What are common mistakes developers make with equals and hashCode?

When implementing the equals and hashCode methods in Java, developers often make several common mistakes that can lead to unexpected behavior when using collections like HashSet or HashMap. Here are some of these mistakes:

• Inconsistent Equals and HashCode: If the equals method indicates that two objects are equal, their hashCode must also return the same value. Failing to maintain this contract can lead to inconsistent behavior in hash-based collections.
• Using Mutable Fields: Including mutable fields in the equality check can lead to issues if their values change after the object is added to a collection. This can invalidate the object's position in a HashMap or HashSet.
• Not Following Reflexivity, Symmetry, Transitivity: The equals method must satisfy properties like reflexivity (an object must be equal to itself), symmetry (if one object equals another, the second must equal the first), and transitivity (if one object equals a second that equals a third, the first must equal the third).
• Not Handling Null Properly: The equals method should handle null comparisons gracefully, usually by returning false.
• Incorrect Type Checks: Always check the type of the object being compared in the equals method to prevent ClassCastException.

By avoiding these common pitfalls, developers can create more reliable and stable Java applications.