EXCEPTIONS

Why Runtime Exceptions are Not Checked

11.2.2

11.2   Compile Time Checking of Exceptions

The Java language checks, at compile time, that a Java program contains handlers

for

checked exceptions

, by analyzing which checked exceptions can result from

execution of a method or constructor. For each checked exception which is a pos 

sible result, the

throws

 clause for the method ( 8.4.4) or constructor ( 8.6.4)

must mention the class of that exception or one of the superclasses of the class of

that exception. This compile time checking for the presence of exception handlers

is designed to reduce the number of exceptions which are not properly handled.

The

unchecked exceptions classes

 are the class

RuntimeException

 and its

subclasses, and the class

Error

 and its subclasses. All other exception classes are

checked exception classes

. The standard Java API defines a number of exception

classes, both checked and unchecked. Additional exception classes, both checked

and unchecked, may be declared by Java programmers. See  11.5 for a descrip 

tion of the Java exception class hierarchy and the exception classes defined by the

standard Java API and Java Virtual Machine.

The checked exception classes named in the

throws

 clause are part of the

contract between the implementor and user of the method or constructor. The

throws

 clause of an overriding method may not specify that this method will

result in throwing any checked exception which the overridden method is not per 

mitted, by its

throws

 clause, to throw. When interfaces are involved, more than

one method declaration may be overridden by a single overriding declaration. In

this case, the overriding declaration must have a

throws

 clause that is compatible

with

all

 the overridden declarations ( 9.4).

Variable initializers for fields ( 8.3.2) and static initializers ( 8.5) must not

result in a checked exception; if one does, a compile time error occurs.

11.2.1   Why Errors are Not Checked

Those unchecked exception classes which are the

error classes

  (

Error

 and its

subclasses) are exempted from compile time checking because they can occur at

many points in the program and recovery from them is difficult or impossible. A

Java program declaring such exceptions would be cluttered, pointlessly.

11.2.2   Why Runtime Exceptions are Not Checked

The

runtime exception classes

  (

RuntimeException

 and its subclasses) are

exempted from compile time checking because, in the judgment of the designers

of Java, having to declare such exceptions would not aid significantly in establish 

ing the correctness of Java programs. Many of the operations and constructs of the

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