The hashCode() method returns a hash code value (an integer number) for the object. It is supported for the benefit of hashtables such as those provided by java.util.Hashtable.
The general contract of hashCode is:
- Whenever it is invoked on the same object more than once during an execution of a Java application, the hashCode method must consistently return the same integer, provided no information used in equals comparisons on the object is modified. This integer need not remain consistent from one execution of an application to another execution of the same application.
- 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.
- It is notrequired that if two objects are unequal according to the equals(java.lang.Object) method, then calling the hashCode method on each of the two objects must produce distinct integer results. However, the programmer should be aware that producing distinct integer results for unequal objects may improve the performance of hashtables.
The hashCode() is used for bucketing in Hash implementations like HashMap, HashTable, HashSet, etc.
The value received from hashCode() is used as the bucket number for storing elements of the set or map. This bucket number is the address of the element inside the set or map.
When we call contains() it will take the hash code of the element, then look for the bucket where hash code points to. If more than 1 element is found in the same bucket (multiple objects can have the same hash code), then it uses the equals() method to evaluate if the objects are equal, and then decide if contains() is true or false, or decide if element could be added in the set or not.
Java interview questions on collections
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