Errata for OCP: Oracle Certified Professional Java SE 8 Programmer II Study Guide - page 146

[T Vergilio]

Page 146, "compareTo() and equals() Consistency" box.

In the last paragraph, "You might be sorting Product objects by name, but names are not unique. Therefore, the return value of compareTo() might not be 0 when comparing two equal Product objects (...)"

should read: "You might be sorting Product objects by name, but names are not unique. Therefore, the return value of compareTo() might be 0 when comparing two non-equal Product objects (...)"

public static void main(String[] args) {
        Product p1 = new Product();
        p1.id = 1;
        p1.name = "A";
        Product p2 = new Product();
        p2.id = 2;
        p2.name = "B";
        Product p3 = new Product();
        p3.id = 3;
        p3.name = "A";
        System.out.println(p1.equals(p1)); // true
        System.out.println(p1.compareTo(p1)); // 0 - comparing two equal objects still returns zero
        System.out.println(p1.equals(p3)); // false
        System.out.println(p1.compareTo(p3)); // 0 - comparing non-equal objects returns zero, but it shouldn't 
    }

[T Vergilio]

And a small typo on page 149: "If two squirrels are from the species" should read "If two squirrels are from the same species".

[Jeanne Boyarsky]

I agreed with the comment on page 149 and have logged it.

I think the one on page 146 is correct. Consider:

public static void main(String[] args) {

   Product p1 = new Product();
   p1.id = 1;
   p1.name = "A";
   Product p2 = new Product();
   p2.id = 1;
   p2.name = "B";
   System.out.println(p1.equals(p2));  // true
   System.out.println(p1.compareTo(p2)); // -1
}

The two objects are equal but have a compareTo value other than 0. Your statement is true. And equally a problem. But the way the book phrases it isn't wrong. It just goes with a different example than yours. And either way, the point is the same. Don't do that!

[T Vergilio]

Indeed, the message is the same either way. I assumed id would be unique, but there is nothing in the code to suggest that. Thanks for checking.