) {
                return authorCmp;
            }
            return descriptionCmp;
        }
        return dateCmp;
    }
...
}

Btw. the final part can be refactored, much nicer this way:

        if (dateCmp != 0) return dateCmp;
        if (descriptionCmp != 0) return descriptionCmp;
        return authorCmp;

(Note: I've combined my previous responses in this updated answer.)

I refused to continue the course till i got this, this discussion was very helpful, thanks guys.

When you say "we are comparing this particular instance (a Treet object) to another object (other)" do you mean comparing treet to secondTreet.

Again thanks for the help.

I understand it now, thank for the help!

hey yea, thanks man, i was the same as douglas and this was what was throwing me for a loop