1. CS Week 13
Jim Williams, PhD

2. This Week Team Lab: Instantiable Class BP2 Strategy
Lecture: Classes as templates

3. BP2 Strategy M1: 2 of 3 milestone tests didn't require reading a file.
M2: matchCase and translate (2 of 3 tests)
don't require file I/O
Opportunities to reuse code
make supporting method rather than copy code.
No change in Milestone deadlines
However, 2 more days (Saturday 12/9) before final program grading.

4. Class as a template A template for objects/instances.
attributes/fields
accessors/getters
mutators/setters
constructors
this (implicit parameter)
public, private, protected, <package>

5. 'this' implicit parameter
The way to access the current instance from within the instance methods.

6. Constructor Called when instance created
Used to initialize instance fields

7. Default Constructor class Person { private String name; }
Can we create an instance?
Person p = new Person();

8. Constructor Overloading
class Person {
private String name;
Person() {
this("no name"); }
Person(String name) {
this.name = name;

9. toppings is a(n) instance (non-static) variable class Pizza {
private String toppings;
public String getToppings() {
if ( toppings == null)
toppings = "no toppings";
return toppings; }
instance (non-static) variable
class (static) variable
parameter
local variable
A is true,
B, C and D are false

10. getToppings() is a(n) method class Pizza { getter
accessor
provides read-only access to toppings field
class Pizza {
private String toppings;
public String getToppings() {
if ( toppings == null)
toppings = "no toppings";
return toppings; }
All are true.

11. Constructors
yes - the default, no argument constructor
yes - a really good one. no
error
If there is no constructor in a class is a constructor automatically provided by the compiler?
best answer: A, yes - the default, no argument constructor.

12. this class Picture { private boolean hasFrame;
public Picture( boolean hasFrame) {
this.hasFrame = hasFrame; }

13. Does this print true or false?
class Person {
private boolean something = false;
boolean getThing(boolean something) {
return this.something; }
public static void main(String []args) {
Person p = new Person();
System.out.println( p.getThing( true));
true
false
error/other
try it and see.

14. Circle Class Design a Circle Class Field: radius
Constructor: radius is the argument
Methods: getArea(), getCircumference(), toString()
Recall: Area = π * radius * radius; Circumference = π × diameter
Draw a UML Class diagram
Create TestCircle Class
Create circles with radius 3.5 and 34.1
Print out area, circumference, and radius

15. Rectangle Design a Rectangle class
Fields: width & height as double with default of 1.0 and private
Constructors: no-arg constructor & a constructor with specified width and height, public
Methods: getArea() and getPerimeter(), public
Draw a UML diagram for the class then implement the class.
Write a TestRectangle program that:
Creates 2 rectangles (4 by 10) and (3.5 by 25.4)
Display width, height, area and perimeter

16. UML Diagrams Examples of Diagrams: http://agilemodeling.com
Simple Drawing Tool:

17. Use Case
How external users (actors) interact with the system.

18. More Classes
derived classes, overriding member methods, the Object class, polymorphism, ArrayLists of Objects, is-a vs has-a relationships.

19. Visibility Modifiers For members of a class: public private protected
<package>
Demo

20. Can methodA call methodB?
//classes in different files in same package
class A {
public void methodA() {
B b = new B();
b.methodB(); }
class B {
void methodB() {
yes no
depends
error
try it and see.

21. Can a method outside the package call methodA()?
//classes in different files in same package
class A {
public void methodA() {
B b = new B();
b.methodB(); }
class B {
void methodB() {
yes no
depends
error
try it and see

22. The visibility modifier private is
appropriate
should be public
doesn't allow a user of this class to change the field (attribute) directly
allows only methods within this class to change the field.
class Pizza {
private String toppings;
public String getToppings() {
if ( toppings == null)
toppings = "no toppings";
return toppings; }
A, C & D are true
B is false

23. Will the default constructor be provided in this case?
class Cup {
private String contents;
Cup(String contents) {
this.contents = contents; }
yes no
compile time error
runtime error
try it and see.

24. Which are true? class Light { } // in some method
error - no constructor
Object classes' toString() will be called
an instance of Light has nothing in it
error
class Light { }
// in some method
Light aLight = new Light();
System.out.println( aLight);
best answer: B: Object classes' toString() will be called

25. How many Bug instances in list?2
2 copies of reference to 1 bug
none, error, no list 3
ArrayList<Bug> list;
list = new ArrayList<Bug>();
list.add( new Bug());
Bug aBug = new Bug();
list.add( aBug);
list.add( 0, aBug);
import java.util.ArrayList;
class Bug {
private static int count = 0;
private final int id;
Bug() {
id = ++count; }
public String toString() {
return "Bug:" + id;
public class ClassNameHere {
public static void main(String[] args) {
ArrayList<Bug> list = new ArrayList<>();
list.add( new Bug());
Bug aBug = new Bug();
list.add( aBug);
list.add( 0, aBug);
System.out.println( list);

26. class (static) vs instance (non-static)
fields
methods

27. What will print out? 1 can't access a private field outside the class1
can't access a private field outside the class
error
class Employee {
private static int employeeCount = 0;
private final int id;
Employee() {
this.id = ++employeeCount; }
public static void main(String []args) {
Employee anEmployee = new Employee();
System.out.println( anEmployee.id);
try it and see

28. What is the value of num? num: 10 Stuff.num: 6 Stuff.num: 10
class Stuff {
final static int MAX_VALUE = 10; //allowed BP2
static int num = 6; //NOT allowed in BP2
static void change( int n) {
num = n + 1; }
public static void main( String [] args) {
int num = MAX_VALUE;
change( num);
System.out.println("num:" + num);
System.out.println("Stuff.num:" + Stuff.num);
num: 10
Stuff.num: 6
Stuff.num: 10
Stuff.num: 11
error
try it.

29. Class Diagram
Shows structure of a designed system at level of classes.
Independent of time.

30. Animals in Zoo

31. Object Diagram A snapshot of a system at a point in time.
Instances of classes with specific attribute values.

32. Bike Design a bike class. Instance Fields: numWheels, Color, unique id
Class Field: numBikesCreated, used to assign unique id’s to each bike.
Constructor: numWheels and Color, automatically sets the unique identifier.
Instance Methods: Number of Wheels and id can be accessed but not changed. Color can be changed. Add a toString() method to return all instance field values in String form.
Class Method: returns the number of bikes created.
Draw the UML diagram and then write the code.
Create a BikeShop class that creates 10 bikes and stores in an array.
Print out each bike’s number of wheels, color and id using the toString method.