In this article, I`ll first show an example of a common error, and then explain how equals() and hashCode() contracts work. This is because you are not correctly replacing the equals() method. The correct way is to pass Object as a method argument. The best way to recognize this is to add an annotation @override. It will tell you that the method is overridden or local. Two objects can be in a separate location on Heap, but they can still be the same. But the Equals hash code contract states that if two objects are identical, they must have the same hash code. public boolean equals( Object o ) { // suppose hashCode() has been successfully implemented as the examples in this article return ( o == this || ( o != null && o.getClass() == getClass() && o.hashCode() == hashCode() ) ); } Replacing equals() alone will result in your company using hash data structures such as HashSet, HashMap, HashTable. fails, etc. The Object class defines both the equals() and hashCode() methods, which means that these two methods are implicitly defined in every Java class, including the ones we created: this type of problem occurs when you use a hash data structure in the collection framework such as HashTable, HashSet.

Especially with a collection like HashSet, you end up with a duplicate item and violate the defined contract. You are comparing two strings with the operator == Don`t you need to use the Equals method of strings? Thank you Everything works well until you do not replace any of these methods in your classes. However, the application sometimes needs to change the default behavior of certain objects. Let`s understand why we need to replace the equals and hashcode methods. Second, this doesn`t necessarily mean that their hash code has to be different. Okay, so we replace equals() and get the expected behavior – although the hash code of the two objects is different. So what is the purpose of hashcode() crushing? HashSet stores its items in storage buckets. Each bucket is associated with a specific hash code. When students.add(alex1) is called, Java stores alex1 in a bucket and associates it with the value alex1.hashcode().

Whenever an item with the same hash code is inserted into the set, it only replaces alex1. However, since alex2 has a different hash code, it is stored in a separate bucket and considered a completely different object. If you run the code without the hasCode implementation, the equals method is not even called. It checks the equalities only when you compare the objects (with comparator or comparable…) or in the case where the hashCode is implemented. If you use a code editor, most editors are also able to generate a good structure for you. For example, Eclipse IDE has the ability to generate a very good implementation of hashCode() and equals() for you. If we want to check if our implementations comply with Java SE contracts and some best practices, we can use the EqualsVerifier library. The Team class replaces only equals(), but implicitly uses the default implementation of hashCode() as defined in the Object class. And it returns a different hashCode() for each instance of the class. This violates the second rule.

The Lombok project also provides a @EqualsAndHashCode rating. Again, note how equals() and hashCode() “belong together” and even have a common annotation. If the criterion for equals() is such common sense, how can we harm it at all? Well, violations most often occur when we extend a class that has replaced equals(). Consider a coupon class that extends our Money class: in this case, the unexpected behavior of the HashSet collection is represented – duplicate entries have been inserted into this set. HashSet looks for duplicates using object hash code, which in this case derives from memory addresses instead of object frame values. If two objects have the same hash codes, the equals() method checks to see if they are identical. In this example, two identical objects have different hashcodes, so the hashSet does not check if they are duplicated. Try using your item as a key in HashMap (edited after comment by joachim-sauer), and you`ll get into trouble. A contract is a policy, not something imposed on you. The assertion test failed because the objects returned different hash codes even though they are identical.

The desire to avoid such behavior leads us to the need to introduce rules between these methods. A set of these rules is called a contract. Read this tutorial: EqualsVerifier is much stricter than the Java SE contract. For example, this ensures that our methods cannot trigger a NullPointerException. It also requires that both methods or the class itself are final. See the magic of hashcode()! The two items are now considered identical and stored in the same storage bucket, so whenever you call contains() and pass a student object with the same hash code, the sentence can find the item. In the following sections, we provide several examples that show how important it is to replace both methods and the disadvantages of replacing equals() without hashcode(). The static equalsInvocations field was created in the User class to count calls to the equals() method in all instances of the class. It shows how the complexity of finding objects increases when they have the same hash codes. Excuse me, I don`t have a question, but I don`t know what it`s like to ask. If two objects are identical according to the equals(Object) method, calling the hashCode method must produce the same integer result for each of the two objects.

The equals() method is used to compare two objects to see if they are identical. .