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Why should you care about equals and hashcode

Equals and hash code are fundamental elements of every Java object. Their correctness and performance are crucial for your applications. However often we see how even experienced programmers are ignoring this part of class development. In this post, I will go through some common mistakes and issues related to those two very basic methods.
Contract What is crucial about mentioned methods is something called "contract." There are three rules about hashCode and five about equals (you can find them in the Java doc for Object class), but we'll talk about three essential. Let's start from hashCode():
"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."
That means the hash code of an object doesn't have to be immutable. So let's take a look at the code of really simple J…