What is the difference between hashcode and equals

What is hashCode in Java 4. The equals method is used to compare two objects. The default equals method is defined in the object class. The two object references are equal only if they are pointing to the same object.

It is possible to override the equals method. The statement System. It was similar to the statement, System. There is no equals method in the Student class. Therefore, the equals in the Object class is called. True is displayed only if the object reference is pointing to the same object. According to the above program, the equals method is overridden. An object is passed to the method, and it is type casted to Student.

Then, the id values are checked. We want to store all the students in a HashSet , so we update HashcodeEquals as the following:. We already override equals and verify that alex1 and alex2 are equal, and we all know that HashSet stores unique objects, so why did it consider them as different objects?

HashSet stores its elements in memory buckets. Each bucket is linked to a particular hash code. When calling students.

Now any time an element with the same hash code is inserted into the set, it will just replace alex1. However, since alex2 has a different hash code, it will be stored in a separate bucket and will be considered a totally different object.

Now when HashSet searches for an element inside it, it first generates the element's hash code and looks for a bucket which corresponds to this hash code. Here comes the importance of overriding hashcode , so let's override it in Student and set it to be equal to the ID so that students who have the same ID are stored in the same bucket:. See the magic of hashcode! The two elements are now considered as equal and stored in the same memory bucket, so any time you call contains and pass a student object holding the same hash code, the set will be able to find the element.

The same is applied for HashMap, HashTable , or any data structure that uses a hashing mechanism for storing elements. In order to achieve a fully working custom equality mechanism, it is mandatory to override hashcode each time you override equals. Follow the tips below and you'll never have leaks in your custom equality mechanism:.

See the original article here. Reason : Reference obj can also refer to the Object of subclass of Geek. Line b ensures that it will return false if passed argument is an Object of subclass of class Geek.

But the instanceof operator condition does not return false if it found the passed argument is a subclass of the class Geek. Read InstanceOf operator. This method must be overridden in every class which overrides equals method. The general contract of hashCode is: During the execution of the application, if hashCode is invoked more than once on the same Object then it must consistently return the same Integer value, provided no information used in equals Object comparison on the Object is modified.

It is not necessary that this Integer value to be remained same from one execution of the application to another execution of the same application. If two Objects are equal, according to the equals Object method, then hashCode method must produce the same Integer on each of the two Objects.

If two Objects are unequal, according to the equals Object method, It is not necessary the Integer value produced by hashCode method on each of the two Objects will be distinct. It can be same but producing the distinct Integer on each of the two Objects is better for improving the performance of hashing based Collections like HashMap, HashTable…etc.

Note: Equal objects must produce the same hash code as long as they are equal, however unequal objects need not produce distinct hash codes. If you like GeeksforGeeks and would like to contribute, you can also write an article using contribute.

Two objects with equal hashCode have equal not necessarily equal, that is, hashCode is not absolutely reliable. In general, the default equals comparison rule of the Object class is to compare the memory addresses of two objects.

The hashcode is a local method. Java's memory is safe. Therefore, the memory address of the object cannot be obtained from the hash code. In fact, the hashcode is obtained by a hash algorithm based on the memory address of the object, so two objects cannot be guaranteed Whether the memory addresses of the two are the same. Today, I recorded the knowledge of Hashcode.

I haven't been touched before, I feel very strange, I have been studying it. First of all, my biggest problem is what is Hashcode is doing, and now I know