What it means if you don't override equals() ? [Preparing for OCP]

[Bobby Kiran]

Hi Everyone, I am preparing for OCP and I am not able to understand the context here.. Can someone help me to understand this.

What it means if you don't override equals() ?... Of course, overriding the equals() method for Car also allows the potential for more than one
object representing a single unique car to exist, which might not be safe in your design.

Page #578
OCA/OCP Java® SE 7 Programmer I & II Study Guide
(Exams 1Z0-803 & 1Z0-804)

Thanks in advance.

[Junilu Lacar]

It means that when you decide to override equals(), think hard about what the consequences are to the semantics of equality.

For example, if your Car had a VIN property, which is unique for each and every car, then think twice before using VIN as part of the definition of equals(). Because that would mean that this is possible:

public static final String VIN = "BOGUS1234VIN";

Car myCar = new Car(VIN);
Car cloneCar = new Car(VIN);

System.out.println(myCar.equals(cloneCar)); // true

This could be fine but it could also cause some unwanted surprises elsewhere in your code. You need to make sure there are no surprises.