Programmer's Guide to Java™ Certification, A: A Comprehensive Primer, Second Edition

6.7. Polymorphism and Dynamic Method Lookup

Which object a reference will actually denote during runtime, cannot always be determined at compile time. Polymorphism allows a reference to denote objects of different types at different times during execution. A supertype reference exhibits polymorphic behavior, since it can denote objects of its subtypes.

When a non-private instance method is invoked on an object, the method definition actually executed is determined both by the type of the object at runtime and the method signature. Dynamic method lookup is the process of determining which method definition a method signature denotes during runtime, based on the type of the object. However, a call to a private instance method is not polymorphic. ...