The throw Statement in Java

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68 The throw Statement
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Earlier examples in this chapter have shown how an exception can be thrown implicitly by the JVM during execution Now we look at how an application can programmatically throw an exception using the throw statement The general format of the throw statement is as follows:
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throw
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<object reference expression>;
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CHAPTER 6: CONTROL FLOW
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The compiler ensures that the <object reference expression> is of the type Throwable class or one of its subclasses At runtime a NullPointerException is thrown by the JVM if the <object reference expression> is null This ensures that a Throwable will always be propagated A detail message is often passed to the constructor when the exception object is created
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throw new ArithmeticException("Integer division by 0");
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When an exception is thrown, normal execution is suspended The runtime system proceeds to find a catch block that can handle the exception The search starts in the context of the current try block, propagating to any enclosing try blocks and through the runtime stack to find a handler for the exception Any associated finally block of a try block encountered along the search path is executed If no handler is found, then the exception is dealt with by the default exception handler at the top level If a handler is found, normal execution resumes after the code in its catch block has been executed, barring any rethrowing of an exception In Example 616, an exception is thrown using a throw statement at (17) This exception is propagated to the main() method where it is caught and handled by the catch block at (3) Note that the finally blocks at (6) and (14) are executed Execution continues normally from (7)
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Example 616 Throwing Exceptions public class Average7 { public static void main(String[] args) { try { printAverage(100, 0); } catch (ArithmeticException ae) { aeprintStackTrace(); Systemoutprintln("Exception handled in " + "main()"); } finally { Systemoutprintln("Finally in main()"); } Systemoutprintln("Exit main()"); }
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// // // // //
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(1) (2) (3) (4) (5)
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// (6) // (7)
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public static void printAverage(int totalSum, int totalNumber) { try { // (8) int average = computeAverage(totalSum, totalNumber); // (9) Systemoutprintln("Average = " + // (10) totalSum + " / " + totalNumber + " = " + average); } catch (IllegalArgumentException iae) { // (11) iaeprintStackTrace(); // (12) Systemoutprintln("Exception handled in " + // (13) "printAverage()"); } finally { Systemoutprintln("Finally in printAverage()"); // (14) }
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69: THE throws CLAUSE Systemoutprintln("Exit printAverage()"); } public static int computeAverage(int sum, int number) { Systemoutprintln("Computing average"); if (number == 0) throw new ArithmeticException("Integer division by 0"); return sum/number; } } // (15)
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// (16) // (17) // (18)
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Output from the program:
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Computing average Finally in printAverage() javalangArithmeticException: Integer division by 0 at Average7computeAverage(Average7java:35) at Average7printAverage(Average7java:19) at Average7main(Average7java:6) Exception handled in main() Finally in main() Exit main()
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69 The throws Clause
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A throws clause can be specified in the method header
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someMethod() throws <ExceptionType1>,
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<ExceptionType2>,, <ExceptionTypen>
{ }
Each <ExceptionTypei> declares an exception, normally only checked exceptions are declared The compiler enforces that the checked exceptions thrown by a method are limited to those specified in its throws clause Of course, the method can throw exceptions that are subclasses of the checked exceptions in the throws clause This is permissible since exceptions are objects and a subclass object can polymorphically act as an object of its superclass (see Section 71, p 284) The throws clause can specify unchecked exceptions, but this is seldom done and the compiler does not verify them In a method, a checked exception can be thrown directly by using the throw statement, or indirectly by calling other methods that can throw a checked exception If a checked exception is thrown in a method, it must be handled in one of three ways: By using a try block and catching the exception in a handler and dealing with it By using a try block and catching the exception in a handler, but throwing another exception that is either unchecked or declared in its throws clause By explicitly allowing propagation of the exception to its caller by declaring it in the throws clause of its method header