If the criterion for equals() is such common sense, how can we even violate it? Well, violations most often occur when we extend a class that has replaced equals(). Consider a Voucher class that extends our Money class: the problem you will have is with collections where the uniformity of items is calculated by both .equals() and .hashCode(), for example, the keys in a HashMap. Also, I wrote my equals so that it always returns true and hashcode() always returns a diff number. 1.) Does the above set contain 3 entries of the double obj? 2.) In what order are equals() and hashcode() called? Because, if the hash code is diff, then it is not even necessary to consider the equal method 3.) How often are equal values and hash code called in the above case? Please explain this somewhat equal (Object obj): a method provided by java.lang.Object that indicates whether another object passed as an argument is “equal” to the current instance. The default implementation provided by the JDK is location-based: two objects are exactly the same when stored in the same memory address. The HashCode() and equals() methods are defined in the Object class, which is the parent class of Java objects. For this reason, all Java objects inherit a default implementation of these methods. Very nice article. I understood the contract between the equal code and hash methods. Simply put, the contract is that if obj1.equals(obj2) then obj1.hashCode() == obj2.hashCode() The idea behind a map is to be able to find an object faster than a linear search. Using hash keys to find objects is a two-step process. Internally, hashMap is implemented as an array of Entry objects. Each entry has a pair and a pointer pointing to the next entry.

The hash code for the key object is the address index of the array. You can find the linked list of entries in this cell of the table. The linked list in the cell is then searched linearly using equals() to determine whether two objects are equal. `> A very popular use of equals() is to define a list of students and search for a specific student in it. So we changed our test class to achieve this. public boolean equals(Object obj) public int hashCode() That`s all I know about the hashCode() and equals() methods. I hope this helps someone somewhere. WAIT!! We already replace equals() and check if alex1 and alex2 are the same, and we all know that HashSet stores unique items, so why were they considered different items? Note: For each non-zero reference value a, a.equals(null) must return false. If you do not replace the method correctly, your object may not function correctly in hash-based collections and will still try to use an effective hash code implementation. IDEs like Eclipse, IntelliJ Idea provide the implementation as they are usually good enough to generate good hash code. For IntelliJ Idea, you can press Command+N (Code -> Generate) and select the hashCode() method.

But why is breach of contract a problem in our team class? Well, the problem starts when some hash-based collections are involved. Let`s try using our Team class as the key to a HashMap: hashcode(): a method provided by java.lang.Object that returns an entire representation of the object store address. By default, this method returns a random integer that is unique to each instance. This integer can change between multiple runs of the application and does not remain the same. The basic rule of the contract is that if two objects are identical based on the equals() method, the hash code must be the same, but if the hash code is the same, equals() can return false. To ensure that the contract is fulfilled, the methods must use the same fields and must always be overwritten together. In this case, the unexpected behavior of the HashSet collection is represented – duplicate entries have been inserted into this set. HashSet checks for duplicates using object hash code, which in this case derives from memory addresses instead of object frame values.

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