Several system objects support exception handling within PowerBuilder.
The object type Throwable is the root datatype for all user-defined exception and system error types. Two other system object types, RuntimeError and Exception, derive from Throwable.
PowerBuilder runtime errors are represented in the RuntimeError object type. For more robust error-handling capabilities, the RuntimeError type has its own system-defined descendants; but the RuntimeError type contains all information required for dealing with PowerBuilder runtime errors.
One of the descendants of RuntimeError is the NullObjectError type that is thrown by the system whenever a null object reference is encountered. This allows you to handle null-object-reference errors explicitly without having to differentiate them from other runtime errors that might occur.
Error types that derive from RuntimeError are typically used by the system to indicate runtime errors. RuntimeErrors can be caught in a try-catch block, but it is not necessary to declare where such an error condition might occur. (PowerBuilder does that for you, since a system error can happen anywhere anytime the application is running.) It is also not a requirement to catch these types of errors.
The system object Exception also derives from Throwable and is typically used as an ancestor object for user-defined exception types. It is the root class for all checked exceptions. Checked exceptions are user-defined exceptions that must be caught in a try-catch block when thrown, or that must be declared in the prototype of a method when thrown outside of a try-catch block.
The PowerScript compiler checks the local syntax where you throw checked exceptions to make sure you either declare or catch these exception types. Descendants of RuntimeError are not checked by the compiler, even if they are user defined or if they are thrown in a script rather than by the runtime system.
