CHAPTER 15: Expressions

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15.4 Expressions and Run-Time Checks

If the type of an expression is a primitive type, then the value of the expression is of that same primitive type. But if the type of an expression is a reference type, then the class of the referenced object, or even whether the value is a reference to an object rather than null , is not necessarily known at compile time. There are a few places in the Java language where the actual class of a referenced object affects program execution in a manner that cannot be deduced from the type of the expression. They are as follows:

• Method invocation (S15.11). 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 specific 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 (S15.19.2). An expression whose type is a reference 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 compatible (S5.2) with some other reference type.
• Casting (S5.4, S15.15). The class of the object referenced by the run-time value of the operand expression might not be compatible with the type specified 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 (S5.2) with the target type.
• Assignment to an array component of reference type (S10.10, S15.12, S15.25.1). 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 (S14.18). 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 (S5.4, S15.15); 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 (S10.10, S15.12, S15.25.1); in this case an ArrayStoreException is thrown.
• When an exception is not caught by any catch handler (S11.3); in this case the thread of control that encountered the exception first invokes the method uncaughtException (S20.21.31) for its thread group and then terminates.