CS 200 - Week 13 Jim Williams, PhD

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

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.

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

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

Constructor Called when instance created Used to initialize instance fields

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

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

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

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.

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.

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

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.

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

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

UML Diagrams Examples of Diagrams: http://agilemodeling.com https://www.uml-diagrams.org/ Simple Drawing Tool: http://www.umlet.com/umletino/umletino.html

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

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

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

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.

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

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

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.

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

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);

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

What will print out? 1 can't access a private field outside the class 1 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

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.

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

Animals in Zoo

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

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.