Syntax of throw
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;
What if object is not Throwable ?
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. Example
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
Thethrows
clause specifies that the method can throw anEmptyStackException
. 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 thethrows
clause of the method declaration. For more information about this requirement see Java's catching or throwing exception. Also, Specifying the Exceptions Thrown by a Method shows you in more detail how a method can specify the exceptions it can throw.
throw vs throws
See here for the difference between throw and throws.
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