14.16

The

throw

 Statement

BLOCKS AND STATEMENTS

The

throw

 statement is contained in the

try

 block of a

try

 statement

( 14.18) and the type of the

Expression

 is assignable ( 5.2) to the type of the

parameter of at least one

catch

 clause of the

try

 statement. (In this case we

say the thrown value is

caught

 by the

try

 statement.)

The

throw

 statement is contained in a method or constructor declaration and

the type of the

Expression

 is assignable ( 5.2) to at least one type listed in the

throws

 clause ( 8.4.4,  8.6.4) of the declaration.

A

throw

 statement first evaluates the

Expression

. If the evaluation of the

Expression

 completes abruptly for some reason, then the

throw

 completes

abruptly for that reason. If evaluation of the

Expression

 completes normally, pro 

ducing a value

V

, then the

throw

 statement 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 ( 14.18) whose

try

 blocks contain

the

throw

 statement, then any

finally

 clauses of those

try

 statements are exe 

cuted 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 (or the method invocation of method

newInstance

 of class

Class

)

that invoked the constructor will complete abruptly because of the

throw

.

If a

throw

 statement is contained in a static initializer ( 8.5), then a compile 

time check ensures that either its value is always an unchecked exception or its

value is always caught by some

try

 statement that contains it. If, 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).

By convention, user declared throwable types should usually be declared to

be subclasses of class

Exception

, which is a subclass of class

Throwable

 ( 11.5,

 20.22).

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