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Java Exception Handling Best Practices

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  1. Java Exceptions SWE 332 Last Modified Spring 2010 Paul Ammann

  2. Rationale for Exceptions • Preconditions document undefined behavior • Undefined behavior  Partial specification • In any implementation, something happens • Should clients rely on undocumented behavior? • What happens in next release? • Bottom line: • Preconditions are usually undesirable • But sometimes unavoidable… • Exception handling: • Transform preconditions to defined behavior • One of several possible mechanisms…

  3. Transforming a Precondition to an Exceptional Postcondition Key: Postcondition defines behavior for inputs excluded by precondition public double sqrt (double x) // precondition: x >= 0 // postcondition: … public double sqrt (double x) // precondition: // postcondition: If x < 0 throw IllegalArgumentException // else … public double sqrt (double x) /** * @param x the square of the intended result * @return approximate square root of x… * @throws IllegalArgumentException if x < 0 */

  4. Java Exception Mechanism Checked Exceptions Unchecked Exceptions Ammann 2008

  5. Exceptions • Unchecked: client does not need to explicitly write special code • Checked: client has to pay attention to these. Must write special code blocks • Many different types of exceptions are provided. • You can provide your own. Ammann 2008

  6. Custom Exceptions public MyFavoriteException extends Exception { public MyFavoriteException() { super(); } public MyFavoriteException(String s) {super(s);} can add more • like state at the time of throwing the exception • methods that let the invalid state be printed and other useful information Ammann 2008

  7. Throwing Exceptions • Explicit throws throw new NullPointerException(“”); throw new NPE(“class, method”); • System throws String [ ] a = {“1”,”2”,”3”,”4”,”5”}; print(a[-1]); System throws IndexOutOfBoundsException • Passing through (goes up the call chain) • keeps going till it finds a handler Ammann 2008

  8. Catching Exceptions • In a special code block called try-block try{ }catch(ExceptionType instance){ } Ammann 2008

  9. Catching Exceptions • Can have multiple catch blocks, with different execution logic for different exceptions raised • Class hierarchy matters in catching! • If you catch an exception of type Exception, all subtypes are also caught! • catching Exception is dangerous! WHY? Ammann 2008

  10. Remember! • try-catch block should be as short as possible. • catch the right type of exception: to be sure of what’s the right thing to do Ammann 2008

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