11.5

The Exception Hierarchy

EXCEPTIONS

This example declares an exception class

TestException

. The

main

 method

of class

Test

 invokes the

thrower

 method four times, causing exceptions to be

thrown three of the four times. The

try

 statement in method

main

 catches each

exception that the

thrower

 throws. Whether the invocation of

thrower

 completes

normally or abruptly, a message is printed describing what happened.

The declaration of the method

thrower

 must have a

throws

 clause because

it can throw instances of

TestException

, which is a checked exception class

( 11.2). A compile time error would occur if the

throws

 clause were omitted.

Notice that the

finally

 clause is executed on every invocation of

thrower

,

whether or not an exception occurs, as shown by the  

[thrower(

...

) done]

  out 

put that occurs for each invocation

11.5   The Exception Hierarchy

The possible exceptions in a Java program are organized in a hierarchy of classes,

rooted at class

Throwable

 ( 11.5,  20.22), a direct subclass of

Object

. The

classes

Exception

 and

Error

 are direct subclasses of

Throwable

. The class

RuntimeException

 is a direct subclass of

Exception

.

The exception classes declared by the standard packages

java.lang

,

java.util

,

java.io

 and

java.net

 are called the

standard exception classes

.

Java programs can use the pre existing exception classes in

throw

 statements,

or define additional exception classes, as subclasses of

Throwable

 or of any of its

subclasses, as appropriate. To take advantage of Java's compile time checking for

exception handlers, it is typical to define most new exception classes as checked

exception classes, specifically as subclasses of

Exception

 that are not subclasses

of

RuntimeException

.

11.5.1   The Classes

Exception

 and

RuntimeException

The class

Exception

 is the superclass of all the exceptions that ordinary pro 

grams may wish to recover from.

11.5.1.1

Standard Runtime Exceptions

The class

RuntimeException

 is a subclass of class

Exception

. The subclasses

of

RuntimeException

 are unchecked exception classes.

Package

java.lang

 defines the following standard unchecked runtime excep 

tions, which, like all other classes in package

java.lang

, are implicitly imported

and therefore may be referred to by their simple names:

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