Скачать или смотреть Equals and HashCode Method Contract

Creating an "equals" and "hashCode" method contract in Java is a fundamental topic related to object-oriented programming. To explain it effectively, in this video detailed description of the "equals" and "hashCode" methods and their contract in Java is provided.

Equals Method:
The equals method in Java is used to compare the equality of two objects. By default, the equals method is inherited from the Object class and simply performs a reference equality check (i.e., it checks if two references point to the same memory location). However, in many cases, you'll want to override this method to define your custom equality comparison.
Contract for the equals method:

Reflexive: For any non-null object x, x.equals(x) should return true.
Symmetric: For any non-null objects x and y, if x.equals(y) returns true, then y.equals(x) should also return true.
Transitive: For any non-null objects x, y, and z, if x.equals(y) returns true and y.equals(z) returns true, then x.equals(z) should also return true.
Consistent: For any non-null objects x and y, multiple invocations of x.equals(y) should consistently return the same result, provided that no information used in the comparison has been modified.
Null comparison: For any non-null object x, x.equals(null) should return false.
Remember to consider overriding the hashCode method whenever you override the equals method to maintain the contract.

HashCode Method:
The hashCode method in Java is used to produce a hash code, an integer value that represents an object's state. Hash codes are used in hash-based data structures like hash maps to efficiently store and retrieve objects.
Contract for the hashCode method:

Consistency with equals: If two objects are equal according to the equals method, they should produce the same hash code when the hashCode method is invoked on them.
Uniqueness: While it's not required that two unequal objects have different hash codes, having unique hash codes for different objects can improve the performance of hash-based data structures.