10 Equals In Addition To Hashcode Interview Questions Inward Java

Equals in addition to HashCode methods inward Java are 2 primal methods from java.lang.Object class, which is used to compare equality of objects, primarily within hash based collections such every bit Hashtable in addition to HashMap. Both equals() in addition to hashCode() are defined inward java.lang.Object shape in addition to at that spot default implementation is based upon Object information e.g. default equals() method provide truthful if 2 objects are just same i.e. they are pointing to the same retention address spell default implementation of hashcode method provide int in addition to implemented every bit a native method. The similar default implementation of toString() method, returns type of class, followed yesteryear retention address inward hex String.

It's advised to override these methods, based on logical in addition to describe concern rules e.g. String overrides equals to cheque equality of  two String based upon content, nosotros accept too seen in addition to the instance implementation of equals() in addition to hashCode for custom classes. Because of their usefulness in addition to usage, they are too real pop inward diverse degree of Java Interviews, and

In this tutorial, I am going to portion around of the actually interesting questions from equals() in addition to hashCode() method inward Java. This inquiry non alone tests your concept on both method just too gives an chance to explore them more. By the way, if you lot are preparing for Java Interviews hence I too propose taking a human face at Java Programming Interview Exposed book, i of the best books to fix for Java programming interview. You volition larn close everything you lot could await inward Java interviews.

Equals in addition to HashCode Interview questions inward Java

Here is my listing of 10 interesting questions on both of these methods.  I accept seen, programmer struggles to write equals() in addition to hashCode() yesteryear hands, for a rich class, which contains unlike information types e.g. int, float, date etc. Reading those items in addition to trying examples volition give you lot plenty confidence to human face upwardly whatsoever inquiry on equals in addition to hashCode methods. I too propose to read Effective Java Items on equals() and hashCode() to create sum your gaps inward cognition of this 2 critical methods.

When you lot are writing equals() method, which other method or methods you lot demand to override?

hashcode is the right answer. Since equals in addition to hashCode accept their contract, hence overriding i in addition to non other volition interruption the contract betwixt them. By the way this inquiry tin Pb to an interesting discussion, if Interviewer likes to acquire on deep e.g. he may inquire what are those contracts, what happens if those contracts interruption etc. I similar to give an instance How equals in addition to hashcode are used inward hash based collections e.g. Hashtable, that leaves positive impression to a greater extent than often. You tin too scream close compareTo() hither to grade around additional point, this method should too demand to live on consistent alongside equals, which is around other interesting inquiry on our list.

Can 2 objects which are non equal accept the same hashCode?

YES, 2 objects, which are non equal to equals() method tin withal provide same hashCode. By the way, this is i of the confusing fighting of equals in addition to hashcode contract. See Core Java, Volume 1 ninth Edition yesteryear Cay S. Horstmann for  more details.

How does get() method of HashMap works, if 2 keys accept the same hashCode?

This is the follow-up of previous interview questions on equals in addition to hashcode, inward fact, sometimes this leads to a give-and-take of the before point. When 2 key provide same hashcode, they terminate upwardly inward the same bucket. Now, inward lodge to discovery the right value, you lot used keys.equals() method to compare alongside key stored inward each Entry of linked listing there. Remember to indicate out keys.equals() method, because that's what interviewer is looking for. You tin too come across hither for full list of interview inquiry on Java HashMap.

Where accept you lot written equals() in addition to hashCode inward your project?

This is to come across if the developer has fifty-fifty written these methods or not. Of course, almost all of Java programmer are exposed to this, you lot tin indicate out value objects, Hibernate entities from your domain, where you lot accept overridden equals in addition to hashCode. Always gives examples from your domain in addition to from your project, rather than a piffling instance from a bear witness program, because if Interviewer is scream for this question, it way he is interested inward examples from your domain.

Suppose your Class has an Id field, should you lot include inward equals()? Why?

This inquiry is asked to i of my readers every bit Hibernate Interview question, well including id is non a proficient thought inward equals() method because this method should cheque equality based upon content in addition to describe concern rules. Also including id, which is generally a database identifier in addition to non available to transient object until they are saved into the database.

What happens if equals() is non consistent alongside compareTo() method?

This is an interesting questions, which asked along alongside equals() in addition to hashCode() contract. Some java.util.Set implementation e.g. SortedSet or it's concrete implementation TreeSet uses compareTo() method for comparison objects. If compareTo() is non consistent way doesn't provide zero, if equals() method returns true, it may interruption Set contract, which is non to avoid whatsoever duplicates.

What happens if you lot compare an object to nil using equals()?

When a nil object is passed every bit an declaration to equals() method, it should provide false, it must non throw NullPointerException, just if you lot telephone vociferation upwardly equals method on reference, which is null it volition throw NullPointerException. That’s why it’s improve to role == operator for comparison nil e.g. if(object != null) object.equals(anohterObject). By the way, if you lot comparison String literal alongside around other String object hence you lot improve telephone vociferation upwardly equals() method on the String literal rather than known object to avoid NPE, i of those elementary tricks to avoid NullPointerException inward Java.

What is the divergence inward using instanceof in addition to getClass() method for checking type within equals?

This inquiry was asked multiple times, sometimes yesteryear looking at your equals() in addition to hashCode implementation. Well, key divergence comes from the indicate that instanceof operator returns true, fifty-fifty if compared alongside subclass e.g. Subclass instanceof Superclass is true, just alongside getClass() it's false. By using getClass() you lot ensure that your equals() implementation doesn't provide truthful if compared alongside subclass object. While if you lot role instanceof operator, you lot terminate upwardly breaking symmetry dominion for equals which says that if a.equals(b) is true than b.equals(a) should too live on true. Just supersede a in addition to b alongside an event of Superclass in addition to Subclass, in addition to you lot volition terminate upwardly breaking symmetry dominion for equals() method.

How create you lot avoid NullPointerException, spell comparison 2 Strings inward Java?

Since when compared to null, equals provide faux in addition to doesn't throw NullPointerException, you lot tin role this belongings to avoid NPE spell using comparison String. Suppose you lot accept a known String "abc" in addition to you lot are comparison alongside an unknown String variable str, hence you lot should telephone vociferation upwardly equals every bit "abc".equals(str), this volition non throw Exception inward thread Main: java.lang.NullPointerException, fifty-fifty if str is null. On the other hand, if you lot call str.equals("abc"), it volition throw NPE. So live on careful alongside this. By the way this is i of the Java coding best practices, which Java developer should follow, spell using equals() method.

What is the divergence betwixt "==" in addition to equals() method inward Java?

One of the most classic interview inquiry on equals(). It has been asked numerous times during inward yesteryear decade. I accept too covered this inquiry already. See here for a detailed give-and-take on how it acquit on equality checking of String in addition to Integer inward the autoboxing world.

That's all on this list of Java interview Questions on Equals in addition to HashCode methods inward Java. It's i of the primal concepts of Java programming language, just yet has several subtle things, which is unknown to many Java programmers. I strongly propose to acquire yourself actually proficient on equals(), hashCode(), compareTo() in addition to compare() method, non alone to create good on Java Interviews just too to write right code inward Java.

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