610: ASSERTIONS in Java

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610: ASSERTIONS
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Assertions should also not be used to validate information supplied by a client A typical example is argument checking in public methods Argument checking is part of such a method s contract, which could be violated if the assertions were disabled A special case is program arguments on the command line Their validation should be enforced by exception handling, and not by assertions Another drawback is that assertion failures can only provide limited information about the cause of any failure, in the form of an AssertionError Appropriate argument checking can provide more suitable information about erroneous arguments, in the form of specific exceptions such as IllegalArgumentException, IndexOutOfBoundsException, or
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The rest of this section illustrates useful idioms that employ assertions
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Internal Invariants
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Very often assumptions about the program are documented as comments in the code The following code at (1) makes the assumption that the variable status must be negative for the last else clause to be executed
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int status = ref1compareTo(ref2); if (status == 0) { } else if (status > 0) { } else { // (1) status must be negative }
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This assumption is an internal invariant and can be verified using an assertion, as shown at (2) below
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int status = ref1compareTo(ref2); if (status == 0) { } else if (status > 0) { } else { assert status < 0 : status; // (2) }
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Often an alternative action is chosen, based on a value that is guaranteed to be one of a small set of predefined values A switch statement with no default clause is a typical example The value of the switch expression is guaranteed to be one of the case labels and the default case is omitted, as the following code shows
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switch (trinityMember) { case THE_FATHER: break; case THE_SON: break;
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274 case THE_HOLY_GHOST: break; }
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CHAPTER 6: CONTROL FLOW
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A default clause that executes an assertion can be used to formulate this invariant
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default: assert false : trinityMember;
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If assertions are enabled, an AssertionError will signal the failure in case the trinity no longer holds Note that using enum constants in the switch statement above makes the default clause unnecessary However, the previous code causes a compile-time error in a non-void method if all case labels return a value and no return statement follows the switch statement
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switch (trinityMember) { case THE_FATHER: return psalm101; case THE_SON: return psalm102; case THE_HOLY_GHOST: return psalm103; default: assert false: trinityMember; } return psalm100; // (3) Compile time error if omitted
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Without the return statement at (3) and with assertions disabled, the method could return without a value, violating the fact that it is a non-void method Explicitly throwing an AssertionError rather than using an assert statement in the default clause, would be a better option in this case
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default: throw new AssertionError(trinityMember);
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Control Flow Invariants
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Control flow invariants can be used to test assumptions about the flow of control in the program The following idiom can be employed to explicitly test that certain locations in the code will never be reached
assert false : "This line should never be reached";
If program control does reach this statement, assertion failure will detect it In the following code, the assumption is that execution never reaches the end of the method declaration indicated by (1)
private void securityMonitor() { // while (alwaysOnDuty) { // if (needMaintenance) return;
610: ASSERTIONS // } // (1) This line should never be reached }
The previous assertion can be inserted after (1) to check the assumption Care should be taken in using this idiom, as the compiler can flag the assert statement at this location as being unreachable For example, if the compiler can deduce that the while condition will always be true, it will flag the assert statement as being unreachable
Preconditions and Postconditions
The assertion facility can be used to practice a limited form of programming-bycontract For example, the assertion facility can be used to check that methods comply with their contract Preconditions define assumptions for the proper execution of a method when it is invoked As discussed earlier, assertions should not be used to check arguments in public methods For non-public methods, preconditions can be checked at the start of method execution
private void adjustReactorThroughput(int increment) { // Precondition: assert isValid(increment) : "Throughput increment invalid"; // Proceed with the adjustment // }
Postconditions define assumptions about the successful completion of a method Postconditions in any method can be checked by assertions executed just before returning from the method For example, if the method adjustReactorThroughPut() guarantees that the reactor core is in a stable state after its completion, we can check this postcondition using an assertion
private void adjustReactorThroughput(int increment) { // Precondition: assert isValid(increment) : "Throughput increment invalid"; // Proceed with the adjustment // // Postcondition -- the last action performed before returning assert isCoreStable() : "Reactor core not stable"; }
Section 84 (p 371) provides an example using a local class where data can be saved before doing a computation, so that it can later be used to check a postcondition