Should You Use `instanceof` or `getClass()` in Java\'s `.equals()` Method?

Preferring instanceof over getClass() in .equals() Implementations

When generating .equals() methods in Eclipse, developers are presented with the choice between using "getClass()" or "instanceof" to compare object types. While Eclipse defaults to using "getClass()", it is generally preferable to opt for "instanceof" instead.

Reasons for Preferring instanceof

• Adherence to Liskov Substitution Principle: Using "getClass()" restricts object equality to objects of the exact same class. This violates the Liskov Substitution Principle, which states that subclasses should be substitutable for their superclasses in all respects. As a result, using "getClass()" can lead to unexpected behavior when comparing objects of different classes but the same logical type.
• Improved Extensibility: By using "instanceof", the .equals() method can correctly identify equality between objects of different subclasses. This allows for greater extensibility and flexibility in the code, as new subclasses can be introduced without affecting the equality comparisons.

Best Practices for Null Checks

Regarding null checks, it is indeed a good practice to remove the "if (obj == null)" statement when using "instanceof". This is because "instanceof" guarantees that "obj" is an instance of the relevant class, and a null object cannot be an instance of any class.

Supporting Arguments

Josh Bloch, the author of "Effective Java," advocates for using "instanceof" in .equals() implementations. He argues that it ensures the proper adherence to the Liskov Substitution Principle and prevents surprising behavior in collections that rely on the equals method.

Additional support for this approach can be found in an answer on Stack Overflow and in the third chapter of Bloch's book, "Effective Java."

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