Inheritance

1
Inheritance

• Java organizes classes in a hierarchy. Classes
  inherit the instance variables and methods of the
  classes above them in the hierarchy. A class can
  extend its inherited characteristics by adding
  instance variables and methods and by overriding
  inherited
• methods. Thus inheritance provides a mechanism
  for reusing code and can greatly reduce the
  effort required to implement a new class.

2
Abstract Classes

• Some classes in a hierarchy must never be
  instantiated. They are called abstract classes.
  Their sole purpose is to define features and
  behavior common to their subclasses.

3
Polymorphism

• Methods in different classes with a similar
  function are usually given the same name. This
  is called polymorphism. Polymorphism makes
  classes easier to use because programmers need to
  memorize fewer methods names. In a well-designed
  class hierarch, polymorphism is employed as much
  as possible. A good example of a polymorphic
  message is toString. Every object no matter
  which class it belongs to understands the
  toString message and responds by returning a
  string that describes the object.

4
Pre and Post conditions

• Clients need to know how to use a method
  correctly and what results to expect if it is so
  used. Preconditions specify the correct use of
  a method and post conditions describe what will
  result if the preconditions are satisfied.

5
Exceptions for error handling

• When a methods preconditions are violated, a
  foolproof way to catch the errors is to throw
  exceptions, thereby halting program execution at
  the point of the errors.

6
Reference Types

• The identity of an object and the fact that there
  can be multiple references to the same object are
  issues that arise when comparing two objects for
  equality and when copying an object. There are
  also subtle rules to master when manipulating
  objects of different but related types in a
  hierarchy.

7

• Class variablebelongs to a class
• Class method is activated when a message is sent
  to the class rather than to an object.
• Static is used to designate class variables and
  methods.

8
Example static variable and method

• public class Student
• private String name
• int test new int3
• public Student()
• studentCount
• name" "
• for(int x0xlttest.lengthx)
• testx0
• public void setName(String n)
• namen
• static private int studentCount

9
Rules for Using Static variables

• Class methods can reference only the static
  variables, and never the instance variables.
• Instance methods can reference static and
  instance variables.

10

• import java.util.Scanner
• public class Convert
• private static Scanner scanner new
  Scanner(System.in)
• public static void main(String args)
• while(true)
• printCelsius(convertToCelsius(readFah
  renheit()))
• private static double readFahrenheit()
• System.out.println("Enter degrees
  Fahrenheit ")
• double xscanner.nextInt()
• return x
• private static double convertToCelsius(dou
  ble fahrenheit)

11
Why not use static all the time?

• Makes for awkward and difficult to maintain
  programs. Rather than break up into static
  methods etc, break tasks up into a system of
  cooperating classes.

12
Exercises

• 1. Describe the manner in which Java allocates
  memory for instance variables and static
  variables.
• 2. List three possible situations in which
  static methods might me included in classes
• 3. what error occurs in the following code?

public class SomeClass private int
instVar0 static public void someMethod(int
x) classVar instVar classVar
x static private int classVar0
13

• Exercise 9.1
• Java allocates a separate set of memory cells in
  each instance for the instance variables of a
  class. By contrast, only one set of cells is
  allocated for the class variables.
• First, a static method can be included to allow
  the user to call the method by sending a message
  to the class. This method can serve as a public
  accessor or mutator of a class variable. Second,
  a static method can be included to perform some
  useful calculation. The Math class methods are
  good examples. Third, a static method can be
  included in a set of cooperating helper methods
  to decompose responsibilities for tasks in a
  class.
• The reference to instVar in the static method is
  a syntax error because static methods cannot
  reference instance variables.
• A public constant is accessible to all clients,
  and a static constant is accessible by using the
  class name as a reference.

14
What is an interface?

• 1. State the purpose of a Java interface.
• 2. Why dont all classes have interfaces?

15

• A Java interface specifies the set of public
  methods for classes that implement that
  interface. It serves as a form of glue that
  holds the parts of a software system together.
• Some classes are so simple that they do not need
  interfaces moreover, an interface is commonly
  used when there is more than one implementing
  class.

16

• An interface contains only methods, never
  variables.
• The methods in a n interface are usually public
• If more than one class implements an interface,
  its methods are polymorphic
• A class can implement methods in addition to
  those listed in the interface, as we will
  illustrate soon.
• A class can implement more than one interface, a
  feature that we are not going to explore
• Interfaces can be organized in an inheritance
  hierarchy, another feature we are not going to
  explore.

17
Exercises

• Write an interface for Student
• When the programmer uses the expression
  implements ltan interfacegtin a class definition,
  how does the compiler react?

18

• public interface StudentSpecs
• public void setName (String nm)
• public String getName ()
• public void setScore (int i, int score)
• public int getScore (int i)
• public int getAverage()
• public int getHighScore()
• public String toString()
• The expression implements ltan interfacegt causes
  the compiler to check the implementing class for
  the presence of all the methods in the specified
  interface. If any of them are missing, a syntax
  error occurs.

19
Exercises

• 1. What is class hierarchy? Give an example
• 2. What syntax is used to get one class to
  inherit data and behavior form another class?
• 3. How does the keyword super work with
  constructors?
• How does the keyword super work with methods
  other than constructors?
• What is the role of the visibility modifier
  protected in a class hierarchy?
• Find the error in the following code and suggest
  a remedy for the problem
• Shape s new Wheel()
• S.setSpokes(5)

20

• A class hierarchy is a set of classes that is
  organized by the relationship of subclass and
  superclass. An example is the hierarchy
  Object/Circle/Wheel, where Circle is a subclass
  of Object and Wheel is a subclass of Circle.
• The expression extends lta classgt is used to
  inherit the data and behavior of another class.
• To run a constructor of the same form in the
  superclass, one uses the form super(ltparametersgt)
  at the beginning of the corresponding constructor
  in the subclass.
• With methods other than constructors, one uses
  the form super.ltmethod namegt (ltparametersgt).
• The visibility modifier protected allows
  subclasses in a hierarchy to access instance and
  class variables or methods of superclasses
  without allowing other clients in the system to
  access these items.
• There is a syntax error. The method setSpokes is
  not included in the Shape interface. The
  programmer attempts to send this message to an
  instance of Wheel, but this object is
  masquerading as a Shape. The variable s must be
  cast to a Wheel before sending it the setSpokes
  message.

21
Exercises contd

• 1. What is an abstract class? Give an example.
• Why do we declare abstract methods?
• What is a final method? Give an example.

22

• An abstract class serves as a repository of data
  and behavior common to a set of subclasses.
  Because this class is abstract, it cannot be
  instantiated. An example is the AbstractShape
  class, which is abstract and defines the behavior
  common to circles and rectangles.
• We declare abstract methods for two possible
  reasons. First, the abstract class in which they
  are declared implements an interface, but the
  implementation of these methods is deferred to
  the concrete classes. Second, even if there is
  no interface, these methods must be implemented
  in all subclasses, so declaring them abstract
  enforces this.
• A final method cannot be overridden in a
  subclass. Examples are the accessors for
  variables that are defined in an abstract class.
  These methods should not be overridden by
  subclasses.

23
Observations

• A java interface has a name and consists of a
  list of method headers.
• One or more classes can implement the same
  interface.
• If a variable is declared to be of an interface
  type, then it can be associated with an object
  form any class that implements the interface.
• If a class implements an interface, then all its
  subclasses do so implicitly.
• A subclass inherits all the characteristics of
  its superclass. To this basis the subclass can
  add new variables and methods or modify inherited
  methods.
• Characteristics common to several classes can be
  collected in a common abstract surperclass that
  is never instantiated
• An abstract class can contain headers for
  abstract methods that are implemented in the
  subclass.
• A classs constructors and methods can utilize
  constructors and methods that are implemented in
  the subclasses.
• A classs constructors and methods can utilize
  constructors and methods in the superclass
• Inheritance reduces repetition and promotes the
  reuse of code.
• Interfaces and inheritance promote the use of
  polymorphism.

24
How does it work?

• When a message is sent to an object, Java looks
  for a matching method. The search starts in the
  objects class and if necessary continues up the
  class hierarchy.

25

• Do fill in the blank on page 312
• Do project 9-1 and 9-5

26

• 1. Methods and variables that belong to a class
  rather than an instance are called class or
  static variables and methods.
• 2. The keyword used to invoke a constructor or
  method in a superclass is called super.
• 3. The visibility modifier protected makes a
  method or variable visible to subclasses but not
  to other clients.
• 4. A(n) abstract class contains common behavior
  and data for a set of subclasses but is not
  instantiated.
• 5. A(n) abstract method consists of just a head
  and forces all subclasses to implement it.
• 6. Preconditions and postconditions state the
  assumptions that are true before a method
  executes correctly and after it executes
  correctly.
• 7. The method equals is the conventional Java
  method for comparing two objects for equality,
  whereas the method clone is the conventional Java
  method for copying an object.