CS Week 13. Jim Williams, PhD

Transcription

1 CS Week 13 Jim Williams, PhD

2 This Week 1. Team Lab: Instantiable Class 2. BP2 Strategy 3. Lecture: Classes as templates

3 BP2 Strategy 1. M1: 2 of 3 milestone tests didn't require reading a file. 2. M2: matchcase and translate (2 of 3 tests) a. don't require file I/O 3. Opportunities to reuse code a. make supporting method rather than copy code. 4. No change in Milestone deadlines a. 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) 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

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

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

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

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 1. 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 2. Draw a UML diagram for the class then implement the class. 3. 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: 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

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

22 The visibility modifier private is class Pizza { private String toppings; public String gettoppings() { if ( toppings == null) toppings = "no toppings"; return toppings; 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.

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

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

25 How many Bug instances in list? ArrayList<Bug> list; list = new ArrayList<Bug>(); list.add( new Bug()); Bug abug = new Bug(); list.add( abug); list.add( 0, abug); 2 2 copies of reference to 1 bug none, error, no list 3

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

27 What will print out? 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); 0 1 can't access a private field outside the class error

28 What is the value of num? 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 num: 10 Stuff.num: 10 num: 10 Stuff.num: 11 error

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.