Specifying the Exceptions Thrown by a Method - Java Tutorial 5.0 英文版

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Specifying the Exceptions Thrown by a Method

The previous section showed how to write an exception handler for the writeList method in the ListOfNumbers class. Sometimes, it's appropriate for code to catch exceptions that can occur within it. In other cases, however, it's better to let a method further up the call stack handle the exception. For example, if you were providing the ListOfNumbers class as part of a package of classes, you probably couldn't anticipate the needs of all the users of your package. In this case, it's better to not catch the exception and to allow a method further up the call stack to handle it.

If the writeList method doesn't catch the checked exceptions that can occur within it, the writeList method must specify that it can throw these exceptions. Let's modify the original writeList method to specify the exceptions it can throw instead of catching them. To remind you, here's the original version of the writeList method that won't compile.

// Note: This method won't compile by design!
public void writeList() {
    PrintWriter out = 
               new PrintWriter(new FileWriter("OutFile.txt"));
    for (int i = 0; i < SIZE; i++) {
        out.println("Value at: " + i + " = " 
                     + victor.elementAt(i));
    }
    out.close();
}

To specify that writeList can throw two exceptions, add a throws clause to the method declaration for the writeList method. The throws clause comprises the throws keyword followed by a comma-separated list of all the exceptions thrown by that method. The clause goes after the method name and argument list and before the brace that defines the scope of the method; here's an example.

public void writeList() throws IOException,
                               ArrayIndexOutOfBoundsException {

Remember that ArrayIndexOutOfBoundsException is a runtime exception, so you don't have to specify it in the throws clause, although you can. You could just write the following.

public void writeList() throws IOException {

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