Which of the following members of a superclass can be extended by a subclass

Answer: C

Class A & class B both contain display() method, class B inherits class A, when display() method is called by object of class B, display() method of class B is executed rather than that of Class A. output:
$ javac inheritance_demo.java $ java inheritance_demo 2

Creating a subclass can be as simple as including the extends clause in your class declaration (such as in the declaration in ImaginaryNumber above). However, you usually have to make other provisions in your code when subclassing a class, such as overriding methods.

What Member Variables Does a Subclass Inherit?

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Rule:A subclass inherits all of the member variables within its superclass that are accessible to that subclass (unless the member variable is hidden by the subclass).

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That is, subclasses

• inherit those member variables declared as public or protected
• inherit those member variables declared with no access specifier as long as the subclass is in the same package as the superclass. These variables are sometimes known as "friendly".
• don't inherit a superclass's member variable if the subclass declares a member variable using the same name. The subclass's member variable is said to hide the member variable in the superclass.
• don't inherit private member variables

Hiding Member Variables

As mentioned in the previous section, member variables defined in the subclass hide member variables of the same name in the superclass.

While this feature of the Java language is powerful and convenient, it can be a fruitful source of errors: hiding a member variable can be done deliberately or by accident. So, when naming your member variables be careful to only hide those member variables that you actually wish to hide.

One interesting feature of Java member variables is that a class can access a hidden member variable through its superclass. Consider this superclass and subclass pair:

In fact, in Java, all classes must be derived from some class. Which leads to the question "Where does it all begin?" The top-most class, the class from which all other classes are derived, is the Object class defined in java.lang. Object is the root of a hierarchy of classes.

The subclass inherits state and behavior in the form of variables and methods from its superclass. The subclass can just use the items inherited from its superclass as is, or the subclass can modify or override it. So, as you drop down in the hierarchy, the classes become more and more specialized:

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Definition: A subclass is a class that derives from another class. A subclass inherits state and behavior from all of its ancestors. The term superclass refers to a class's direct ancestor as well as all of its ascendant classes.

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Now would be a good time to review the discussion in What Is Inheritance?.

Creating Subclasses

To create a subclass of another class use the extends clause in your class declaration. (The Class Declaration explains all of the components of a class declaration in detail.) As a subclass, your class inherits member variables and methods from its superclass. Your class can choose to hide variables or override methods inherited from its superclass.

Writing Final Classes and Methods

Sometimes, for security or design reasons, you want to prevent your class from being subclassed. Or, you may just wish to prevent certain methods within your class from being overriden. In Java, you can achieve either of these goals by marking the class or the method as final.

Writing Abstract Classes and Methods

On the other hand, some classes are written for the sole purpose of being subclassed (and are not intended to ever be instantiated). These classes are called abstract classes and often contain abstract methods.

The Object Class

All objects in the Java environment inherit either directly or indirectly from the Object class. This section talks about the interesting methods in Object--methods that you may wish to invoke or override.

11.59  A class design requires that a particular member variable must be accessible by any subclasses of this class, but otherwise not by classes which are not members of the same package. What should be done to achieve this?

There are two ways to reuse existing classes, namely, composition and inheritance. With composition (aka aggregation), you define a new class, which is composed of existing classes. With inheritance, you derive a new class based on an existing class, with modifications or extensions.

Composition

We shall begin with reusing classes via composition - through examples.

Composition EG. 1: The Author and Book Classes

Let's start with the Author class

A class called

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3 is designed as shown in the class diagram. It contains:

• Three

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  4 member variables:

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  6),

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  7 (

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  6), and

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  0 of either

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  2 - you might also use a

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  3 variable called

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  4 having value of

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  6).
• A constructor to initialize the

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  9 with the given values.
  (There is no default constructor, as there is no default value for

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  9.)
• Public getters/setters:

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  9, as these properties are not designed to be changed.)
• A

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  9 method that returns "

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  0", e.g., "

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  1".

The Author Class (Author.java)

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A Test Driver for the Author Class (TestAuthor.java)

A Book is written by one Author - Using an "Object" Member Variable

Let's design a

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2 class. Assume that a book is written by one (and exactly one) author. The Book class (as shown in the class diagram) contains the following members:

• Four

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  4 member variables:

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  5 (

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  6),

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  6 (an instance of the

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  3 class we have just created, assuming that each book has exactly one author),

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  8 (

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  9), and

  class Goalkeeper extends SoccerPlayer {......}
  class MyApplet extends java.applet.Applet {.....}
  class Cylinder extends Circle {......}
  

  0 (

  class Goalkeeper extends SoccerPlayer {......}
  class MyApplet extends java.applet.Applet {.....}
  class Cylinder extends Circle {......}
  

  1).
• The

  class Goalkeeper extends SoccerPlayer {......}
  class MyApplet extends java.applet.Applet {.....}
  class Cylinder extends Circle {......}
  

  2 getters and setters:

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  3,

  class Goalkeeper extends SoccerPlayer {......}
  class MyApplet extends java.applet.Applet {.....}
  class Cylinder extends Circle {......}
  

  4,

  class Goalkeeper extends SoccerPlayer {......}
  class MyApplet extends java.applet.Applet {.....}
  class Cylinder extends Circle {......}
  

  5,

  class Goalkeeper extends SoccerPlayer {......}
  class MyApplet extends java.applet.Applet {.....}
  class Cylinder extends Circle {......}
  

  6,

  class Goalkeeper extends SoccerPlayer {......}
  class MyApplet extends java.applet.Applet {.....}
  class Cylinder extends Circle {......}
  

  7,

  class Goalkeeper extends SoccerPlayer {......}
  class MyApplet extends java.applet.Applet {.....}
  class Cylinder extends Circle {......}
  

  8.
• A

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  9 that returns "

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  0". You could reuse the

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  3's

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  9 method, which returns "

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  3".

The Book Class (Book.java)

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A Test Driver Program for the Book Class (TestBook.java)

Notes: In this example, I used "

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5" for

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2 class instead of "

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6" to illustrate that you can have a variable

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5 in both the

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3 and

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2 classes, but they are distinct.

Composition EG. 2: The Point and Line Classes

As an example of reusing a class via composition, suppose that we have an existing class called

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0, defined as shown in the above class diagram. The source code is HERE.

Suppose that we need a new class called

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1, we can design the

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1 class by re-using the

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0 class via composition. We say that "A line is composed of two points", or "A line has two points". Composition exhibits a "has-a" relationship.

UML Notation: In UML notations, composition is represented as a diamond-head line pointing to its constituents.

The Line Class via Composition (Line.java)

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A Test Driver for Line Class (TestLine.java)

Exercise: Try writing these more complex methods for the

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1 class:

Composition EG. 3: The Point and Circle Classes

Suppose that we have an existing class called

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0, defined as shown in the class diagram. The source code is HERE.

A class called

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6 is designed as shown in the class diagram.

It contains:

• Two

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  4 member variables: a

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  0 class, which we created earlier).
• The constructors,

  class Goalkeeper extends SoccerPlayer {......}
  class MyApplet extends java.applet.Applet {.....}
  class Cylinder extends Circle {......}
  

  2 getters and setters.
• Methods

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  2,

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  3,

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  4,

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  6,

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  7, etc.
• A

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  9 method that returns a string description of

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  0". You should re-use the

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  9 to print "

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  3".
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  4 method that returns the distance from the center of

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  9 instance to the center of the given

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  6 instance (called

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  7).

The Circle class (Circle.java)

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A Test Driver for the Circle Class (TestCircle.java)

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Exercises

LINK TO EXERCISES

In OOP, we often organize classes in hierarchy to avoid duplication and reduce redundancy. The classes in the lower hierarchy inherit all the variables (static attributes) and methods (dynamic behaviors) from the higher hierarchies. A class in the lower hierarchy is called a subclass (or derived, child, extended class). A class in the upper hierarchy is called a superclass (or base, parent class). By pulling out all the common variables and methods into the superclasses, and leave the specialized variables and methods in the subclasses, redundancy can be greatly reduced or eliminated as these common variables and methods do not need to be repeated in all the subclasses. For example,

A subclass inherits all the variables and methods from its superclasses, including its immediate parent as well as all the ancestors. It is important to note that a subclass is not a "subset" of a superclass. In contrast, subclass is a "superset" of a superclass. It is because a subclass inherits all the variables and methods of the superclass; in addition, it extends the superclass by providing more variables and methods.

In Java, you define a subclass using the keyword "

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8", e.g.,

class Goalkeeper extends SoccerPlayer {......}
class MyApplet extends java.applet.Applet {.....}
class Cylinder extends Circle {......}

UML Notation: The UML notation for inheritance is a solid line with a hollow arrowhead leading from the subclass to its superclass. By convention, superclass is drawn on top of its subclasses as shown.

Inheritance EG. 1: The Circle and Cylinder Classes

In this example, we derive a subclass called

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9 from the superclass

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6, which we have created in the previous chapter. It is important to note that we reuse the class

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6. Reusability is one of the most important properties of OOP. (Why reinvent the wheels?) The class

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9 inherits all the member variables (

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8 and

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04) and methods (

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05,

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06, among others) from its superclass

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6. It further defines a variable called

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08, two public methods -

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09 and

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10 and its own constructors, as shown:

Circle.java (Re-produced)

Cylinder.java

A Test Drive for the Cylinder Class (TestCylinder.java)

Keep the "

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11" and "

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12" in the same directory as "

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13" (because we are reusing the class

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6). Compile and run the program.

Method Overriding & Variable Hiding

A subclass inherits all the member variables and methods from its superclasses (the immediate parent and all its ancestors). It can use the inherited methods and variables as they are. It may also override an inherited method by providing its own version, or hide an inherited variable by defining a variable of the same name.

For example, the inherited method

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06 in a

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9 object computes the base area of the cylinder. Suppose that we decide to override the

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06 to compute the surface area of the cylinder in the subclass

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9. Below are the changes:

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If

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06 is called from a

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6 object, it computes the area of the circle. If

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06 is called from a

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9 object, it computes the surface area of the cylinder using the overridden implementation. Note that you have to use public accessor method

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05 to retrieve the

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8 of the

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6, because

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8 is declared

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4 and therefore not accessible to other classes, including the subclass

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9.

But if you override the

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06 in the

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9, the

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10 (

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32) no longer works. It is because the overridden

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06 will be used in

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9, which does not compute the base area. You can fix this problem by using

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35 to use the superclass' version of

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06. Note that

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35 can only be issued from the subclass definition, but no from an instance created, e.g.

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38, as it break the information hiding and encapsulation principle.

Annotation @Override (JDK 1.5)

The "

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39" is known as annotation (introduced in JDK 1.5), which asks compiler to check whether there is such a method in the superclass to be overridden. This helps greatly if you misspell the name of the method to be overridden. For example, suppose that you wish to override method

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9 in a subclass. If

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39 is not used and

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9 is misspelled as

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43, it will be treated as a new method in the subclass, instead of overriding the superclass. If

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39 is used, the compiler will signal an error.

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39 annotation is optional, but certainly nice to have.

Annotations are not programming constructs. They have no effect on the program output. It is only used by the compiler, discarded after compilation, and not used by the runtime.

Keyword "super"

Recall that inside a class definition, you can use the keyword

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9 to refer to this instance. Similarly, the keyword

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47 refers to the superclass, which could be the immediate parent or its ancestor.

The keyword

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47 allows the subclass to access superclass' methods and variables within the subclass' definition. For example,

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49 and

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50 can be used invoke the superclass’ constructor. If the subclass overrides a method inherited from its superclass, says

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06, you can use

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35 to invoke the superclass' version within the subclass definition. Similarly, if your subclass hides one of the superclass' variable, you can use

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53 to refer to the hidden variable within the subclass definition.

More on Constructors

Recall that the subclass inherits all the variables and methods from its superclasses. Nonetheless, the subclass does not inherit the constructors of its superclasses. Each class in Java defines its own constructors.

In the body of a constructor, you can use

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54 to invoke a constructor of its immediate superclass. Note that

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54, if it is used, must be the first statement in the subclass' constructor. If it is not used in the constructor, Java compiler automatically insert a

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49 statement to invoke the no-arg constructor of its immediate superclass. This follows the fact that the parent must be born before the child can be born. You need to properly construct the superclasses before you can construct the subclass.

Default no-arg Constructor

If no constructor is defined in a class, Java compiler automatically create a no-argument (no-arg) constructor, that simply issues a

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49 call, as follows:

Take note that:

• The default no-arg constructor will not be automatically generated, if one (or more) constructor was defined. In other words, you need to define no-arg constructor explicitly if other constructors were defined.
• If the immediate superclass does not have the default constructor (it defines some constructors but does not define a no-arg constructor), you will get a compilation error in doing a

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  49 call. Note that Java compiler inserts a

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  49 as the first statement in a constructor if there is no

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  60.

Single Inheritance

Java does not support multiple inheritance (C++ does). Multiple inheritance permits a subclass to have more than one direct superclasses. This has a serious drawback if the superclasses have conflicting implementation for the same method. In Java, each subclass can have one and only one direct superclass, i.e., single inheritance. On the other hand, a superclass can have many subclasses.

Common Root Class - java.lang.Object

Java adopts a so-called common-root approach. All Java classes are derived from a common root class called

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61. This

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62 class defines and implements the common behaviors that are required of all the Java objects running under the JRE. These common behaviors enable the implementation of features such as multi-threading and garbage collector.

Inheritance EG. 2: The Point2D and Point3D Classes

The Superclass Point2D.java

The Subclass Point3D.java

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A Test Driver for Point2D and Point3D Classes (TestPoint2DPoint3D.java)

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Inheritance EG. 3: Superclass Person and its Subclasses

Suppose that we are required to model students and teachers in our application. We can define a superclass called

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63 to store common properties such as

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5 and

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65, and subclasses

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66 and

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67 for their specific properties. For students, we need to maintain the courses taken and their respective grades; add a course with grade, print all courses taken and the average grade. Assume that a student takes no more than 30 courses for the entire program. For teachers, we need to maintain the courses taught currently, and able to add or remove a course taught. Assume that a teacher teaches not more than 5 courses concurrently.

We design the classes as follows.

The Superclass Person.java

The Subclass Student.java

The Subclass Teacher.java

A Test Driver (TestPerson.java)

Exercises

LINK TO EXERCISES

Composition vs. Inheritance

"A line is composed of 2 points" vs. "A line is a point extended by another point"

Recall that there are two ways of reusing existing classes: composition and inheritance. We have seen that a

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1 class can be implemented using composition of

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0 class - "A line is composed of two points", in the previous section.

A

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1 can also be implemented, using inheritance from the

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0 class - "A line is a point extended by another point". Let's call this subclass

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72 (to differentiate from the

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1 class using composition).

The Superclass Point.java

As above.

The Subclass LineSub.java

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A Test Driver (TestLineSub.java)

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0

Notes: This is the same test driver used in the earlier example on composition, except change in classname.

Study both versions of the Line class (

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1 and

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72). I suppose that it is easier to say that "A line is composed of two points" than that "A line is a point extended by another point".

Rule of Thumb: Use composition if possible, before considering inheritance. Use inheritance only if there is a clear hierarchical relationship between classes.

Exercises

LINK TO EXERCISES ON COMPOSITION VS INHERITANCE

The word "polymorphism" means "many forms". It comes from Greek word "poly" (means many) and "morphos" (means form). For examples, in chemistry, carbon exhibits polymorphism because it can be found in more than one form: graphite and diamond. But, each of the form has it own distinct properties (and price).

Substitutability

A subclass possesses all the attributes and operations of its superclass (because a subclass inherited all attributes and operations from its superclass). This means that a subclass object can do whatever its superclass can do. As a result, we can substitute a subclass instance when a superclass instance is expected, and everything shall work fine. This is called substitutability.

In our earlier example of

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6 and

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9:

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9 is a subclass of

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6. We can say that

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9 "is-a"

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6 (actually, it "is-more-than-a"

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6). Subclass-superclass exhibits a so called "is-a" relationship.

Circle.java

Cylinder.java

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9, and assign it to a

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6 (its superclass) reference, as follows:

You can invoke all the methods defined in the

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6 class for the reference

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86, (which is actually holding a

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9 object), e.g.

This is because a subclass instance possesses all the properties of its superclass.

However, you CANNOT invoke methods defined in the

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9 class for the reference

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86, e.g.

This is because

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86 is a reference to the

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6 class, which does not know about methods defined in the subclass

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9.

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86 is a reference to the

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6 class, but holds an object of its subclass

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9. The reference

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86, however, retains its internal identity. In our example, the subclass

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9 overrides methods

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06 and

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9.

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00 or

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01 invokes the overridden version defined in the subclass

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9, instead of the version defined in

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6. This is because

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86 is in fact holding a

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9 object internally.

Summary

1. A subclass instance can be assigned (substituted) to a superclass' reference.
2. Once substituted, we can invoke methods defined in the superclass; we cannot invoke methods defined in the subclass.
3. However, if the subclass overrides inherited methods from the superclass, the subclass (overridden) versions will be invoked.

Polymorphism EG. 1： Shape and its Subclasses

Polymorphism is very powerful in OOP to separate the interface and implementation so as to allow the programmer to program at the interface in the design of a complex system.

Consider the following example. Suppose that our program uses many kinds of shapes, such as triangle, rectangle and so on. We should design a superclass called

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06, which defines the public interfaces (or behaviors) of all the shapes. For example, we would like all the shapes to have a method called

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06, which returns the area of that particular shape. The

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06 class can be written as follow.

Superclass Shape.java

Take note that we have a problem writing the

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06 method in the

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06 class, because the area cannot be computed unless the actual shape is known. We shall print an error message for the time being. In the later section, I shall show you how to resolve this problem.

We can then derive subclasses, such as

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11 and

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12, from the superclass

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06.

Subclass Rectangle.java

Subclass Triangle.java

The subclasses override the

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06method inherited from the superclass, and provide the proper implementations for

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06.

A Test Driver (TestShape.java)

In our application, we could create references of

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06, and assigned them instances of subclasses, as follows:

The beauty of this code is that all the references are from the superclass (i.e., programming at the interface level). You could instantiate different subclass instance, and the code still works. You could extend your program easily by adding in more subclasses, such as

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6,

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18, etc, with ease.

Nonetheless, the above definition of

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06 class poses a problem, if someone instantiate a

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06 object and invoke the

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06 from the

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06 object, the program breaks.

This is because the

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06 class is meant to provide a common interface to all its subclasses, which are supposed to provide the actual implementation. We do not want anyone to instantiate a

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06 instance. This problem can be resolved by using the so-called

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25 class.

Polymorphism EG. 2： Monster and its Subclasses

Polymorphism is a powerful mechanism in OOP to separate the interface and implementation so as to allow the programmer to program at the interface in the design of a complex system. For example, in our game app, we have many types of monsters that can attack. We shall design a superclass called

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26 and define the method

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27 in the superclass. The subclasses shall then provides their actual implementation. In the main program, we declare instances of superclass, substituted with actual subclass; and invoke method defined in the superclass.

Superclass Monster.java

Subclass FireMonster.java

Subclass WaterMonster.java

Subclass StoneMonster.java

A Test Driver TestMonster.java

Upcasting & Downcasting

Upcasting a Subclass Instance to a Superclass Reference

Substituting a subclass instance for its superclass is called "upcasting". This is because, in a UML class diagram, subclass is often drawn below its superclass. Upcasting is always safe because a subclass instance possesses all the properties of its superclass and can do whatever its superclass can do. The compiler checks for valid upcasting and issues error "incompatible types" otherwise. For example,

Downcasting a Substituted Reference to Its Original Class

You can revert a substituted instance back to a subclass reference. This is called "downcasting". For example,

Downcasting requires explicit type casting operator in the form of prefix operator

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28. Downcasting is not always safe, and throws a runtime

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29 if the instance to be downcasted does not belong to the correct subclass. A subclass object can be substituted for its superclass, but the reverse is not true.

Another Example on Upcasting and Downcasting

The following program tests the upcasting an downcasting (refer to the above instance diagram):

Casting Operator

Compiler may not be able to detect error in explicit cast, which will be detected only at runtime. For example,

The "instanceof" Operator

Java provides a binary operator called

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30which returns

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5 if an object is an instance of a particular class. The syntax is as follows:

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1

An instance of subclass is also an instance of its superclass. For example,

Summary of Polymorphism

1. A subclass instance processes all the attributes operations of its superclass. When a superclass instance is expected, it can be substituted by a subclass instance. In other words, a reference to a class may hold an instance of that class or an instance of one of its subclasses - it is called substitutability.
2. If a subclass instance is assign to a superclass reference, you can invoke the methods defined in the superclass only. You cannot invoke methods defined in the subclass.
3. However, the substituted instance retains its own identity in terms of overridden methods and hiding variables. If the subclass overrides methods in the superclass, the subclass's version will be executed, instead of the superclass's version.

Exercises

LINK TO EXERCISES

Abstract Classes & Interfaces

The abstract Method and abstract class

In the above examples of

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06 and

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26, we encountered a problem when we create instances of

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06 and

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26 and run the

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06 or

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27. This can be resolved via

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25 method and

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25 class.

An

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25 method is a method with only signature (i.e., the method name, the list of arguments and the return type) without implementation (i.e., the method’s body). You use the keyword

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25 to declare an

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25 method.

For example, in the

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06 class, we can declare

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25 methods

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06,

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46, etc, as follows:

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2

Implementation of these methods is NOT possible in the

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06 class, as the actual shape is not yet known. (How to compute the area if the shape is not known?) Implementation of these

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25 methods will be provided later once the actual shape is known. These

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25 methods cannot be invoked because they have no implementation.

A class containing one or more

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25 methods is called an

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25 class. An

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25 class must be declared with a class-modifier

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25. An

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25 class CANNOT be instantiated, as its definition is not complete.

UML Notation:

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25 class and method are shown in italic.

Abstract Class EG. 1: Shape and its Subclasses

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06 class as an

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25 class, containing an

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25 method

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06 as follows:

The abstract Superclass Shape.java

An

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25 class is incomplete in its definition, since the implementation of its

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25 methods is missing. Therefore, an

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25 class cannot be instantiated. In other words, you cannot create instances from an

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25 class (otherwise, you will have an incomplete instance with missing method's body).

To use an

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25 class, you have to derive a subclass from the

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25 class. In the derived subclass, you have to override the

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25 methods and provide implementation to all the

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25 methods. The subclass derived is now complete, and can be instantiated. (If a subclass does not provide implementation to all the

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25 methods of the superclass, the subclass remains

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25.)

This property of the

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25 class solves our earlier problem. In other words, you can create instances of the subclasses such as

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11 and

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12, and upcast them to

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06 (so as to program and operate at the interface level), but you cannot create instance of

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06, which avoid the pitfall that we have faced. For example,

In summary, an

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25 class provides a template for further development. The purpose of an abstract class is to provide a common interface (or protocol, or contract, or understanding, or naming convention) to all its subclasses. For example, in the

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25 class

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06, you can define abstract methods such as

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06 and

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46. No implementation is possible because the actual shape is not known. However, by specifying the signature of the

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25 methods, all the subclasses are forced to use these methods' signature. The subclasses could provide the proper implementations.

Coupled with polymorphism, you can upcast subclass instances to

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06, and program at the

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06 level, i,e., program at the interface. The separation of interface and implementation enables better software design, and ease in expansion. For example,

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06 defines a method called

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06, which all the subclasses must provide the correct implementation. You can ask for a

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06 from any subclasses of Shape, the correct area will be computed. Furthermore, you application can be extended easily to accommodate new shapes (such as

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6 or

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18) by deriving more subclasses.

Rule of Thumb: Program at the interface, not at the implementation. (That is, make references at the superclass; substitute with subclass instances; and invoke methods defined in the superclass only.)

Notes:

• An abstract method cannot be declared

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  88, as

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  88 method cannot be overridden. An

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  25 method, on the other hand, must be overridden in a descendant before it can be used.
• An

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  25 method cannot be

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  4 (which generates a compilation error). This is because

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  4 method are not visible to the subclass and thus cannot be overridden.

Abstract Class EG. 2: Monster

We shall define the superclass

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26 as an

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25 class, containing an

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25 method

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27. The

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25 class cannot be instantiated (i.e., creating instances).

The Java's interface

A Java

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99 is a 100% abstract superclass which define a set of methods its subclasses must support. An

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99 contains only

class Goalkeeper extends SoccerPlayer {......}
class MyApplet extends java.applet.Applet {.....}
class Cylinder extends Circle {......}

2 abstract methods (methods with signature and no implementation) and possibly constants (

class Goalkeeper extends SoccerPlayer {......}
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2

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03

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88 variables). You have to use the keyword "

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99" to define an

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99 (instead of keyword "

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07" for normal classes). The keyword

class Goalkeeper extends SoccerPlayer {......}
class MyApplet extends java.applet.Applet {.....}
class Cylinder extends Circle {......}

2 and

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25 are not needed for its abstract methods as they are mandatory.

(JDK 8 introduces default and static methods in the interface. JDK 9 introduces private methods in the interface. These will not be covered in this article.)

Similar to an

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25 superclass, an

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99 cannot be instantiated. You have to create a "subclass" that implements an interface, and provide the actual implementation of all the

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25 methods.

Unlike a normal class, where you use the keyword "

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8" to derive a subclass. For interface, we use the keyword "

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14" to derive a subclass.

An interface is a contract for what the classes can do. It, however, does not specify how the classes should do it.

An interface provides a form, a protocol, a standard, a contract, a specification, a set of rules, an interface, for all objects that implement it. It is a specification and rules that any object implementing it agrees to follow.

In Java,

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25 class and

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99 are used to separate the public interface of a class from its implementation so as to allow the programmer to program at the interface instead of the various implementation.

Interface Naming Convention: Use an adjective (typically ends with "

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17") consisting of one or more words. Each word shall be initial capitalized (camel-case). For example,

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18,

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19,

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20,

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21,

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22, etc.

Interface EG. 1: Shape Interface and its Implementations

We can re-write the

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25 superclass

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06 into an

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99, containing only

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25 methods, as follows:

UML Notations: Abstract classes, Interfaces and abstract methods are shown in italics. Implementation of interface is marked by a dash-arrow leading from the subclasses to the interface.

A test driver is as follows:

Interface EG. 2: Movable Interface and its Implementations

Suppose that our application involves many objects that can move. We could define an interface called

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27, containing the signatures of the various movement methods.

Interface Moveable.java

Similar to an

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25 class, an

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99 cannot be instantiated; because it is incomplete (the abstract methods' body is missing). To use an interface, again, you must derive subclasses and provide implementation to all the abstract methods declared in the interface. The subclasses are now complete and can be instantiated.

MovablePoint.java

To derive subclasses from an

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99, a new keyboard "

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14" is to be used instead of "

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8" for deriving subclasses from an ordinary class or an

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25 class. It is important to note that the subclass implementing an interface need to override ALL the abstract methods defined in the interface; otherwise, the subclass cannot be compiled. For example,

Other classes in the application can similarly implement the

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20 interface and provide their own implementation to the

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25 methods defined in the interface

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20.

TestMovable.java

We can also upcast subclass instances to the

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20 interface, via polymorphism, similar to an

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25 class.

Implementing Multiple Interfaces

As mentioned, Java supports only single inheritance. That is, a subclass can be derived from one and only one superclass. Java does not support multiple inheritance to avoid inheriting conflicting properties from multiple superclasses. Multiple inheritance, however, does have its place in programming.

A subclass, however, can implement more than one interfaces. This is permitted in Java as an interface merely defines the abstract methods without the actual implementations and less likely leads to inheriting conflicting properties from multiple interfaces. In other words, Java indirectly supports multiple inheritances via implementing multiple interfaces. For example,

interface Formal Syntax

The formal syntax for declaring interface is:

All methods in an interface shall be

class Goalkeeper extends SoccerPlayer {......}
class MyApplet extends java.applet.Applet {.....}
class Cylinder extends Circle {......}

2 and

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25 (default). You cannot use other access modifier such as

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4,

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42 and default, or modifiers such as

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03,

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88.

All fields shall be

class Goalkeeper extends SoccerPlayer {......}
class MyApplet extends java.applet.Applet {.....}
class Cylinder extends Circle {......}

2,

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03 and

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88 (default).

An

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99 may "

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8" from a super-interface.

UML Notation: The UML notation uses a solid-line arrow linking the subclass to a concrete or abstract superclass, and dashed-line arrow to an interface as illustrated. Abstract class and abstract method are shown in italics.

Why interfaces?

An interface is a contract (or a protocol, or a common understanding) of what the classes can do. When a class implements a certain interface, it promises to provide implementation to all the abstract methods declared in the interface. Interface defines a set of common behaviors. The classes implement the interface agree to these behaviors and provide their own implementation to the behaviors. This allows you to program at the interface, instead of the actual implementation. One of the main usage of interface is provide a communication contract between two objects. If you know a class implements an interface, then you know that class contains concrete implementations of the methods declared in that interface, and you are guaranteed to be able to invoke these methods safely. In other words, two objects can communicate based on the contract defined in the interface, instead of their specific implementation.

Secondly, Java does not support multiple inheritance (whereas C++ does). Multiple inheritance permits you to derive a subclass from more than one direct superclass. This poses a problem if two direct superclasses have conflicting implementations. (Which one to follow in the subclass?). However, multiple inheritance does have its place. Java does this by permitting you to "implements" more than one interfaces (but you can only "extends" from a single superclass). Since interfaces contain only abstract methods without actual implementation, no conflict can arise among the multiple interfaces. (Interface can hold constants but is not recommended. If a subclass implements two interfaces with conflicting constants, the compiler will flag out a compilation error.)

Interface vs. Abstract Superclass

Which is a better design: interface or abstract superclass? There is no clear answer.

Use abstract superclass if there is a clear class hierarchy. Abstract class can contain partial implementation (such as instance variables and methods). Interface cannot contain any implementation, but merely defines the behaviors.

As an example, Java's thread can be built using interface

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22 or superclass

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51.

Exercises

LINK TO EXERCISES ON POLYMORPHISM, ABSTRACT CLASSES AND INTERFACES

(Advanced) Dynamic Binding or Late Binding

We often treat an object not as its own type, but as its base type (superclass or interface). This allows you to write codes that do not depends on a specific implementation type. In the

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06 example, we can always use

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06 and do not have to worry whether they are triangles or circles.

This, however, poses a new problem. The compiler cannot know at compile time precisely which piece of codes is going to be executed at run-time (e.g.,

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06 has different implementation for

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12 and

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11).

In the procedural language like C, the compiler generates a call to a specific function name, and the linkage editor resolves this call to the absolute address of the code to be executed at run-time. This mechanism is called static binding (or early binding).

To support polymorphism, object-oriented language uses a different mechanism called dynamic binding (or late-binding or run-time binding). When a method is invoked, the code to be executed is only determined at run-time. During the compilation, the compiler checks whether the method exists and performs type check on the arguments and return type, but does not know which piece of codes to execute at run-time. When a message is sent to an object to invoke a method, the object figures out which piece of codes to execute at run-time.

Although dynamic binding resolves the problem in supporting polymorphism, it poses another new problem. The compiler is unable to check whether the type casting operator is safe. It can only be checked during runtime (which throws a

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29 if the type check fails).

JDK 1.5 introduces a new feature called generics to tackle this issue. We shall discuss this problem and generics in details in the later chapter.

Exercises

LINK TO EXERCISES

(Advanced) Object-Oriented Design Issues

Encapsulation, Coupling & Cohesion

In OO Design, it is desirable to design classes that are tightly encapsulated, loosely coupled and highly cohesive, so that the classes are easy to maintain and suitable for re-use.

Encapsulation refers to keeping the data and method inside a class such users do not access the data directly but via the

class Goalkeeper extends SoccerPlayer {......}
class MyApplet extends java.applet.Applet {.....}
class Cylinder extends Circle {......}

2 methods. Tight encapsulation is desired, which can be achieved by declaring all the variable

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4, and providing

class Goalkeeper extends SoccerPlayer {......}
class MyApplet extends java.applet.Applet {.....}
class Cylinder extends Circle {......}

2 getter and setter to the variables. The benefit is you have complete control on how the data is to be read (e.g., in how format) and how to the data is to be changed (e.g., validation).

[TODO] Example: Time class with private variables hour (0-23), minute (0-59) and second (0-59); getters and setters (throws

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61). The internal time could also be stored as the number of seconds since midnight for ease of operation (information hiding).

Information Hiding: Another key benefit of tight encapsulation is information hiding, which means that the users are not aware (and do not need to be aware) of how the data is stored internally.

The benefit of tight encapsulation out-weights the overhead needed in additional method calls.

Coupling refers to the degree to which one class relies on knowledge of the internals of another class. Tight coupling is undesirable because if one class changes its internal representations, all the other tightly-coupled classes need to be rewritten.

[TODO] Example: A class uses Time and relies on the variables hour, minute and second.

Clearly, Loose Coupling is often associated with tight encapsulation. For example, well-defined public method for accessing the data, instead of directly access the data.

Cohesion refers to the degree to which a class or method resists being broken down into smaller pieces. High degree of cohesion is desirable. Each class shall be designed to model a single entity with its focused set of responsibilities and perform a collection of closely related tasks; and each method shall accomplish a single task. Low cohesion classes are hard to maintain and re-use.

[TODO] Example of low cohesion: Book and Author in one class, or Car and Driver in one class.

Again, high cohesion is associated with loose coupling. This is because a highly cohesive class has fewer (or minimal) interactions with other classes.

"Is-a" vs. "has-a" relationships

"Is-a" relationship: A subclass object processes all the data and methods from its superclass (and it could have more). We can say that a subclass object is-a superclass object (is more than a superclass object). Refer to "polymorphism".

"has-a" relationship: In composition, a class contains references to other classes, which is known as "has-a" relationship.

You can use "is-a" and 'has-a" to test whether to design the classes using inheritance or composition.

Does a subclass extend a superclass?

class SubClass extends SuperClass { . . . } This declares that SubClass is the subclass of the Superclass class. It also implicitly declares that SuperClass is the superclass of SubClass. A subclass also inherits variables and methods from its superclass's superclass, and so on up the inheritance tree.

How many Superclasses a subclass can extend?

A class can extend only one other class. To use the proper terminology, Java allows single inheritance of class implementation. Later in this chapter, we'll talk about interfaces, which take the place of multiple inheritance as it's primarily used in other languages. A subclass can be further subclassed.

Which member of the superclass is accessible to the subclass?

Yes, a subclass has access to all nonprivate members of its superclass. 2.

In what ways can a subclass extend the functionality of a superclass?

The subclass inherits state and behavior in the form of variables and methods from its superclass. The subclass can just use the items inherited from its superclass as is, or the subclass can modify or override it.