Grokking The Java Developer Interview: More Than 200 Questions To Crack The Java, Spring, SpringBoot & Hibernate Interview

scenarios in HashMap when the key is a custom
class
Answer: equals and hashCode methods are called when we store
and retrieve values from hashmap. So, when the interviewer asks
this question, it is mostly asked with an example, where the
hashmap’s key is a custom class and you are given some situations
where either equals() is implemented or hashCode() is implemented,
sometimes properly, sometimes not. We will discuss all combinations
with programs below so you can give the correct answer in any
situation.
Before we continue, just remember if in your custom class, you are
not implementing equals() and hashCode(), then the Object class
equals() and hashCode() will be called, and also remember about
the contract between these 2 methods. It says when 2 objects are

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equal according to equals() method, then their hashCode must be
same, reverse may not be true.
Scenario 1: when custom class does not implement both equals and
hashCode methods
Here, Employee class has not given equals() and hashCode()
method implementation, so Object’s class equals() and hashCode()
methods will be used when we use this Employee class as
hashmap’s key, and remember, equals() method of Object class
compares the reference.
TestHashMap.java:

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Can you predict the output of this one?
Output:

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Here, Employee objects e1 and e2 are same but they are both
inserted in the HashMap because both are created using new
keyword and holding a different reference, and as the Object’s
equals() method checks reference, they both are unique.
And as for objects e3 and e4, they both are pointing to same
reference (e4 = e3), so they are equal according to Object’s equals()
method hence the value of e3 which was 300 gets replaced with the
value 400 of the same key e4, and finally size of HashMap is 3.