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Exception Declaration and the Public Interface
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So, how do we know that some method throws an exception that we have to catch Just as a method must specify what type and how many arguments it accepts and what is returned, the exceptions that a method can throw must be declared (unless the exceptions are subclasses of RuntimeException). The list of thrown exceptions is part of a method s public interface. The throws keyword is used as follows to list the exceptions that a method can throw:
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void myFunction() throws MyException1, MyException2 { // code for the method here }
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This method has a void return type, accepts no arguments, and declares that it throws two exceptions of type MyException1 and MyException2. (Just because the method declares that it throws an exception doesn t mean it always will. It just tells the world that it might.) Suppose your method doesn t directly throw an exception, but calls a method that does. You can choose not to handle the exception yourself and instead just declare it, as though it were your method that actually throws the exception. If you do declare the exception that your method might get from another method, and you don t provide a try/catch for it, then the method will propagate back to the method that called your method, and either be caught there or continue on to be handled by a method further down the stack. Any method that might throw an exception (unless it s a subclass of RuntimeException) must declare the exception. That includes methods that aren t actually throwing it directly, but are ducking and letting the exception pass down to the next method in the stack. If you duck an exception, it is just as if you were the one actually
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4: Flow Control, Exceptions, and Assertions
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throwing the exception. RuntimeException subclasses are exempt, so the compiler won t check to see if you ve declared them. But all non-RuntimeExceptions are considered checked exceptions, because the compiler checks to be certain you ve acknowledged that bad things could happen here. Remember this: Each method must either handle all checked exceptions by supplying a catch clause or list each unhandled checked exception as a thrown exception. This rule is referred to as Java s handle or declare requirement. (Sometimes called catch or declare.)
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Look for code that invokes a method declaring an exception, where the calling method doesn t handle or declare the checked exception. The following code has two big problems that the compiler will prevent:
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void doStuff() { doMore(); } void doMore() { throw new IOException(); }
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First, the doMore() method throws a checked exception, but does not declare it! But suppose we fix the doMore() method as follows:
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void doMore() throws IOException { }
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The doStuff() method is still in trouble because it, too, must declare the IOException, unless it handles it by providing a try/catch, with a catch clause that can take an IOException.
Again, some exceptions are exempt from this rule. An object of type RuntimeException may be thrown from any method without being specified as part of the method s public interface (and a handler need not be present). And even if a method does declare a RuntimeException, the calling method is under no obligation to handle or declare it. RuntimeException, Error, and all of their subtypes are unchecked exceptions and unchecked exceptions do not have to be specified or handled. Here is an example:
import java.io.*; class Test { public int myMethod1() throws EOFException { return myMethod2(); } public int myMethod2() throws EOFException {
Handling Exceptions (Exam Objectives 2.3 and 2.4)
// Some code that actually throws the exception goes here return 1; } }
Let s look at myMethod1(). Because EOFException subclasses IOException and IOException subclasses Exception, it is a checked exception and must be declared as an exception that may be thrown by this method. But where will the exception actually come from The public interface for method myMethod2() called here declares that an exception of this type can be thrown. Whether that method actually throws the exception itself or calls another method that throws it is unimportant to us; we simply know that we have to either catch the exception or declare that we throw it. The method myMethod1() does not catch the exception, so it declares that it throws it. Now let s look at another legal example, myMethod3().
public void myMethod3() { // Some code that throws a NullPointerException goes here }
According to the comment, this method can throw a NullPointerException. Because RuntimeException is the immediate superclass of NullPointerException, it is an unchecked exception and need not be declared. We can see that myMethod3() does not declare any exceptions. Runtime exceptions are referred to as unchecked exceptions. All other exceptions, meaning all those that do not derive from java.lang.RuntimeException, are checked exceptions. A checked exception must be caught somewhere in your code. If you invoke a method that throws a checked exception but you don t catch the checked exception somewhere, your code will not compile. That s why they re called checked exceptions; the compiler checks to make sure that they re handled or declared. A number of the methods in the Java 2 Standard Edition libraries throw checked exceptions, so you will often write exception handlers to cope with exceptions generated by methods you didn t write. You can also throw an exception yourself, and that exception can be either an existing exception from the Java API or one of your own. To create your own exception, you simply subclass Exception (or one of its subclasses) as follows:
class MyException extends Exception { }
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