When to Throw IllegalArgumentException or NullPointerException for a Null Parameter

When defining setter methods, developers often face the dilemma of whether to throw an IllegalArgumentException or a NullPointerException for a null parameter. Both exceptions appear relevant based on their JavaDoc descriptions:

• IllegalArgumentException: Indicates an illegal or inappropriate argument.
• NullPointerException: Thrown when null is used inappropriately.

However, the appropriate choice is IllegalArgumentException (IAE) for the following reasons:

• Intended Usage: NPE is intended for runtime errors related to null dereferencing, while IAE is designated for validating parameters.
• Misleading Trace: NPE in a stack trace may suggest a null dereference instead of an incorrect parameter value.
• Consistency: IAE is commonly used for other invalid parameter errors, so consistency is preferred.
• Java API Inconsistencies: While the Java API sometimes uses NPE for null parameters, its inconsistency should not be emulated.

Therefore, for setter methods where null is not appropriate, IllegalArgumentException should be used to convey the intent of parameter validation.