Collecstions.sort(ArrayList) - Please Explain

Question

Hello,
This is a rewrite of an earlier question. Hopefully, I better formed the question in this post.
Can anyone explain why the first code example requires that I write a compareTo() method (called by Collections.sort(bidList)) and a toString() method (called by System.out.println(bidList), while the second code example requires no such additional code in my program?
Thanks for all responses.
Karl
First Code Example (Requires comparTo() and toString() methods in the program.)
     public static void main(String[] args) {
        // fill a list with 5 bids
        ArrayList bidList = new ArrayList();
        for(int i=0; i<5; i++){
            //generate random number 0-100 for a bid amount
            bidList.add(new AuctionBid("Bidder" + i, (int)(Math.random()*100)));
        //sort the bids in natural order and display them
        Collections.sort(bidList); //Why does this call method compareTo()
        System.out.println("Natural order sort by bid amount");
        System.out.println(bidList); //Why does this call method toString()
}Second Code Example (Does not require compareTo() or toString() methods in the program.)
        public static void main(String[] args) {
        // fill a list with 5 bids
        System.out.println("Printing bidList from Array: ");
        String[] bidList = new String[5]; 
        for(int i=0; i<5; i++){
            if(i==0){bidList[i] = "Zoey";}
            else if (i==1){bidList[i] = "Megan";}
             else if (i==2){bidList[i] = "Larrry";}
              else if (i==3){bidList[i] = "Brian";}
               else if (i==4){bidList[i] = "Abigail";}
   List list2 = Arrays.asList(bidList); // Does NOT requre a toString() method to be coded.
   System.out.println("Printing Unsorted list2: ");
   System.out.println(list2);
   Collections.sort(list2); // Does NOT require a compareTo() method to be coded.
   System.out.println("Printing Sorted list2: ");
   System.out.println(list2);
}

Answer 1

To answer your first question, Collections doesn't
know how to sort any ArrayList unless the objects
implement the Comparable interface and define the
compareTo method. The String class already defines
the compareTo method for you, and that's why your
second ArrayList doesn't require you to write one.
In your first list you're loading AuctionBid
references, and Collections can't sort that list
unless your AuctionBid class implements Comparable
and defines the compareTo method.
To answer your second question, System.out.println
calls toString on the object reference you pass to
it, unless the reference actually IS a String
reference. How else could it get a string
representation of an object without calling toString
on it?Thank you!
That makes sense.
Karl