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synchronization or is subject to externally induced state transitions, it may be essential to use a distinguished return value, as the object's state could change in the interval between the invocation of a state-testing method and its corresponding state-dependent method Performance concerns may dictate that a distinguished return value be used if a separate statetesting method would, of necessity, duplicate the work of the state-dependent method All other things being equal, however, a state-testing method is mildly preferable to a distinguished return value It offers slightly better readability, and inappropriate use is likely to be easier to detect and correct
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Item 40:Use checked exceptions for recoverable conditions and run-time exceptions for programming errors
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The Java programming language provides three kinds of throwables: checked exceptions, runtime exceptions, and errors There is some confusion among programmers as to when each kind of throwable is appropriate While the decision is not always clear-cut, there are some general rules that go a long way toward easing the choice The cardinal rule in deciding whether to use a checked or unchecked exception is: Use checked exceptions for conditions from which the caller can reasonably be expected to recover By throwing a checked exception, you force the caller to handle the exception in a catch clause or to propagate it outward Each checked exception that a method is declared to throw is thus a potent indication to the API user that the associated condition is a possible outcome of invoking the method By confronting the API user with a checked exception, the API designer presents a mandate to recover from the condition The user can disregard this mandate by catching the exception and ignoring it, but this is usually a bad idea (Item 47) There are two kinds of unchecked throwables: run-time exceptions and errors They are identical in their behavior: Both are throwables that needn't, and generally shouldn't, be caught If a program throws an unchecked exception or an error, it is generally the case that recovery is impossible and continued execution would do more harm than good If a program does not catch such a throwable, it will cause the current thread to halt with an appropriate error message Use run-time exceptions to indicate programming errors The great majority of run-time exceptions indicate precondition violations A precondition violation is simply a failure by the client of an API to adhere to the contract established by the API specification For example, the contract for array access specifies that the array index must be between zero and the array length minus one ArrayIndexOutOfBoundsException indicates that this precondition was violated While the JLS does not require it, there is a strong convention that errors are reserved for use by the JVM to indicate resource deficiencies, invariant failures, or other conditions that make it impossible to continue execution [Chan98, Horstman00] Given the almost universal acceptance of this convention, it's best not to implement any new Error subclasses All of the unchecked throwables you implement should subclass RuntimeException (directly or indirectly)
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It is possible to define a throwable that is not a subclass of Exception, RuntimeException, or Error The JLS does not address such throwables directly, but specifies implicitly that they are behaviorally identical to ordinary checked exceptions (which are subclasses of Exception but not RuntimeException) So when should you use such a beast In a word, never It has no benefits over an ordinary checked exceptionality would serve merely to confuse the user of your API To summarize, use checked exceptions for recoverable conditions and run-time exceptions for programming errors Of course, the situation is not always black and white For example, consider the case of resource exhaustion, which can be caused by a programming error such as allocating an unreasonably large array or by a genuine shortage of resources If resource exhaustion is caused by a temporary shortage or by temporarily heightened demand, the condition may well be recoverable It is a matter of judgment on the part of the API designer whether a given instance of resource exhaustion is likely to allow for recovery If you believe a condition is likely to allow for recovery, use a checked exception; if not, use a run-time exception If it isn't clear whether recovery is possible, you're probably better off using an unchecked exception, for reasons discussed in Item 41 API designers often forget that exceptions are full-fledged objects on which arbitrary methods can be defined The primary use of such methods is to provide the code that catches the exception with additional information concerning the condition that caused the exception to be thrown In the absence of such methods, programmers have been known to parse the string representation of an exception to ferret out additional information This is extremely bad practice Classes seldom specify the details of their string representations; thus string representations may differ from implementation to implementation and release to release Therefore code that parses the string representation of an exception is likely to be nonportable and fragile Because checked exceptions generally indicate recoverable conditions, it's especially important for such exceptions to provide methods that furnish information that could help the caller to recover For example, suppose a checked exception is thrown when an attempt to make a call on a pay phone fails because the caller has not deposited a sufficient quantity of money The exception should provide an accessor method to query the amount of the shortfall so the amount can be relayed to the user of the phone
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