CLASSES

Method Throws

8.4.4

See  17 for more discussion of threads and locks.

8.4.4   Method Throws

A

throws clause

 is used to declare any checked exceptions ( 11.2) that can result

from the execution of a method or constructor:

Throws:

throws

ClassTypeList

ClassTypeList:

ClassType

ClassTypeList

 ,

ClassType

A compile time error occurs if any

ClassType

 mentioned in a

throws

 clause is not

the class

Throwable

 ( 20.22) or a subclass of

Throwable

. It is permitted but not

required to mention other (unchecked) exceptions in a

throws

 clause.

For each checked exception that can result from execution of the body of a

method or constructor, a compile time error occurs unless that exception type or a

superclass of that exception type is mentioned in a

throws

 clause in the declara 

tion of the method or constructor.

The requirement to declare checked exceptions allows the compiler to ensure

that code for handling such error conditions has been included. Methods or con 

structors that fail to handle exceptional conditions thrown as checked exceptions

will normally result in a compile time error because of the lack of a proper excep 

tion type in a

throws

 clause. Java thus encourages a programming style where

rare and otherwise truly exceptional conditions are documented in this way.

The predefined exceptions that are not checked in this way are those for which

declaring every possible occurrence would be unimaginably inconvenient:

Exceptions that are represented by the subclasses of class

Error

, for example

OutOfMemoryError

, are thrown due to a failure in or of the virtual machine.

Many of these are the result of linkage failures and can occur at unpredictable

points in the execution of a Java program. Sophisticated programs may yet

wish to catch and attempt to recover from some of these conditions.

The exceptions that are represented by the subclasses of the class

RuntimeException

, for example

NullPointerException

, result from

runtime integrity checks and are thrown either directly from the Java program

or in library routines. It is beyond the scope of the Java language, and perhaps

beyond the state of the art, to include sufficient information in the program to

reduce to a manageable number the places where these can be proven not to

occur.

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