Re: IllegalArgumentException vs. NullPointerException and collection behavior

Lew <lew@lewscanon.nospam>
Fri, 10 Aug 2007 17:38:17 -0400
Karsten Wutzke wrote On 08/10/07 10:47,:

When passing nulls to my methods and constructors (for required
fields), is it better to use IllegalArgumentException("Blah is null!")
or NullPointerException("Blah is null!")?

Eric Sosman wrote:

    There's no hard-and-fast rule, but it seems to me more
straightforward to throw IllegalArgumentException (or one
of its subclasses) when an argument fails a validity check.
Sometimes, though, I will let Java do the validity checking
for me, as in

    void method(String bass) {
        String lower = bass.toLowerCase();

This idiom has the disadvantage that you do not control logging the error.

TABs in a Usenet post are a Bad Thing.

.... instead of

  void method(String bass) {
    if (bass == null)

       { // good practice: always use braces in your ifs, etc.
          String msg = "suitable error message";
          logger.error( msg );

       throw new IllegalArgumentException(




  String lower = bass.toLowerCase();

.... in which case the caller will get NPE instead of IAE.
Whatever you do, make sure the Javadoc explains all the
requirements you impose on the argument values: non-nullity,
no zero-length strings, all alphabetic, whatever.

Like so much of programming, Java gives you the tools but no restriction on
which to use; it is a judgment call. There is an advantage to declaring a
checked exception - it forces client programmers to pay attention to it. The
trouble with throwing any RuntimeException is that the API clients might fail
abruptly because their programmers didn't pay close enough attention to the

Fail without throwing any exception when it makes sense to return a value
within the range of the method. For example, a database lookup for a name
might reasonably return null for a database failure as well as for the name
not being there; from the application standpoint it might not matter why the
name couldn't be found. The lookup method should log the db error and return

Fail with an exception when it makes sense that the failure condition is "out
of band" for the method. In the preceding example, if the database failure
meant that the rest of the application would have trouble (e.g., an insert is
next), it would be better to throw the exception so the application doesn't
try something already known to be doomed to fail.

Use a RuntimeException if the situation is sufficiently under the client's
control that they shouldn't have let it happen. Throw a checked Exception if
the client should have been able to count on getting a result, and you want to
force awareness of any out-of-band problems. So an IllegalArgumentException
(or NPE) is appropriate when the client passed a null value, but shouldn't
have, whereas a checked FooException is a better choice when the client is
accessing a resource that becomes unavailable for extrinsic reasons.

Do not use assertions for run-time data checks. Assertions are to check
program invariants that are under your control (e.g., preconditions for
private methods).

Designing wisely is the goal; mastery comes from not ever assuming one has
mastered it.


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