Blocks and Statements - The Java Language Specification - page 399

A throw statement first evaluates the Expression . Then:
• If evaluation of the Expression completes abruptly for some reason, then the throw
completes abruptly for that reason.
• If evaluation of the Expression completes normally, producing a non- null value V ,
then the throw statement completes abruptly, the reason being a throw with value V .
• If evaluation of the Expression completes normally, producing a null value, then an
instance V' of class NullPointerException is created and thrown instead of null . The
throw statement then completes abruptly, the reason being a throw with value V' .
It can be seen, then, that a throw statement always completes abruptly.
If there are any enclosing try statements (§ 14.20 ) whose try blocks contain the throw state-
ment, then any finally clauses of those try statements are executed as control is transferred
outward, until the thrown value is caught. Note that abrupt completion of a finally clause
can disrupt the transfer of control initiated by a throw statement.
If a throw statement is contained in a method declaration, but its value is not caught by some
try statement that contains it, then the invocation of the method completes abruptly because
of the throw .
If a throw statement is contained in a constructor declaration, but its value is not caught by
some try statement that contains it, then the class instance creation expression that invoked
the constructor will complete abruptly because of the throw (§ 15.9.4 ) .
If a throw statement is contained in a static initializer (§ 8.7 ), then a compile-time check
(§ 11.2.3 ) ensures that either its value is always an unchecked exception or its value is al-
ways caught by some try statement that contains it. If at run time, despite this check, the
value is not caught by some try statement that contains the throw statement, then the value is
rethrown if it is an instance of class Error or one of its subclasses; otherwise, it is wrapped
in an ExceptionInInitializerError object, which is then thrown (§ 12.4.2 ).
If a throw statement is contained in an instance initializer (§ 8.6 ) , then a compile-time check
(§ 11.2.3 ) ensures that either its value is always an unchecked exception or its value is al-
ways caught by some try statement that contains it, or the type of the thrown exception (or
one of its superclasses) occurs in the throws clause of every constructor of the class.
By convention, user-declared throwable types should usually be declared to be sub-
classes of class Exception , which is a subclass of class Throwable (§ 11.1.1 ) .