INTRODUCTION:

Inheritance is a major component of a powerful and popular programming technique known as object-oriented-programming. Inheritance is a technique that will allow you to define a very general class and then later define more specialized classes by simply adding some new details to the older more general class definitions. This save work because the more specialized class inherits all the properties of the general class and you, the programmer, need only program the new features.

The key topics for this lesson are:

A. Single Inheritance
B. Class Hierarchies
C. Using Inheritance
D. Method Overriding

VOCABULARY:

PARENT CLASS
BASE CLASS
CHILD CLASS

SUPERCLASS
SUBCLASS
DERIVED CLASS

DISCUSSION:

A. Single Inheritance

1.Inheritance enables you to define a new class based on a class that already exists. The new class will be similar to the existing class, but will have some new characteristics. This makes programming easier, because you can build upon your previous work instead of starting out from scratch.

2. The class that is used as a basis for defining a new class is called a superclass (or parent class or base class). The new class based on the superclass is called a subclass (or child class or derived class.)

3. In Java, (unlike with humans) children inherit characteristics from just one parent. This is called single inheritance. Some languages allow a child to inherit from more than one parent. This is called multiple inheritance. With multiple inheritance, it is sometimes hard to tell which parent will contribute what characteristics to the child (as with humans.) Java avoids these problems by using single inheritance.

4. The figure shows a superclass and a subclass. The line between them shows the "is a kind of" relationship. Notice that the arrow points to the superclass. The picture can be read as "a Movie is a kind of Video." The clouds represent classes. That is, the picture does not show any particular Car or any particular Ford, but shows that the class Ford is a subclass of the Car class.

5. Inheritance is between classes, not between objects. A superclass is a blueprint that is followed when an object is constructed. A subclass of the superclass is another blueprint (that looks much like the original), but with added features. The subclass is used to construct objects that look like the superclass ' objects, but with added features.

6. The figure shows a superclass and a subclass, and some objects that have been constructed from each. These objects are shown as rectangles (to convey the idea that they are more real than the classes, which are only designs.) In the picture, "Dilbert's Movie," "Ada's Star Wars," and "Owen's Star Wars" represent actual objects. The cloudy classes represent designs that were used to construct the objects.

B. Hierarchies

1. 1. In a hierarchy, each class has at most one superclass, but might have several subclasses. There is one class, at the "top" of the hierarchy that has no superclass. This is sometimes called the root of the hierarchy.

The figure shows a hierarchy of classes. It shows that "MusicVideo is a kind of Video," "Movie is a kind of Video," and that "HomeMovie is a kind of Video." It also shows that "StarWars is a kind of Movie."

2. A derived class is a class defined by adding instance variables and methods to an existing class. The existing class that the derived class is built upon is called the base class. In our example, the class Video is the base class and the classes MusicVideo, Movie, HomeMovie, and StarWars are derived classes.

3. The terms "superclass" and "subclass" are not absolute. Any class may be the superclass for a subclass derived from it. Just as with human relationships, an individual is a child to some relatives and a parent to others.

4. In Java, the syntax for deriving a child class from a parent class is:

class subclass extends superclass
{
   // new characteristics of the subclass go here
} 

5. Several classes can be declared as subclasses of the same superclass. The subclasses, which might be referred to as "sibling classes," share some structures and behaviors - namely, the ones they inherit from their common superclass. The superclass expresses these shared structures and behaviors. In the diagram above, classes MusicVideo, Movie, and HomeMovie are sibling classes. Inheritance can also extend over several "generations" of classes. This is shown in the diagram, where class StarWars is a subclass of class Movie which is itself a subclass of class Video. In this case, class StarWars is considered to be a subclass of class Video, even though it is not a direct subclass.

C. Using Inheritance

1. Here is a program that uses a class Video to represent movies available at a video rental store. The Video class has basic information in it, and would be OK for documentaries and instructional tapes. But movies need more information. An additional class, Movie, is created that is similar to Video, but has the name of the director and a rating.

class Video
{
  protected String  myTitle;     // name of the item
  protected int     myLength;    // number of minutes
  protected boolean myIsAvail;   // is the tape in the store?

  public Video(String title, int len)
  {
    myTitle = title; myLength = len ; myIsAvail = true; 
  }

  public String toString()
  {
    return myTitle + ", " + myLength +
           " min. available: " + myIsAvail;
  }
}

class Movie extends Video
{
  protected String  myDirector;     // name of the director
  protected String  myRating;       // G, PG, R, or X

  // constructor
  public Movie(String title, int len, String dir, String rating)
  {
    // use the super class' constructor
    super(title, len);

    // initialize what's new to Movie
    myDirector = dir;  myRating = rating;
  }
}

class VideoStore
{

  public static void main (String args[])
  {
    Video item1 = new Video("Juiced on Java", 90 );
    Movie item2 = new Movie("Just Java", 120, "Gosling", "PG" );
    System.out.println(item1.toString());
    System.out.println(item2.toString());
  }
}

2. The class Movie is a derived class (subclass) of Video. An object of type Movie has the following members in it:

Both classes are defined: the Video class can be used to construct objects of that type, and now the Movie class can be used to construct objects of the Movie type.

3. The class definition for Video has a constructor that initializes the member data of Video objects. The class Movie has a constructor that initializes the data of Movie objects. The constructor for class Movie looks like this:

// constructor
public Movie(String title, int len, String dir, String rating)
{
  // use the super class' constructor
  super(title, len);

  // initialize what's new to Movie
  myDirector = dir;  myRating = rating;
}

The statement super(title, len) invokes the base class' constructor to initialize some of the data. Then the next two statements (on one line) initialize the members that only Movie has. When super is used in this way, it must be the first statement in the subclass' constructor.

4. It is not necessary to use super; the following would also work as a constructor for Movie:

// constructor
public Movie(String title, int len, String dir, String rating)
{
  // initialize the inherited members
  myTitle = title; myLength = len ; myIsAvail = true;

  // initialize members unique to Movie
  myDirector = dir;  myRating = rating;
}

In this constructor, each variable of the newly created Movie object is set to an initial value.

5. So far, we have only seen the public and private access modifiers. There is a third access modifier that can be applied to an instance variable or method. If it is declared to be protected, then it can be used in the class where it is defined and in any subclass of that class. This is obviously less restrictive than private and more restrictive than public. Classes that are written specifically to be used as a basis for making subclasses often have protected members. The protected members are there to provide a foundation for the subclasses to build on. But they are still invisible to the public at large.

D. Method Overriding

1. A derived class can override a method from its base class by defining a replacement method with the same signature. For example in our Movie subclass, the toString() method contained in the Video superclass can not reference the new variables that have been added to objects of type Movie, so nothing new is printed out. We need a new toString() method in the class Movie:

// overrides the toString method in the parent class
public String toString()
{
  return myTitle + ", " + myLength +
         " min. available: " + myIsAvail +
         ", dir: " + myDirector + ", " + myRating;  
}

2. Even though the base class has a toString() method, the new definition of toString() in the derived class will override the base class' version . The base class has its method, and the derived has its own method with the same name. With the change in the class Movie the following program will print out the full information for both items.

class VideoStore
{
  public static void main (String args[])
  {
    Video item1 = new Video("Samurai Jack", 90 );
    Movie  item2 = new Movie("Star Wars", 120, "Lucas", "PG");
    System.out.println(item1.toString());
    System.out.println(item2.toString());
  }
}

Samurai Jack, 90 min. available: true
Star Wars, 120 min. available: true, dir: Lucas, PG

The line item1.toString() calls the toString() method defined in Video, and the line item2.toString() calls the toString() method defined in Movie.

3. Sometimes (as in the example) you want a derived class to have its own method, but that method includes everything the derived class' method does. You can use the super reference in this situation to first invoke the original toString() method in the base class as follows:

public String toString()
{
  return super.toString()+ ", dir: " + myDirector + ", " + myRating;  
}

Unlike the case when super is used in a constructor, inside a method super does not have to be used in the first statement.

SUMMARY/ REVIEW:

Inheritance represents the "is a kind of" relationship between types of objects. In practice it may be used to a new features to an existing class. It is the primary tool for reusing your own and standard library classes. Inheritance allows a programmer to derive a new class (called a derived class or a subclass) from another class (called a base class or superclass). A derived class inherits all the data fields and methods (but not constructors) from the base class and can add its own methods or redefine some of the methods of the base class.