INHERITANCE AND POLYMORPHISM - Java Programming: From Problem Analysis to Program Design - page 711

18 . A reference variable of a class can refer to either an object of its own class or
an object of its subclass.
19 . In early binding, a method's definition is associated with its invocation
when the code is compiled.
20 . In late binding, a method's definition is associated with its invocation at
execution time, that is, when the method is executed.
21 . Except for a few (special) cases, Java uses late binding for all methods.
22 . The term polymorphism means assigning multiple meanings to the same
method. In Java, polymorphism is implemented using late binding.
23 . You cannot automatically consider a superclass object to be an object of a
subclass. In other words, you cannot automatically make a reference variable
of a subclass type point to an object of a superclass type.
24 . Suppose that supRef is a reference variable of a superclass type. Moreover,
suppose that supRef points to an object of a subclass. You can use an
appropriate cast operator on supRef and make a reference variable of the
subclass point to the object. On the other hand, if supRef does not point to
a subclass object, and you use a cast operator on supRef to make a
reference variable of the subclass point to the object, then Java will throw
a ClassCastException —indicating that the class cast is not allowed.
25 . An abstract method is a method that has only the heading, not the body.
Moreover, the heading of an abstract method is terminated with a semicolon.
26 . An abstract class is a class that is declared with the reserved word abstract
in its heading.
27 . The following are some of the facts about abstract classes:
￿ An abstract class can contain instance variables, constructors, a finalizer,
and nonabstract methods.
￿ An abstract class can contain an abstract method(s).
￿ If a class contains an abstract method, then the class must be declared abstract.
￿ You cannot instantiate an object of an abstract class. You can only
declare a reference variable of an abstract class type.
￿ You can instantiate an object of a subclass of an abstract class, but only if
the subclass gives the definitions of all the abstract methods of the
superclass.
28 . An interface is a class that contains only abstract methods and/or named
constants.
29 . Java allows a class to implement more than one interface. This is, in fact, the
way to implement a form of multiple inheritance in Java.
30 . In composition, one or more members of a class are objects of one or more
other classes.
31 . Composition (aggregation) can be viewed as a ''has-a'' relationship.

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