Objectives Object Oriented Programming OOP review Black-box - PDF document

9/4/20 Objectives • Object Oriented Programming Ø OOP review Ø Black-box programming Ø Creating classes in Java • State • Constructor • Methods Sep 4, 2020 Sprenkle - CSCI209 1 1 Review • True or False: you can call methods on an array, e.g., int int[] [] fibNums fibNums = {1, 1, 2, 3, 5}; = {1, 1, 2, 3, 5}; • What are some Python à Java Gotchas? • What does == mean in Java? (i.e., what question does == ask?) • What does static mean? • When should we make a method static? • What does a static method have access to? Sep 4, 2020 Sprenkle - CSCI209 2 2 1

9/4/20 Assignment 3 Feedback: Remember good programming practices • No side effects to methods Ø If method does not say that it is printing something, don’t print something • Printing while debugging is fine; remove print statements before final submission Ø Want method to reusable à print statements may not be what others want • Leverage existing APIs Ø StringBuilder has replace and reverse methods • Likely more efficient than anything you write Sep 4, 2020 Sprenkle - CSCI209 3 3 Assignment 3 Discussion Move from this (when you were new to programming): if if (isPalindrome(potentialPalindrome) == true true ) { System. out out .println(potentialPalindrome + " is a palindrome."); } else else { System. out out .println(potentialPalindrome + " is not a palindrome"); } To this (at least your 3 rd programming course): if if (isPalindrome(potentialPalindrome)) { System. out out .println(potentialPalindrome + " is a palindrome."); } else else { System. out out .println(potentialPalindrome + " is not a palindrome"); } Sep 4, 2020 Sprenkle - CSCI209 4 4 2

9/4/20 Assignment 3 Discussion if if (string.equals(string2)) { return return true true; } return return false false; Rewrite the above code in one statement. Sep 4, 2020 Sprenkle - CSCI209 5 5 Assignment 3 Discussion if if (string.equals(string2)) { return return true true; } return return false false; Much more concise and still understandable return return string.equals(string2)); Sep 4, 2020 Sprenkle - CSCI209 6 6 3

9/4/20 Review: Object-Oriented Programming • What is OO programming? Ø What are its components? • What are its benefits? Sep 4, 2020 Sprenkle - CSCI209 7 7 Review: Classes & Objects • Classes define template from which objects are made Ø “Cookie cutters” Ø Define state (aka fields or attributes) Ø Define behavior • Many objects can be created of a class Ø Object: the cookie! Ø Ex: Many Mustangs created from Ford’s “blueprint” Ø Object is an instance of the class Sep 4, 2020 Sprenkle - CSCI209 8 8 4

9/4/20 Constructors • Constructor: a special method that constructs and initializes an object Ø After construction, can call methods on object Sep 4, 2020 Sprenkle - CSCI209 9 9 Black-Box Programming • How object does something doesn’t matter Ø Example: if object sorts , does not matter if uses merge or quick sort • What object does matters (its functionality ) Ø What object exposes to other objects Ø Referred to as “ black-box programming” or encapsulation Object • Has public interface that others can use • Hides state from others Sep 4, 2020 Sprenkle - CSCI209 10 10 5

9/4/20 Discussion What is the problem with white-box programming? Object Object Others can see and manipulate Java’s structure helps us object’s internals enforce black-box • May have unintended programming consequences Sep 4, 2020 Sprenkle - CSCI209 11 11 Access Modifiers • A public public method (or instance field) means that any object of any class can directly access the method (or field) Ø Least restrictive • A private private method (or instance field) means that any object of the same class can directly access this method (or field) Ø Most restrictive • Additional access modifiers will be discussed with inheritance In general, what access modifiers will we use for methods? For instance fields? Sep 4, 2020 Sprenkle - CSCI209 12 12 6

9/4/20 CREATING CLASSES Sep 4, 2020 Sprenkle - CSCI209 13 13 Classes and Objects • Java is pure object-oriented programming Ø All data and methods in a program must be contained within a class • But, for data, can use objects as well as primitive types (e.g., int, double, char ) Sep 4, 2020 Sprenkle - CSCI209 14 14 7

9/4/20 Example: Chicken class • State Ø Name, weight, height • Behavior Ø Accessor methods • getWeight, getHeight, getName • Convention: “get” for “getter” methods Ø Mutator methods • feed : adds weight and height when bird eats • setName Sep 4, 2020 Sprenkle - CSCI209 15 15 General Java Class Structure public public class class ClassName { ClassName { // --------- INSTANCE VARIABLES --------------- // define variables that represent object’s state private int private int inst_var inst_var; // --------- CONSTRUCTORS --------------- public public ClassName() { ClassName() { // initialize data structures } // ----------- METHODS ------------ public public int int getInfo() { getInfo() { return return inst_var inst_var; } Note: instance variables are private } and methods are public Sep 4, 2020 Sprenkle - CSCI209 16 16 8

9/4/20 Example: Chicken class Discussion : data types for • State state variables? Ø Name, weight, height • Behavior Ø Accessor methods • getWeight, getHeight, getName • Convention: “get” for “getter” methods Ø Mutator methods • feed : adds weight, height • setName Ø Convention: “set” for “setter” methods Sep 4, 2020 Sprenkle - CSCI209 17 17 Instance Variables: Chicken.java public class public class Chicken { Chicken { // --------- INSTANCE VARIABLES --------------- private String private String name; name; private int private int height; height; // in cm private double private double weight; weight; // in lbs Instance variables are declared, with access modifier All instance variables are private private Sep 4, 2020 Sprenkle - CSCI209 18 18 9

9/4/20 Constructor: Chicken.java public public class class Chicken { Chicken { // --------- INSTANCE VARIABLES --------------- private String private String name; name; private private int int height; height; // in cm private double weight; private double weight; // --------- CONSTRUCTORS --------------- public Chicken( public Chicken( String name, int int h, h, double weight) { double weight) { this this.name name = name; = name; this this.height height = h; = h; this this.weight weight = weight; = weight; } … Observations? Thoughts? Questions? Sep 4, 2020 Sprenkle - CSCI209 19 19 Constructor: Chicken.java public public class class Chicken { Chicken { // --------- INSTANCE VARIABLES --------------- private String private String name; name; private int private int height; height; // in cm Type and name for private double private double weight; weight; Constructor name same as class’s name each parameter // --------- CONSTRUCTORS --------------- public public Chicken( Chicken( String name, int int h, h, double double weight) { weight) { this this.name name = name; = name; Params don’t need to be same this this.height height = h; = h; this this.weight weight = weight; = weight; names as instance var names } this this : Special name for the constructed object, … like self in Python (differentiate from parameters) Sep 4, 2020 Sprenkle - CSCI209 20 20 10

9/4/20 Constructors • Constructor: a special method that constructs and initializes an object Ø After construction, can call methods on object • A constructor has the same name as its class Sep 4, 2020 Sprenkle - CSCI209 21 21 Example: Chicken class • State Ø Name, weight, height • Behavior Ø Accessor methods • getWeight, getHeight, getName • Convention: “get” for “getter” methods Ø Mutator methods • feed : adds weight, height • setName Discussion : What are the methods’ input (parameters) and output (what is returned)? Sep 4, 2020 Sprenkle - CSCI209 22 22 11

9/4/20 Methods: Chicken.java Type the method returns … // --------- Getter Methods --------------- public String getName() { public return this. return this. name ; Chicken object’s } instance variables // --------- Mutator Methods --------------- void feed() { public public void weight += .3; height += 1; } … } Note that you don’t have to use this this when variables are unambiguous Sep 4, 2020 Sprenkle - CSCI209 23 23 Constructing objects • Given the Chicken constructor Chicken( String name, int height, double weight ) create a chicken with the following characteristics Ø “Fred”, weight: 2.0, height: 38 Chicken chicken = new new Chicken("Fred", 38, 2.0); Sep 4, 2020 Sprenkle - CSCI209 24 24 12

9/4/20 Object References • Variable of type Object: value is memory location new Chicken("Fred", 38, 2.0); Chicken one = new Chicken weight = 2.0 Memory Location 38 height = one = name = "Fred" Sep 4, 2020 Sprenkle - CSCI209 25 25 Object References • Variable of type Object: value is memory location If I haven’t called the constructor, only declared the variables, e.g., one = Chicken one; Chicken two; two = Both one and two are equal to null null This is the case for objects. Primitive types are not null . Sep 4, 2020 Sprenkle - CSCI209 26 26 13