CompSci 125 Lecture 07. Chapter 4: Object-Oriented Development

Transcription

1 CompSci 125 Lecture 07 Chapter 4: Object-Oriented Development

2 Homework Update HW3 Due 9/20 HW4 Due 9/27 Exam-1 10/2

3 Programming Assignment Update p1: Traffic Applet due Sept 21 p2: Find Parking due Oct 5

4 Review: Objects Can model/represent aspects/components of the real world An instance of a CheckingAccount An instance of a Customer Exhibit behaviors (methods) Hold information (instance variables) Interact with one another

5 A Motivation for the Object Oriented Approach: Encapsulation Object s encourage healthy program structure Object s data is modified only by that object s methods Object interacts with the remainder of a program only through the interface defined by its methods Wish to minimize coupling between objects Wish to maximum cohesion within an object Co-locate related data and methods

6 Coupling: Minimize connections between classes Cohesion: Maximize connections within classes ThisClass AnotherClass ThatClass

7 Review: Class The plan, or blueprint, for an object Defines how to construct objects of that class Many new objects may be constructed from a single class

8 Example: Class Toad Toad -x: int! -y: int! -health: int! +count: byte! +move(int,int)! +injure()! +heal()! +gethealth(): int! public class Toad {! private int x,y;!! }!! private int health;! public static byte count=0;! public void move(int x, int y) { }! public void injure() { }! public void heal() { }! public static int gethealth() { }!

9 Real World Simplified UML x! y! health! count! move()! injure()! heal()! gethealth()! Toad

10 Instance Variables AKA Instance Data, Non-Static Fields, or Attributes Toad -x: int! -y: int! -health: int! +count: byte! +move(int,int)! +injure()! +heal()! +gethealth(): int! public class Toad {! private int x,y;!! }!! private int health;! public static byte count=0;! public void move(int x, int y) { }! public void injure() { }! public void heal() { }! public int gethealth() { }! Every object has its own version of the instance variables in memory Instance variable memory allocated when Java constructs an object

11 Static Variables AKA Class Variables Toad -x: int! -y: int! -health: int! +count: byte! +move(int,int)! +injure()! +heal()! +gethealth(): int! public class Toad {! private int x,y;!! }! private int health;! public static byte count=0;! public void move(int x, int y) { }! public void injure() { }! public void heal() { }! public int gethealth() { }!! One copy of the static variables shared amongst all objects of the class Static variable memory allocated when Java loads the class definition (usually when program starts)

12 Methods AKA Operations, Behaviors Toad -x: int! -y: int! -health: int! +count: byte! +move(int,int)! +injure()! +heal()! +gethealth(): int! public class Toad {! private int x,y;!! }! private int health;! public static byte count=0;! public void move(int x, int y) { }! public void injure() { }! public void heal() { }! public int gethealth() { }!! Java learns about the methods when it loads the class

13 Visibility of Data and Methods Modifier Class Package Subclass Everywhere private! X default X X protected! X X X public! X X X X

14 Visibility Guidelines Use the most restrictive access feasible Use private data except perhaps for final constants Public data couples other classes to your implementation decisions limiting your flexibility to change those decisions in the future

15 Accessors and Mutators Commonly known as Getters and Setters Implement an interface to a class s private data An accessor method retrieves the value of a private variable A mutator method modifies the value of a private variable

16 Accessor/Mutator Example: The color of a Dragon Object public class Dragon {!! private String color;!.!.!.! public String getcolor() {return color;}! public void setcolor(string c) {color=c;}!.!.!.! }!

17 Accessors/Mutators: Why Bother? Imagine you wish to change the internal representation of the Dragon s color state in the future If the color instance variable is public, you will have to locate and update every class using it If the instance variable is private, then you are free to change the internal representation as long as you preserve the original interface methods

18 Yes, It Really Matters A 600,000 line program has been re-written at least three times in the past 20 years Not because it was defective Excessive coupling led it to become difficult to modify Estimated cost of about $15,000,000 for each rewrite Lost opportunity cost beyond your wildest imagination

19 Flow of Control Through Methods Eclipse

20 Review Parameters public void paint(graphics console) {! }!!.!!.!!.!

21 Review: Parameter Variables Transfer information into a method Defined between the parenthesis in the method s definition Memory is allocated when the method is invoked Memory is released when the method exits Visible only within the method. Except The caller can supply a value for a parameter

22 Parameters and Arguments Parameters, AKA formal parameters, appear in the definition of a method Arguments, AKA actual parameters, are the values supplied by the callers (invokers) of the method When Java calls (invokes) a method, it copies the values of each argument into its associated parameter variable

23 Parameters: Pass by Value Various programming languages implement parameters differently Java s parameter implementation is known as pass by value 1. When Java invokes a method, it copies the value of each argument into its corresponding parameter variable 2. Then Java starts the method which can modify the value of a parameter just like it can modify any other variable 3. BUT when the method completes, Java does not copy the last value of the parameter back into the caller s variables

24 Review: Local Variables Defined somewhere inside of a method Variable belongs to a method Memory allocated when enclosing method is invoked Memory released when enclosing method exits Visible only within the enclosing method

25 Default Constructor Dragon mydragon = new Dragon();! This is what Java gives you if you don t write a constructor Doesn t initialize the new object s instance variables In this example, it builds a Dragon object leaving its color uninitialized

26 Custom Constructor Dragon mydragon = new Dragon( green );! Builds a green Dragon object in memory But Dragon class must define a constructor to do this

27 Custom Constructor public class Dragon {! String color;!!//instance variable!.!.!.! Dragon(String c) {! color = c;! }!.!.!.! }!

28 More Constructors Dragon class is allowed to define multiple constructors Reasons: To initialize some instance variables while defaulting others To accept a variety of types (e.g. Color.green) for specifying an initial value