1. Workshop for CS-AP Teachers
Chapter 3
Advanced Object-Oriented Concepts
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Learning Goals
Understand at a conceptual and practical level
Objects
Object variables and methods
Classes
Class variables and methods
Inheritance
Abstract Classes
Interfaces
Polymorphism
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3. Object-Oriented Principles
Objects with data (fields) and operations (methods)
Usually classes too
Inheritance
Hierarchy of types
Generalization / Specialization
Polymorphism
Executing the right method based on the type of the object at run-time
Some object-oriented languages don’t have classes. They use object prototypes instead.
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Advantages to Objects
Data and related operations are combined
No passing data around
Data is protected
Objects are responsible
If code needs to be fixed you can figure out what class needs to be changed
Methods can change as long as they still do the job
Easier to maintain and extend
Objects don’t change as often as procedures
The program is a simulation of the domain
Classes can be reused
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5. Class Definition Exercise
Write the definition for a class Student.
We need to know about the student’s name (first and last), store the name of a file with his or her picture in it, and keep the grades for this student. We want to be able to add grades and compute the grade point average.
Remember to define methods to get and set all fields
Write the main method to create a Student
Create the class definition in the exercises/Student directory.
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6. Steps in a Class Definition
Underline the nouns and verbs
Nouns may be attributes
Sometimes other classes
Verbs may be methods
Do a simple UML class
Class name, attributes (fields), methods
Specify the types of the attributes
Specify the return type and parameter types
Code the class
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Class Definition
A class definition in Java defines
Object Fields (Variables)
Fields that all objects of the class will have
Class Fields (Variables)
Fields that will only be on the class definition class
Constructors
Used to initialize object fields
Object Methods
Methods that have the current object passed implicitly (this)
Class Methods
Methods that operate on class variables
Class fields and methods have the keyword “static” on them. Object fields and methods do not have the keyword “static”.
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Adding a Student Id
What if we want to add an id as an object field?
Each object should have a unique id
One way to assign a unique id is to have a counter that counts how many students we have
Increment the current count each time a new student is created
Use the current count as the id
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Adding Id Exercise
Modify the Student class to add a unique id for each student
Add the field id
Add the field numStudents that starts out as 0
Each time a new student is created
Increment numStudents
Set the id to the numStudents
Modify the toString method to add the id
Create several students in the main method
You can start with your previous Student.java file or use exercises/Student-Id/Student.java.
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What went wrong?
id and numStudents are object fields
A separate copy is in each object
So all objects have a count of 0
But we want there to only be only one copy of the number of students
That all Student objects have access to
Sally :Student
JaKita :Student
Thomas :Student
numStudents
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11. Class Fields (Variables)
Each object has a link to an object that describes the class
An object of the class “java.lang.Class”
So if we create a class field
All objects will have access to the one field
Create class fields
Using the keyword static
Sally:Student Id
firstName
lastName
pictureFile
grades
Student: Class
numStudents
addGrade(grade)
getGradePointAverage() …
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12. An Object “knows” what Class it is
You can check that an object is of a class
instanceof ClassName
You can get the Class object from an object
getClass()
The Class objects holds information from the class definition
Sally:Student
firstName
lastName
pictureFile
grades
Student: Class
numStudents
addGrade(grade)
getGradePointAverage() …
For every class loaded into a Java virtual machine there is a class java.lang.Class that defines it.
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Class Exercise
Loop up the class java.lang.Class
How do you get the parent class of a class?
How do you get the methods of a class?
How do you get the fields of a class?
How do you tell if it is an interface?
Try the following in DrJava
Class stringClass = “Test”.getClass();
System.out.println(stringClass);
System.out.println(stringClass.newInstance());
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14. Class Variable Exercise
Make numStudents a class variable by adding the keyword static
Now run the main method and check that the ids are unique and incrementing
Create another student in the main and run it again
What id does the first student start with?
You can start with your previous Student.java file or use exercises/Student-Class-Id/Student.java.
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15. Object Methods versus Class Methods
Object methods are implicitly passed the current object
And work on the data in that object
Class methods only have access to class fields
They are not passed an object of the class
They can not work on object data
They are useful when you don’t have a current object
They are useful for working with class fields
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Class Method Exercise
When should you use a class method and when an object method?
To read a file and create student objects from the information in the file
To get the grade point average for a student
To get the number of student objects created
To print the information in a student object
Inexperienced programmers and procedural programmers often overuse class methods.
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Inheritance
Did you get features or abilities from your parents?
People inherit physical characteristics
Some people inherit abilities: music
Inheritance in OO means receiving data and methods from your parent
In Java you can only have one parent
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18. Inheritance in our Models
One class can inherit from another
Gets all the fields and methods
The class you inherit from is called
Parent, superclass, base class
The class doing the inheriting is called
Child, subclass, derived class
Person
Parent
Student
Child
This type of diagramming is called UML which is the Unified Modeling Language and is a standard way to show object-oriented designs.
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Inheritance Exercise
Open the Picture class (Picture.java)
See if you can find the show method
Notice that the Picture class extends (inherits) from SimplePicture
See if you can find the show method in SimplePicture
Since Picture inherits from SimplePicture you can ask a Picture object to show itself
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How Inheritance Works
When an object gets a message it checks to see if it has a corresponding method
If it does it executes that method
If it doesn’t it checks the parent class
And keeps looking at parents until the method is found or it reaches Object
The Java compiler makes sure that a method is available
based on the declared type of the object
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Teaching Inheritance
Point out inheritance in common objects
What is a book?
What is a dictionary?
Talk about the things that are the same
Talk about the things that are different
Some other ideas are (teacher, math teacher), (school, elementary school, middle school, high school)
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The Object Class
If you don’t specify a parent for a class using “extends” it will inherit from Object
public class Name {
All objects in Java inherit from Object
Directly or indirectly
Object is in the package java.lang
All objects inherit
toString() and equals() but you will usually override these
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Inheritance Exercise
Look up Object in the api
In package java.lang
What other public methods are in Object?
Look up String in the api
What class does it inherit from?
Look up JButton in the api
In package javax.swing
What are all the classes it inherits from?
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24. Private Visibility and Inheritance
When a class inherits from another class it gets all the fields and methods
But, it can’t directly access fields or methods that are private
Use public methods to ask for changes to private fields
Call public methods that call private methods
This is to allow an object to protect its data and keep private methods private
You can’t directly access private inherited field because these can only be directly accessed in the same class.
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25. Protected and Inheritance
Some books use protected visibility with inheritance for fields
To give children objects direct access to fields
This is a bad idea
Protected gives access to subclasses
Protected also gives access to all classes in the same package
You can’t guarantee the data isn’t messed up by objects of other classes if you use protected fields!
Even though you can declare your class final so that other classes can’t inherit from it you can’t lock a package.
The only use for protected is to use it for methods that you might want your children to override.
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26. When to Use Inheritance
Use inheritance when one class is really a sub-type of another
You must be able to substitute an object of the child class for an object of the parent class and still have it make sense
Don’t use it just to have access to some fields and methods of the parent class
Use an association (has a) link instead
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27. Correct Use of Inheritance
Students and Teachers are people
You could use a student or teacher when you need a person
A course period is not a course
You wouldn’t include a course period in a list of available courses
Instead a course period would have a course associated with it
Have a field that is a Course object
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Substitution
Is a dictionary a book?
If you need a book will a dictionary work?
You can use a child object when a variable refers to a parent object
SimplePicture p = new Picture(fileName);
You can’t substitute a parent for a child
Picture p = new SimplePicture(fileName)
The Picture p = new SimplePicture(fileName) won’t compile since you can’t substitute a parent object for a child object.
If I ask you for a dictionary just a book wouldn’t do.
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Generalization
Generalization means that classes have things in common
If they are several classes that all have the same or similar fields and methods
pull out the common items and put them in a parent class
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30. Generalization - Exercise
Pull out common fields and methods in Student and Teacher and put them in a new class Person
Modify constructors as needed
Have student and teacher inherit from Person
Run the main methods in Student and Teacher after the changes
To make sure they still work
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Specialization
Person
firstName
lastName
pictureFile
A class differs from the parent class in some way
It add fields
It adds methods
It does something different when given the same message as the parent
Student id
nickname
grades
numGrades
numStudents
getGradePointAverage()
Notice that you can’t take away methods or fields.
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Overriding Methods
One way a child class can specialize is to handle the same message in a different way than the parent
If you inherit parent methods how can you do this?
The child can redefine a method with the same name and parameters as the parent method
The new method is called instead of the parent method
This is called overriding
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33. How Does Overriding Work?
When a message is sent to an object
The object checks with its class to see if the corresponding method exists in the class
If it doesn’t exist in the class then it checks in the parent class
It keeps looking up the inheritance tree till it finds a corresponding method
We know it will find a corresponding method since it wouldn’t have compiled otherwise.
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Using Super
What if the method in the child class wants to call the method in the parent class?
I want to do the same operation as my parent but also add something to it
But my method overrides the parent method
Use the keyword super to invoke an overridden parent method
super.method()
It starts looking for a corresponding method in the parent class
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35. Overriding equals and hashCode
All classes inherit from Object the method
public boolean equals(Object o)
the default result is true if they are the same object else false
You will often want to override this method
And check if some field or fields are equal
If you override equals you must also override hashCode
public int hashCode()
The String You should always override hashCode when you override equals for consistent behavior with Collection classes.
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What is a hash code?
An integer used to index into
hashing based collection classes such as Hashtable, HashMap, HashSet
What should you return as a hash code?
Best if each object has a unique hash code
Okay if most objects have a unique hash code
But some have duplicates
Bad if most objects result in duplicate hash codes
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37. What is a Hashing Collection?
Map of key to value
You put a value (object) into a collection for a key (object)
You can get back the value for that key
Similar to a safety deposit box
The hashCode() method is used on the key to find the memory location holding the value
The address of it
A real key would have a location printed on it which locates the box that the key fits.
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Overriding Exercise
Edit Student.java to override equals()
public booelan equals(Object object)
Return true if the passed object isn’t null and is of the class Student
And the id of the current object and passed student object are equal
Override hashCode()
You can return the id since each object will have a unique id
Test equals in the main method
You can start with Student.java from a previous exercise or use the one in exercises/Student-Override.
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39. Advantages to Inheritance
Handles commonality
Common attributes and operations are factored out and put as high as possible in a hierarchy
Not copied to several locations
Handles differences
Children can inherit the parts that are common from the parent
They can add attributes and operations to handle how they differ from the parent
They can override operations (methods) to do something different from the parent
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Constants
The final keyword means that the item will not change.
By using the final keyword along with public and static you can create class fields that are constant.
public static final int MALE = 0;
These can be accessed by using the Class name.attribute name.
this.gender = Person.MALE;
Final variables mean that variable can not change value. Final methods can not be overridden. Final classes can’t be subclassed.
The Java convention is to uppercase all the letters in a constant. Multiple words are separated by underscores: THIS_IS_A_CONSTANT.
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Constant Exercise
Add constants for unknown, male, and female gender to Person.java
public static final int UNKNOWN = 0;
public static final int FEMALE = 1;
public static final int MALE = 2;
Add a gender field to Person.java
private int gender = UNKNOWN; // default
Add get and set gender methods
Add gender to the result of toString()
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Abstract Classes
Abstract classes are classes that can’t be instantiated. Abstract classes can only be subclassed.
Create an abstract class by using the keyword abstract in the class declaration.
public abstract class Food
Food
price
calories
Hamburger
Coke
The compiler will give an error if you try to create an object of an abstract class.
You can use abstract classes as a way of generalizing attributes and methods.
Can your order “food”?
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43. Why use an Abstract Class?
Represents an abstract idea (like Shape)
Holds methods common to several related classes
Holds attributes common to several related classes
Enforce naming convention by abstract methods that must be overridden by children
Allows for general algorithms based on abstract methods with customization by children
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Interfaces
Interfaces are a description of behavior.
They are a special kind of abstract class that has only abstract methods and constants.
public interface ShapeInterface {
public void setShape(int shape);
public void setShapeColor(Color shapeColor); }
You don’t have to declare the methods as abstract
They automatically are
Interfaces are declared in files with the interface name and the .java extension.
Notice that you don’t have to declare the methods to be abstract. This is automatic with interfaces.
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45. Classes Implement Interfaces
Classes that implement interfaces must provide the implementations for the methods specified in the interface.
public class ShapeCanvas implements ShapeInterface {
public void setShape(int shape)
{ code to handle set shape }
public void setShapeColor(Color shapeColor)
{ code to handle set shape color } }
The compiler will give error messages if the ShapeCanvas class doesn’t provide the implementations of the setShape and setShapeColor methods.
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Why use an Interface?
Separates what from who
I don’t care who you are I just need a way to talk to you
Choose from several implementers
A class can implement many interfaces but inherit from only one class
like multiple inheritance but easier to use
thinner than inheritance
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47. Interfaces Versus Inheritance
When a class inherits from a parent class it inherits all the attributes and methods.
With inheritance it inherits the structure and behavior of the parent class.
With an interface it inherits only the method names and parameter lists.
A class can inherit from only one parent class
public class Person extends Object
A class can implement more than one interface.
public class ShapeCanvas implements Interface1,Interface2,…
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Compare Interface
How would you compare any two objects?
And decide if one is less than, equal too, or greater than the other
It would depend on the Class of the objects being compared
For String objects compare the letters in the string
Implement the Comparable interface
public int compareTo(Object object)
The method compareTo returns a negative number if the current object is less than the passed object, 0 if they are equal, and a positive number if the passed object is greater than the current object.
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Comparable Exercise
How would you compare two Person objects?
Implement the Comparable interface
public int compareTo(Object object)
Compare the last names first
If they are equal compare the first names
The String class implements Comparable so you can use the results of comparing the last name and first name
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Polymorphism
Literally: many forms
In Object-Oriented development it means that what happens when a message is sent to an object depends on the type (class) of the object at runtime
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51. How Does Polymorphism Work?
If a class is declared to be final
then the compiler can figure out the location of a method that matches the message
If a class can be subclassed
then a variable declared to be of the parent type can point to an object of the parent class or any subclass at run-time
the compiler can’t determine the method to invoke
the method to invoke is figured out at run-time
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Shape Panel Exercise
Execute the main method of ShapePanel
Click the Rectangle button and then click and drag to position the rectangle
Click the Oval button and click and drag to position the oval
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53. Class Diagram for ShapePanel
ShapeInterface 1 1 1 1
Shape
ShapeCanvas
ButtonPanel 1 *
draw()
paint()
Oval
Rectangle
Class ShapePanel can be used in a application or applet.
A ShapePanel object has one ShapeCanvas and one ButtonPanel in it.
The ShapeCanvas holds 0 to many Shapes.
The ShapeCanvas implements the ShapeInterface.
Shape is an abstract class with the abstract method draw in it.
Oval and Rectangle both inherit from Shape and implement draw.
draw()
draw()
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Interface Exercise
Add a method to the ShapeInterface
public void clearShapes();
Modify the ShapeCanvas class to implement the method and clear the vector of shapes and then call repaint();
Modify the ButtonPanel class to add the clear button and have it call clearShapes() when pressed.
Edit ShapeInterface.java and add the declaration of the method clearShapes.
public void clearShapes();
Edit ShapeCanvas.java and implement the method clearShapes
public void clearShapes() {
shapes. clear(); // remove all shapes from the vector
repaint(); // repaint to see that shapes are gone }
Edit ButtonPanel to add the clear button and have it call clearShapes on the ShapeInterface handler when pressed.
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55. Polymorphism - Many Forms
Polymorphism is overloading that is resolved at execution time, and is also called dynamic or run-time binding.
Say you have an array of Shapes that actually holds objects that are subclasses of shape.
When you ask a shape to draw itself what gets drawn depends on the run-time type.
Shape
draw()
Oval
Rectangle
draw()
draw()
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56. Add Abstract Class Subclass Exercise
Create a new class Line which is a subclass of Shape.
Add a constant for Line to the Shape class.
Add a Line Button to the Button Panel and when the line button is clicked on notify the handler to set the shape to line.
Edit Oval.java and save it as Line.java.
Modify the draw method of Line.java to draw a line.
g.drawLine(p1.x,p1.y,p2.x,p2.y);
Edit Shape.java and add a constant for Line.
public static final String LINE = ”Line”;
Edit ButtonPanel.java and add a line button that when clicked calls the handler’s setShape with a line as the current shape.
handler.setShape(Shape.LINE);
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57. Advantages to Polymorphism
Used to create general algorithms that work on objects of different types
Collections that hold Objects
List, Set, Stack, Queue, Map
Makes it easy to add new types
Just create the new class and implement the required operations
Don’t change existing code
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Summary
Class fields are on an object of the class Class
Not on objects of the class
Class methods can only work on class fields
Not object fields
Objects inherit fields and methods from a parent class
But need to use public methods to access private inherited fields
Polymorphism allows you to write general methods based on a common parent or interface
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