Item 60: Favor the use of standard exceptions

1.  Reusing preexisting exceptions has several benefits. Chief among these, it

makes your API easier to learn and use because it matches established conventions with which programmers are already familiar. A close second is that programs using your API are easier to read because they aren’t cluttered with unfamiliar exceptions. Last (and least), fewer exception classes mean a smaller memory footprint and less time spent loading classes.

 

2.  The most commonly reused exception is IllegalArgumentException. This is generally the exception to throw when the caller passes in an argument whose value is inappropriate. Another commonly reused exception is IllegalStateException. This is generally the exception to throw if the invocation is illegal because of the state of the receiving object. Arguably, all erroneous method invocations boil down to an illegal argument or illegal state, but other exceptions are standardly used for certain kinds of illegal arguments and states.

 

3.  If a caller passes null in some parameter for which null values are prohibited, convention dictates that NullPointerException be thrown rather than IllegalArgumentException.

 

4.  If a caller passes an out-of range value in a parameter representing an index into a sequence, IndexOutOfBoundsException should be thrown rather than IllegalArgumentException.

 

5.  ConcurrentModificationException should be thrown if an object that was designed for use by a single thread or with external synchronization detects that it is being (or has been) concurrently modified.

 

6.  UnsupportedOperationException is the exception to throw if an object does not support an attempted operation. This exception is used by implementations that fail to implement one or more optional operations defined by an interface.

 

7.  The following table summarizes the most commonly reused exceptions:

Exception	Occasion for Use
IllegalArgumentException	Non-null parameter value is inappropriate
IllegalStateException	Object state is inappropriate for method invocation
NullPointerException	Parameter value is null where prohibited
IndexOutOfBoundsException	Index parameter value is out of range
ConcurrentModificationException	Concurrent modification of an object has been detected where it is prohibited
UnsupportedOperationException	Object does not support method

 

8.  It would be appropriate to reuse ArithmeticException and NumberFormatException if you were implementing arithmetic objects such as complex numbers or rational numbers.

 

9.  Reuse must be based on semantics, not just on name. Also, feel free to subclass an existing exception if you want to add a bit more failure-capture information.