Compsci 201 Add yourself to compsci@duke.edu Classes, Arrays, APIs - PowerPoint PPT Presentation

Be in the know … Compsci 201 • Add yourself to compsci@duke.edu Classes, Arrays, APIs • Duke University mailing lists – add yourself https://lists.duke.edu/sympa State • Compsci related events, jobs, research opportunities Constructor • Apply for Data+, CS+, Code+ Susan Rodger • summer research at Duke, paid, hire lots of 1 st January 17, 2020 Methods year students, 2cd year, etc. (behavior) • Apply in January! https://www.cs.duke.edu/undergrad/summer_research 1/17/2020 Compsci 201, Spring 2020 1 1/17/2020 Compsci 201, Spring 2020 2 C is for … Plan for the Day • Class • Review Object concept: classes, P0 • Framework for creating objects • What is a class, object, instance variable • Collections and Collection • See java.util.* for details • Review arrays in Java: methods and concepts • Required for APTs due next week • Move toward ArrayList and other collections • Collaboration • Coding and helper functions • Review the policy • Efficient programming and not efficient programs 1/17/2020 Compsci 201, Spring 2020 3 1/17/2020 Compsci 201, Spring 2020 4

Strings: Example from last time Class • You can’t modify a string, always create new String • Adjective, noun, close to a verb • Show some ___ you're in a great ___, that's a String s = new String(“joy"); ___ act, let's ___-ify that String t = s; • Fundamental part of object-oriented programming • All Java code is in a class, alas the primitives • In Python int is a class, has no upper bound s = t + t; • In Java int is a primitive, 2 31 -1 maximal value // contents of boxes the same 1/15/2020 Compsci 201, Spring 2020 5 1/17/2020 Compsci 201, Spring 2020 9 Class and Object Class encapsulates state and behavior • Class is a template, object has characteristics • If we had a Retriever class we could instantiate an instance of the class, i.e., create an object • Dogs have fur, speed, temperament, size, … Retriever ks = new Retriever("kelsey"); • Typically we don’t use examples like this, but they • Class is an object factory , calling new creates a can help build intuition and understanding new object that is an instance of the class • Class dog, retriever extends dog, method bark() • We could call a method: ks.bark() 1/17/2020 Compsci 201, Spring 2020 10 1/17/2020 Compsci 201, Spring 2020 11

Classes in Java Classes in Java • Define class Foo in • State: instance variables: private Foo.java State • Create object by calling • Constructors: initialize Constructor instance variables new Foo(..) • Access object by calling • Methods: functions Methods methods: aka behavior (behavior) obj.doSomething() • Documentation: • Some methods return a Javadoc and other value, use it! comments 1/17/2020 Compsci 201, Spring 2020 12 1/17/2020 Compsci 201, Spring 2020 13 Work-Flow for Assignments Using a shell • What is the work-flow for P0 and Assignments? • Place to type shell commands • Login to gitlab • On Mac use Terminal, Windows use Bash Git • What is this? • Code URL to P0 in gitlab • Fork it (makes a copy in the cloud) • It’s a path: • Folder named IdeaProjects • Clone with ssh • Folder named spring20 – May have other folders inside it • $ is a prompt, ready for a shell command 1/17/2020 Compsci 201, Spring 2020 15 1/17/2020 Compsci 201, Spring 2020 16

Back to Work-Flow for Assignments A few shell commands • Clone with ssh h • pwd – display current path • cd – change into main folder/directory • Go to your shell • cd name -- change into folder named name • cd (to folder you want to put your P0 in) • cd .. – change back into parent folder • git clone (SSH URL you copied) • ls -- show files in current folder • ls (will show your files) • Using IntelliJ complete the assignment • Let’s see some of those…. • Save code often to gitlab! 1/17/2020 Compsci 201, Spring 2020 18 1/17/2020 Compsci 201, Spring 2020 19 Classes and P0 Work-Flow for Assignments (cont) • How many Person objects created? • Send code back to gitlab (DO OFTEN) • Each has a name and an age, different for each • cd ( into project folder) instance. Thus: instance variables • git add . • Puts code in staging area, tells git you want to update • git commit –m “comment on what you did” • Changes are recorded in local repo • git push • Uploads the changes to your gitlab repo • Now to Gradescope and submit project • Don’t like results - fix code, push code, run on Gradescope again 1/17/2020 Compsci 201, Spring 2020 20 1/17/2020 Compsci 201, Spring 2020 22

Classes and P0 • How many Person objects created? • Each has a name and an age, different for each instance. Thus: instance variables • To create? Call new which invokes a constructor • No return type, initialize instance variables • Access levels: private only within class, public from other classes • Technically there is a package access, we ignore 1/17/2020 Compsci 201, Spring 2020 23 1/17/2020 Compsci 201, Spring 2020 24 What is this ? Constructor • Same name as class • An object instance refers to itself • No return type • Method or constructor: object references itself • Every reference to an instance variable myVar could be written as this.myVar • Overload with different parameters • Each should initialize all instance variables • Code for an object to pass itself: • callMethod(this,"hello"); • Factor out common code into helper method if lengthy • Constructor can call other constructor • Can call another constructor using this(…) • this("hello"); 1/17/2020 Compsci 201, Spring 2020 25 1/17/2020 Compsci 201, Spring 2020 26

Running a Java Program WOTO (3 minutes) • On laptop/desktop launch/run point is the main method in any class http://bit.ly/201spring20-0117-1 • Driver programs in P0, runs/drives the code • Method signature required to run program 1/17/2020 Compsci 201, Spring 2020 27 1/17/2020 Compsci 201, Spring 2020 28 Luis von Ahn Arrays, APTs, and APIs Duke 2000, Math • Duke Honorary Degree 2017 • • Why is alliteration important in writing? CEO Duolingo • • Why are these important in programming? Macarthur Award, 2006 • • APIs create possibilities • MIT-Lemelson Prize, 2018 “It’s amazing how motivating it is to sit with somebody and say, ‘What you’re doing is really important.’ I use that a lot.” 1/17/2020 Compsci 201, Spring 2020 29 1/17/2020 Compsci 201, Spring 2020 30

Array Details Indexing for loops and arrays • Constructing and initializing … • Once array created, it's size is fixed, can't grow! int[] a = new int[100]; • Indexable elements can be changed for(int k=0; k < a.length; k += 1){ a[k] = 99; • Using a[k] we can read/write values } • Instance variable a.length is size of array • No parentheses, hence not a method • Let an API-call fill in array: java.util.Arrays • Notice dot notation: object dot name • Arrays.fill(a,99); • https://docs.oracle.com/en/java/javase/11/docs/api/ java.base/java/util/Arrays.html 1/17/2020 Compsci 201, Spring 2020 31 Compsci 201, Spring 2020 32 1/17/2020 For each loops and arrays For Loop Summary • For each loop: no index, no changing what’s stored • for(init; boolean guard; update) {…} int[] a = {1,2,3,4,5,6,7,8,9,10}; • for(int k=0; k < a.length; k+=1) {…} int sum1 = 0; int sum2 = 0; • Initialization happens once, before guard for(int k=0; k < a.length; k += 1){ checked for the first time, never again sum1 += a[k]; • Initialization can introduce variables: loop scope } for(int value : a){ • Guard checked, if true loop body executes sum2 += value; } • After loop body, update executes, guard System.out.println(sum1 == sum2); checked 1/17/2020 Compsci 201, Spring 2020 33 1/17/2020 Compsci 201, Spring 2020 34

From Control to APIs Control Construct Summary • List and ArrayList similar to array, but …. • Grow as needed, can't use [k] to access • if (boolean) {…} • Powerful APIs, e.g., as follows • Block executed when guard is true • {.} not needed for single statement, use anyway • if (boolean) {…} else {…} • Code in else block when negation true • while(boolean) {…} • Check boolean guard, execute body, repeat • Guard checked again after body executed 1/17/2020 Compsci 201, Spring 2020 35 1/17/2020 Compsci 201, Spring 2020 36 Solving an APT Together Think Before You Code • Totality (see APT page on course site) • Solve by hand … Check your understanding of examples … think about solution you’ll write ... http://www.cs.duke.edu/csed/newapt/totality.html • Then think before fingers on keys • Solve by hand: a = {20,30,40,50,60} stype=“odd” • Use what you know, but implement in Java • Check ideas using jshell (Java 9 and later) • Command line is your friend! 1/17/2020 Compsci 201, Spring 2020 38 1/17/2020 Compsci 201, Spring 2020 39