The JavaTM Tutorial

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Trail: Essential Java Classes
Lesson: Handling Errors with Exceptions

## The throw Statement

> All Java methods use the `throw` statement to throw an exception. The `throw` statement requires a single argument: a throwable object. In the Java system, throwable objects are instances of any subclass of the `Throwable` class. Here's an example of a `throw` statement:

> >     throw someThrowableObject;

> If you attempt to throw an object that is not throwable, the compiler refuses to compile your program and displays an error message similar to the following:

> >     testing.java:10: Cannot throw class java.lang.Integer;
> >     it must be a subclass of class java.lang.Throwable.
> >                 throw new Integer(4);
> >                 ^

> The next page, The `Throwable` Class and Its Subclasses, talks more about the `Throwable` class.

> [PENDING: EmptyStackException is not a checked exception, so technically pop doesn't have to use the throws clause]

> Let's look at the `throw` statement in context. The following method is taken from a class that implements a common stack object. The `pop` method removes the top element from the stack and returns it:

> >     public Object pop() throws EmptyStackException {
> >         Object obj;

> >         if (size == 0)
> >             throw new EmptyStackException();

> >         obj = objectAt(size - 1);
> >         setObjectAt(size - 1, null);
> >         size--;
> >         return obj;
> >     }

> The `pop` method checks to see if there are any elements on the stack. If the stack is empty (its size is equal to 0), then `pop` instantiates a new `EmptyStackException` object and throws it. The `EmptyStackException` class is defined in the `java.util` package. Later pages in this lesson describe how you can create your own exception classes. For now, all you really need to remember is that you can throw only objects that inherit from the `java.lang.Throwable` class.

### The `throws` Clause

> You'll notice that the declaration of the `pop` method contains this clause:

> >     throws EmptyStackException

> The `throws` clause specifies that the method can throw an `EmptyStackException`. As you know, the Java language requires that methods either catch or specify all checked exceptions that can be thrown within the scope of that method. You do this with the `throws` clause of the method declaration. For more information about this requirement see Java's Catch or Specify Requirement. Also, Specifying the Exceptions Thrown by a Method shows you in more detail how a method can specify the exceptions it can throw.

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