It is perfectly valid to override hashCode() without overriding equals() as objects with equal hash codes need not be equal. Please note that even though equal objects must have equal hash codes, the reverse is not true. But since the equals() method is not overridden, when the set hashes e2 and iterates through the bucket looking if there is an Employee e such that e2.equals(e) is true, it won’t find any as e2.equals(e1) will be false. If we only override the hashCode() method, both e1 and e2 will hash to the same bucket as they produce the same hash code. Override only hashCode() without overriding equals() method It is worth noting that if the class instance is never used in any hash-based collections, then it doesn’t really matter if hashCode() is overridden or not. Both the operator and the hashCode property of objects must be consistent in order for a common hash map implementation to function properly. Even though both e1 and e2 are equal, they don’t hash to the same bucket, and both reside in the collection as separate keys. Since the default hashCode implementation in the Object class return distinct integers for distinct objects, if only equals() method is overridden, e1 will be placed in some bucket and e2 will be placed in some other bucket as e1.hashCode() != e1.hashCode(). Overriding only equals() method without overriding hashCode() causes the two equal instances to have unequal hash codes, which violates the hashCode contract (mentioned in Javadoc) that clearly says, if two objects are equal according to the equals(Object) method, then calling the hashCode method on each of the two objects must produce the same integer result. Override only equals() without overriding hashCode() method Now let’s discuss the above program’s behavior if the equals() method is overridden without overriding hashCode(), or vice versa. This is because we have overridden both the equals() and hashCode() method in the Employee class, and both objects now point to the same bucket and hold the same location within the bucket. two site objects which have same name and id are considered to be. void finalize(), Called by garbage collector. NOTE: It is always recommended if you are overriding the equals () then you must override the hashCode () method. We will be covering more about hashCode () in a separate post. If we don’t do so, equal objects may get different hash-values and hash based collections, including HashMap, HashSet, and Hashtable do not work properly (see this for more details). Then it is recommended to override the equals (Object obj) method. As a side note, when we override equals (), it is recommended to also override the hashCode () method. Suppose you want to compare the object’s own equality condition. As evident from the generated output, the set contains only one Employee object even though two different Employee objects are added. Note: As per the Java documentation, both the methods should be overridden to get the complete equality mechanism using equals() alone is not sufficient. The above program return true because we override the equals method based on name and id i.e. Overriding equals() in Java boolean equals (Object obj), Decides whether two objects are meaningfully equivalent. You can override the equals () method as per your functionality.
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