11.3

Handling of an Exception

EXCEPTIONS

Java language can result in runtime exceptions. The information available to a

Java compiler, and the level of analysis the compiler performs, are usually not suf 

ficient to establish that such runtime exceptions cannot occur, even though this

may be obvious to the Java programmer. Requiring such exception classes to be

declared would simply be an irritation to Java programmers.

For example, certain code might implement a circular data structure that, by

construction, can never involve

null

 references; the programmer can then be

certain that a

NullPointerException

 cannot occur, but it would be difficult for a

compiler to prove it. The theorem proving technology that is needed to establish

such global properties of data structures is beyond the scope of this Java Language

Specification.

11.3   Handling of an Exception

When an exception is thrown, control is transferred from the code that caused the

exception to the nearest dynamically enclosing

catch

 clause of a

try

 statement

( 14.18) that handles the exception.

A statement or expression is

dynamically enclosed

 by a

catch

 clause if it

appears within the

try

 block of the

try

 statement of which the

catch

 clause is a

part, or if the caller of the statement or expression is dynamically enclosed by the

catch

 clause.

The

caller

 of a statement or expression depends on where it occurs:

If within a method, then the caller is the method invocation expression

( 15.11) that was executed to cause the method to be invoked.

If within a constructor or the initializer for an instance variable, then the caller

is the class instance creation expression ( 15.8) or the method invocation of

newInstance

 that was executed to cause an object to be created.

If within a static initializer or an initializer for a

static

 variable, then the

caller is the expression that used the class or interface so as to cause it to be

initialized.

Whether a particular

catch

 clause

handles

 an exception is determined by

comparing the class of the object that was thrown to the declared type of the

parameter of the

catch

 clause. The

catch

 clause handles the exception if the type

of its parameter is the class of the exception or a superclass of the class of the

exception. Equivalently, a

catch

 clause will catch any exception object that is an

instanceof

 ( 15.19.2) the declared parameter type.

The control transfer that occurs when an exception is thrown causes abrupt

completion of expressions ( 15.5) and statements ( 14.1) until a

catch

 clause is

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