Java's Inheritance Mechanisms

I. Inheritance

    A. You create a subclass using the extends keyword

    B. Access Modifiers: more on these later when packages are introduced

        1. public
	2. protected
	3. private

    C. Constructors

        1. use super to call the superclass constructor
	2. super must be the first statement in a constructor
	3. if super is omitted then the superclass's default 0-argument
	    constructor is called
	    a. if the superclass does not provide a 0-argument
		constructor, then the java compiler will complain
		
	4. constructors are called in order of derivation, from superclass
	    to subclass
	5. super can be used to access any member of a superclass. It is
	    commonly used to access the superclass's method when you are
	    overriding it in the subclass. For example, if you are
	    overriding a method named "push" and you want to first
	    call the superclass's implementation of push, you would
	    write "super.push(...)"

    D. All methods are virtual by default

        1. Use the final keyword to prevent a method from being
	   overridden

	2. abstract methods with no method body can be created by prefixing
	    the method name with the abstract keyword

	3. classes that contain abstract methods must be declared abstract
	    themselves

	    a. subclasses must be declared abstract if they fail to define
	        an abstract method that they inherit from an abstract 
		superclass

    E. Object class: Implicit superclass for any class that is not defined
        to extend another class--all objects can be cast to Object

II. Interfaces

    A. An interface specifies a set of methods that a class agrees to
       implement. An interface does not provide any implementation for these
       methods. In fact, an interface can provide no implementation whatsoever.
       It cannot even declare instance variables (it can only declare constants).

       1. Similar to an abstract class, but different in that it
		provides no implementation whatsoever, whereas
		an abstract class may provide implementations for
		one or more methods

	    a. Note: Interfaces do support default methods that provide
		a default method body and static methods that provide
		method bodies. However, these are more advanced concepts
		that we will not consider in this course.

       2. Classes can choose to implement as many interfaces as they desire

       3. Syntax: access interface {
                     return-type method-name1(param-list);
		     return-type method-name2(param-list);
		     type var1 = value;
		     type var2 = value;
		     // ...
		     return-type method-nameN(param-list);
		     type varM = value;
		   }

       Example: public interface Iterator {
                    boolean hasNext();
		    Object next();
		    void remove();
		}
 
 	   a. access to an interface is either public or 
		default (i.e., package)
	   b. all methods/variables are implicitly declared public,
		hence you do not need to prefix their declarations
		(if the interface is declared with package level
		access, the methods will only be visible to classes
		within the package, despite their implicit guarantee
		of publicness).
	    c. all variables are implicitly declared public, static,
		and final (i.e,. they are constants shared by all
		classes that implement the interface)

       4. Interfaces can extend other interfaces

    B. Implementing Interfaces: A class that implements an interface must
       declare and define each of the interface's methods and must declare
       those methods public, regardless of whether they are declared public
       in the interface declaration

       1. Use the implements keyword

       2. Specify multiple interfaces by providing a list of names separated by
          commas

       3. A class that specifies multiple interfaces is in effect supporting
	  multiple inheritance. However, since it can only extend one class,
	  it can only inherit implementation from one class.

    C. Variables can be declared to be of an interface type.
	1. A variable declared to be of an interface type may only access the
		methods declared by that interface, even if the variable
		points to an object that supports additional methods.

    D. Variables in interfaces provide a way of declaring a set of global,
    	read-only variables that will be used by many of the classes in an 
	application.

       1. Whenever you would declare a global constant in C++, 
          place a similar variable into an interface in Java.

       2. Classes that want to reference these global variables will implement
          the interface
          
	  Example: public interface GlobalVariables {
	             int MIN = 0;
		     int MAX = 10;
		     string BADOPERATOR = "Error: Bad Operator";
		   }

		   public class Node implements GlobalVariables {
		      int minValue = MIN;
		      ...
		   }

III. Types and Inheritance
    A. Definitions
        1. Type: A set of values + a set of operations that manipulate those values
        2. Abstract Type: A type that specifies what its operations
	   should do, but which provides no implementation of its 
           operations
        3. Subtype: A type that permits the same values as another type T,
           the same operations as T, and provides one or more additional 
           operations.
           a. The type T is often called the supertype 
           b. A subtype can be used whereever its supertype is used, because
              it supports the same set of operations
           c. Despite having the prefix sub, a subtype actually provides a
              superset of T's operations
    B. Relationship between Types, Classes and Inheritance
        1. Classes provide a concrete way to declare and implement
		types
        2. Interfaces represent abstract types
	3. Abstract base classes represent abstract types if they
		provide no default implementations. However, many
		abstract base classes provide some implementation,
		and so lie somewhere on the continuum between a
		concrete type and an abstract type.
        3. Subclasses represent subtypes of their superclasses.