Exceptions should be used only to denote exceptional conditions; they should not be used for ordinary control flow purposes. Catching a generic object such as
Throwable is likely to catch unexpected errors; see ERR08-J. Do not catch NullPointerException or any of its ancestors for examples. When a program catches a specific type of exception, it does not always know from where that exception was thrown. Using a
catch clause to handle an exception that occurs in a distant known location is a poor solution; it is preferable to handle the error as soon as it occurs—or to prevent it if possible.
The nonlocality of
throw statements and corresponding
catch statements can also impede optimizers from improving code that relies on exception handling. Relying on catching exceptions for control flow also complicates debugging because exceptions indicate a jump in control flow from the
throw statement to the
catch clause. Finally, exceptions need not be highly optimized, as it is assumed that they are thrown only in exceptional circumstances. Throwing and catching an exception frequently has worse performance than handling the error with some other mechanism.
Noncompliant Code Example
This noncompliant code example attempts to concatenate the processed elements of the
This code uses an
ArrayIndexOutOfBoundsException to detect the end of the array. Unfortunately, since
ArrayIndexOutOfBoundsException is a
RuntimeException, it could be thrown by
processSingleString() without being declared in a
throws clause. So it is possible for
processStrings() to terminate prematurely before processing all of the strings.
This compliant solution uses a standard
for loop to concatenate the strings.
This code need not catch
ArrayIndexOutOfBoundsException because it is a runtime exception, and such exceptions indicate programmer errors, which are best resolved by fixing the defect.
Use of exceptions for any purpose other than detecting and handling exceptional conditions complicates program analysis and debugging, degrades performance, and can increase maintenance costs.
|[Bloch 2001]||Item 39, "Use Exceptions Only for Exceptional Conditions"|
|[JLS 2013]||Chapter 11, "Exceptions"|