The assert Statement in Java

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The assert Statement
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BaztestAsserts(); // Will execute after Baz is initialized } } class Bar { static { BaztestAsserts(); // Will execute before Baz is initialized! } }
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Invoking BaztestAsserts() causes Baz to get initialized Before this can happen, Bar must get initialized Bar s static initializer again invokes BaztestAsserts() Because initialization of Baz is already in progress by the current thread, the second invocation executes immediately, though Baz is not initialized (JLS 1242)
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If an assert statement executes before its class is initialized, as in the above example, the execution must behave as if assertions were enabled in the class
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DISCUSSION
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In other words, if the program above is executed without enabling assertions, it must print:
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Asserts enabled Asserts disabled
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An assert statement is enabled if and only if the top-level class ( 8) that lexically contains it enables assertions Whether or not a top-level class enables assertions is determined by its de ning class loader before the class is initialized ( 1242), and cannot be changed thereafter An assert statement causes the enclosing top level class (if it exists) to be initialized, if it has not already been initialized ( 1241)
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class Baz extends Bar { static void testAsserts(){ boolean enabled = false; assert enabled = true; Systemoutprintln("Asserts " + (enabled "enabled" : "disabled")); } }
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The assert Statement
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Note that an assertion that is enclosed by a top-level interface does not cause initialization Usually, the top level class enclosing an assertion will already be initialized However, if the assertion is located within a static nested class, it may be that the initialization has nottaken place
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A disabled assert statement does nothing In particular neither Expression1 nor Expression2 (if it is present) are evaluated Execution of a disabled assert statement always completes normally An enabled assert statement is executed by rst evaluating Expression1 If the result is of type Boolean, it is subject to unboxing conversion ( 518) If evaluation of Expression1 or the subsequent unboxing conversion (if any) completes abruptly for some reason, the assert statement completes abruptly for the same reason Otherwise, execution continues by making a choice based on the value of Expression1 : If the value is true, no further action is taken and the assert statement completes normally If the value is false, the execution behavior depends on whether Expression2 is present:
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If Expression2 is present, it is evaluated
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If the evaluation completes abruptly for some reason, the assert statement completes abruptly for the same reason If the evaluation completes normally, the resulting value is converted to a String using string conversion ( 151811) If the string conversion completes abruptly for some reason, the assert statement completes abruptly for the same reason If the string conversion completes normally, an AssertionError instance whose "detail message" is the result of the string conversion is created If the instance creation completes abruptly for some reason, the assert statement completes abruptly for the same reason
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The assert Statement
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If the instance creation completes normally, the assert statement completes abruptly by throwing the newly created AssertionError object
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If Expression2 is not present, an AssertionError instance with no "detail message" is created
If the instance creation completes abruptly for some reason, the assert statement completes abruptly for the same reason
DISCUSSION
For example, after unmarshalling all of the arguments from a data buffer, a programmer might assert that the number of bytes of data remaining in the buffer is zero By verifying that the boolean expression is indeed true, the system corroborates the programmer s knowledge of the program and increases one s con dence that the program is free of bugs Typically, assertion-checking is enabled during program development and testing, and disabled for deployment, to improve performance Because assertions may be disabled, programs must not assume that the expressions contained in assertions will be evaluated Thus, these boolean expressions should generally be free of side effects: Evaluating such a boolean expression should not affect any state that is visible after the evaluation is complete It is not illegal for a boolean expression contained in an assertion to have a side effect, but it is generally inappropriate, as it could cause program behavior to vary depending on whether assertions were enabled or disabled Along similar lines, assertions should not be used for argument-checking in public methods Argument-checking is typically part of the contract of a method, and this contract must be upheld whether assertions are enabled or disabled Another problem with using assertions for argument checking is that erroneous arguments should result in an appropriate runtime exception (such as IllegalArgumentException, IndexOutOfBoundsException or NullPointerException) An assertion failure will not throw an appropriate exception Again, it is not illegal to use assertions for argument checking on public methods, but it is generally inappropriate It is intended that AssertionError never be caught, but it is possible to do so, thus the rules for try statements should treat assertions appearing in a try block similarly to the current treatment of throw statements
If the instance creation completes normally, the assert statement completes abruptly by throwing the newly created AssertionError object