Equals as well as HashCode methods inwards Java are 2 cardinal methods from java.lang.Object class, which is used to compare equality of objects, primarily within hash based collections such every bit Hashtable as well as HashMap. Both equals() as well as hashCode() are defined inwards java.lang.Object degree as well as at that topographic point default implementation is based upon Object information e.g. default equals() method furnish truthful if 2 objects are just same i.e. they are pointing to the same retentivity address spell default implementation of hashcode method furnish int as well as implemented every bit a native method. The similar default implementation of toString() method, returns type of class, followed past times retentivity address inwards hex String.
It's advised to override these methods, based on logical as well as concern rules e.g. String overrides equals to cheque equality of two String based upon content, nosotros accept too seen as well as the representative implementation of equals() as well as hashCode for custom classes. Because of their usefulness as well as usage, they are too rattling pop inwards diverse flat of Java Interviews, and
In this tutorial, I am going to portion roughly of the actually interesting questions from equals() as well as hashCode() method inwards Java. This inquiry non exclusively 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 as well as hence I too advise taking a human face at Java Programming Interview Exposed book, ane of the best books to laid upward for Java programming interview. You volition larn near everything you lot could hold off inwards Java interviews.
It's advised to override these methods, based on logical as well as concern rules e.g. String overrides equals to cheque equality of two String based upon content, nosotros accept too seen as well as the representative implementation of equals() as well as hashCode for custom classes. Because of their usefulness as well as usage, they are too rattling pop inwards diverse flat of Java Interviews, and
In this tutorial, I am going to portion roughly of the actually interesting questions from equals() as well as hashCode() method inwards Java. This inquiry non exclusively 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 as well as hence I too advise taking a human face at Java Programming Interview Exposed book, ane of the best books to laid upward for Java programming interview. You volition larn near everything you lot could hold off inwards Java interviews.
Equals as well as HashCode Interview questions inwards Java
Here is my listing of 10 interesting questions on both of these methods. I accept seen, programmer struggles to write equals() as well as hashCode() past times hands, for a rich class, which contains dissimilar information types e.g. int, float, date etc. Reading those items as well as trying examples volition give you lot plenty confidence to human face upward whatever inquiry on equals as well as hashCode methods. I too advise to read Effective Java Items on equals() and hashCode() to create sum your gaps inwards 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 as well as hashCode accept their contract, hence overriding ane as well as non other volition interruption the contract betwixt them. By the way this inquiry tin give notice Pb to an interesting discussion, if Interviewer likes to become on deep e.g. he may enquire what are those contracts, what happens if those contracts interruption etc. I similar to give an representative How equals as well as hashcode are used inwards hash based collections e.g. Hashtable, that leaves positive impression to a greater extent than often. You tin give notice too cite near compareTo() hither to grade roughly additional point, this method should too demand to live on consistent amongst equals, which is roughly 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 give notice even hence furnish same hashCode. By the way, this is ane of the confusing fleck of equals as well as hashcode contract. See Core Java, Volume 1 ninth Edition past times 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 as well as hashcode, inwards fact, sometimes this leads to a news of the before point. When 2 key furnish same hashcode, they terminate upward inwards the same bucket. Now, inwards guild to respect the right value, you lot used keys.equals() method to compare amongst key stored inwards each Entry of linked listing there. Remember to signal out keys.equals() method, because that's what interviewer is looking for. You tin give notice too meet hither for full list of interview inquiry on Java HashMap.
Where accept you lot written equals() as well as hashCode inwards your project?
This is to meet 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 give notice signal out value objects, Hibernate entities from your domain, where you lot accept overridden equals as well as hashCode. Always gives examples from your domain as well as from your project, rather than a footling representative from a essay out program, because if Interviewer is bespeak this question, it agency he is interested inwards examples from your domain.
Suppose your Class has an Id field, should you lot include inwards equals()? Why?
This inquiry is asked to ane of my readers every bit Hibernate Interview question, well including id is non a proficient thought inwards equals() method because this method should cheque equality based upon content as well as concern rules. Also including id, which is generally a database identifier as well as non available to transient object until they are saved into the database.
What happens if equals() is non consistent amongst compareTo() method?
This is an interesting questions, which asked along amongst equals() as well as 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 agency doesn't furnish zero, if equals() method returns true, it may interruption Set contract, which is non to avoid whatever duplicates.
What happens if you lot compare an object to cipher using equals()?
When a cipher object is passed every bit an declaration to equals() method, it should furnish false, it must non throw NullPointerException, just if you lot telephone band equals method on reference, which is null it volition throw NullPointerException. That’s why it’s amend to role == operator for comparison cipher e.g. if(object != null) object.equals(anohterObject). By the way, if you lot comparison String literal amongst roughly other String object as well as hence you lot amend telephone band equals() method on the String literal rather than known object to avoid NPE, ane of those elementary tricks to avoid NullPointerException inwards Java.
What is the divergence inwards using instanceof as well as getClass() method for checking type within equals?
This inquiry was asked multiple times, sometimes past times looking at your equals() as well as hashCode implementation. Well, key divergence comes from the signal that instanceof operator returns true, fifty-fifty if compared amongst subclass e.g. Subclass instanceof Superclass is true, just amongst getClass() it's false. By using getClass() you lot ensure that your equals() implementation doesn't furnish truthful if compared amongst subclass object. While if you lot role instanceof operator, you lot terminate upward breaking symmetry dominion for equals which says that if a.equals(b) is true than b.equals(a) should too live on true. Just supplant a as well as b amongst an event of Superclass as well as Subclass, as well as you lot volition terminate upward breaking symmetry dominion for equals() method.
How produce you lot avoid NullPointerException, spell comparison 2 Strings inwards Java?
Since when compared to null, equals furnish faux as well as doesn't throw NullPointerException, you lot tin give notice role this holding to avoid NPE spell using comparison String. Suppose you lot accept a known String "abc" as well as you lot are comparison amongst an unknown String variable str, as well as hence you lot should telephone band equals every bit "abc".equals(str), this volition non throw Exception inwards 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 amongst this. By the way this is ane of the Java coding best practices, which Java developer should follow, spell using equals() method.
What is the divergence betwixt "==" as well as equals() method inwards Java?
One of the most classic interview inquiry on equals(). It has been asked numerous times during inwards past times decade. I accept too covered this inquiry already. See here for a detailed news on how it impact equality checking of String as well as Integer inwards the autoboxing world.
That's all on this list of Java interview Questions on Equals as well as HashCode methods inwards Java. It's ane of the cardinal concepts of Java programming language, just yet has several subtle things, which is unknown to many Java programmers. I strongly advise to acquire yourself actually proficient on equals(), hashCode(), compareTo() as well as compare() method, non exclusively to produce good on Java Interviews just too to write right code inwards Java.
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