![]() ![]() The default implementation creates a hashCode value based on the object's reference, which means that books having the same content that are nonetheless different objects get different results from the hashCode method. The reason lies in the default implementation of the hashCode method in the Object class. ![]() However, when searching by the exact same book but with a different object, a borrower isn't found, and we get the null reference instead. We find the borrower when searching for the same object that was given as a key to the hash map's put method. The key is the book and the value attached to the book is a string that tells the borrower's name: We'll implement the book keeping with a HashMap. Let's create an example where this is not the case: we'll continue with the books and keep track of the books that are on loan. So far, we've only used String and Integer-type objects as HashMap keys, which have conveniently had ready-made hashCode methods implemented. This way, the search only needs to consider a fraction of the keys stored in the hash map. The value associated with the key will be returned if, and only if, the exact same value is found in the list (equality comparison is done using the equals method). ![]() The hash value identifies the array index, whereby the list located at the array index is traversed. HashMap's internal behavior is based on the fact that key-value pairs are stored in an array of lists based on the key's hash value. Hash codes are used in HashMaps, for instance. On the other hand, if two objects have different hash values, they are certainly unequal. If two objects have the same hash value, they may be equal. The method creates from the object a "hash code", i.e, a number, that tells a bit about the object's content. In addition to equals, the hashCode method can also be used for approximate comparison of objects. Log in to view the quiz Approximate Comparison With HashMap Now the book comparison returns true if the instance variables of the books are the same. We then create a version of the object that is of the same type as our class, after which the object variables are compared against each other. This is followed by checking the parameter object's type with the instanceof operation - if the object type does not match the type of our class, the object cannot be the same. The comparison first looks at the references. The method created accepts an Object-type reference as a parameter, which can be any object. If we want to compare our own classes using the equals method, then it must be defined inside the class. The String class has replaced the default equals with its own implementation. The second comparison is about two different entities, even though the variables have the same values.įor strings, equals works as expected in that it declares two strings identical in content to be 'equal' even if they are two separate objects. This is because the references are the same in the first case, i.e., the object is compared to itself. The internal structure of the book objects (i.e., the values of their instance variables ) in the previous example is the same, but only the first comparison prints " The books are the same". ![]()
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