The CAST operator

Important consideration has to be taken when assigning object references to different target types or classes. A Widening Reference Conversion occurs when an object reference is converted to a superclass that can accommodate any possible reference of the original type or class. A Narrowing Reference Conversion occurs when an object reference of a superclass is converted to a subtype or subclass of the original object reference. For example, in a vehicle class hierarchy with Vehicle and Car classes, Car is a subclass that inherits from the Vehicle superclass. When assigning a Car object reference to a Vehicle variable, Widening Reference Conversion takes place. When assigning a Vehicle object reference to a Car variable, Narrowing Reference Conversion occurs.

While widening conversion does not require casts and will not produce compilation or runtime errors, narrowing conversion needs the CAST operator to convert to the target type or class:

CAST( object_reference AS type_or_class )

The next example creates a java.lang.StringBuffer object, and assigns the reference to a java.lang.Object variable (implying Widening Reference Conversion); then the object reference is assigned back to the java.lang.StringBuffer variable (implying Narrowing Reference Conversion and CAST operator usage):

IMPORT JAVA java.lang.Object
IMPORT JAVA java.lang.StringBuffer
MAIN
  DEFINE o java.lang.Object
  DEFINE sb java.lang.StringBuffer
  LET sb = StringBuffer.create()
  -- Widening Reference Conversion
  LET o = sb
  -- Narrowing Reference Conversion needs CAST()
  LET sb = CAST( o AS StringBuffer )
END MAIN