Implicit Subclass-Object-to-Superclass-Object Conversion

• Consider the following declarations, where Rectangle is a subclass of Quadrilateral:

  Quadrilateral qObject = new Quadrilateral(), qRef;
  Rectangle rObject = new Rectangle(),rRef;

• Let's consider the 4 ways to combine sub- and super-class objects with sub- and super-class references:
  • Referring to a superclass object (e.g., Quadrilateral) with a superclass reference:

    qRef = qObject;

  • Referring to a subclass object (e.g., Rectangle) with a subclass reference:

    rRef = rObject;

  • Referring to a subclass object (e.g., Rectangle) with a superclass reference:

    qRef = rObject;

    Code using qRef cannot refer to Rectangle members.

  • Referring to a superclass object (e.g., Quadrilateral) with a subclass reference:

    rRef = qObject; // a syntax error

    A cast must be used, and it will be checked at runtime.

    rRef = (Rectangle) qObject; // not a syntax error, but possibly a runtime error