EXPRESSIONS

Expressions and Run Time Checks

15.4

Method invocation ( 15.11). The particular method used for an invocation

o.m(

...

)

 is chosen based on the methods that are part of the class or interface

that is the type of

o

. For instance methods, the class of the object referenced

by the run time value of

o

 participates because a subclass may override a spe 

cific method already declared in a parent class so that this overriding method

is invoked. (The overriding method may or may not choose to further invoke

the original overridden

m

 method.)

The

instanceof

 operator ( 15.19.2). An expression whose type is a refer 

ence type may be tested using

instanceof

 to find out whether the class of the

object referenced by the run time value of the expression is assignment com 

patible ( 5.2) with some other reference type.

Casting ( 5.4,  15.15). The class of the object referenced by the run time

value of the operand expression might not be compatible with the type speci 

fied by the cast. For reference types, this may require a run time check that

throws an error if the class of the referenced object, as determined at run time,

is not assignment compatible ( 5.2) with the target type.

Assignment to an array component of reference type ( 10.10,  15.12,

 15.25.1). The type checking rules allow the array type

S[]

 to be treated as a

subtype of

T[]

 if

S

 is a subtype of

T

, but this requires a run time check for

assignment to an army component, similar to the check performed for a cast.

Exception handling ( 14.18). An exception is caught by a

catch

 clause only

if the class of the thrown exception object is an

instanceof

 the type of the

formal parameter of the

catch

 clause.

The first two of the cases just listed ought never to result in detecting a type error.

Thus, a Java run time type error can occur only in these situations:

In a cast, when the actual class of the object referenced by the value of the

operand expression is not compatible with the target type specified by the cast

operator ( 5.4,  15.15); in this case a

ClassCastException

 is thrown.

In an assignment to an array component of reference type, when the actual

class of the object referenced by the value to be assigned is not compatible

with the actual run time component type of the array ( 10.10,  15.12,

 15.25.1); in this case an

ArrayStoreException

 is thrown.

When an exception is not caught by any

catch

 handler ( 11.3); in this case

the thread of control that encountered the exception first invokes the method

uncaughtException

( 20.21.31) for its thread group and then terminates.

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