Compsc sci 201 201 Classe sses, s, Arrays, s, A APIs State - - PowerPoint PPT Presentation

Compsc sci 201 201 Classe sses, s, Arrays, s, A APIs

1/17/2020 Compsci 201, Spring 2020 1

Susan Rodger January 17, 2020

State Constructor Methods (behavior)

Be in the know …

• Add yourself

elf t to compsci@ ci@duk uke.edu

• Duke University mailing lists – add yourself

https://lists.duke.edu/sympa

• Compsci related events, jobs, research
• pportunities
• Appl

Apply fo for Dat ata+, CS+, Code+ e+

• summer research at Duke, paid, hire lots of 1st

year students, 2cd year, etc.

• Apply in January!

https://www.cs.duke.edu/undergrad/summer_research

1/17/2020 Compsci 201, Spring 2020 2

C is for …

• Class
• Framework for creating objects
• Collections and Collection
• See java.util.* for details
• Collaboration
• Review the policy

1/17/2020 Compsci 201, Spring 2020 3

Plan for the Day

• Revie

iew Obj bject t concept: t: c classes, P P0

• What is a class, object, instance variable
• Revie

iew arrays in J Java: m meth thods ds and c d concepts ts

• Required for APTs due next week
• Move toward ArrayList and other collections
• Codin

ing a and h d helpe lper f functio tions

• Efficient programming and not efficient programs

1/17/2020 Compsci 201, Spring 2020 4

Strings: Example from last time

• You

u can an’t m modif ify a a string, alw always crea eate ne new S String

String s = new String(“joy"); String t = s; s = t + t; // contents of boxes the same

1/15/2020 Compsci 201, Spring 2020 5

Class

• Adject

ctiv ive, n noun, c n, close t to a verb

• Show some ___ you're in a great ___, that's a

___ act, let's ___-ify that

• Fundam

ament ental p al part o

• f object

ct-ori riented p pro rogra grammi mming

• All Java code is in a class, alas the primitives
• In Python int is a class, has no upper bound
• In Java int is a primitive, 231-1 maximal value

1/17/2020 Compsci 201, Spring 2020 9

Class encapsulates state and behavior

• Class i

is a templa late, o , object ect h has c char aract cter eris istics ics

• Dogs have fur, speed, temperament, size, …
• Typic

icall lly w we e don’t us use ex exam amples lik like t thi his, b but ut t the hey can an help help b build uild int intuition and and und understanding

• Class dog, retriever extends dog, method bark()

1/17/2020 Compsci 201, Spring 2020 10

Class and Object

• If w

we e had had a R a Retriever class we c e coul uld instant ntia iate a an instance nce o

• f the c

e class, i. , i.e., ., c create a e an o

• bject

ct

Retriever ks = new Retriever("kelsey");

• Cla

lass is is an an object f t factory, c , calling ling new create tes a s a ne new o

• bjec

ect t tha hat is is an an ins instance o

• f the

he c cla lass

• We could call a method: ks.bark()

1/17/2020 Compsci 201, Spring 2020 11

Classes in Java

• Define class Foo in

Foo.java

• Create object by calling

new Foo(..)

• Access object by calling

methods:

• bj.doSomething()
• Some methods return a

value, use it!

1/17/2020 Compsci 201, Spring 2020 12

State Constructor Methods (behavior)

Classes in Java

• State: instance

variables: private

• Constru

ructors rs: initialize instance variables

• Met

etho hods: functions aka behavior

• Docum

ument entatio ion: Javadoc and other comments

1/17/2020 Compsci 201, Spring 2020 13

Work-Flow for Assignments

• Wha

hat is is the he work-flo low f for P0 and A Assig ignm nment nts?

• Login to gitlab
• Code URL to P0 in gitlab
• Fork it (makes a copy in the cloud)
• Clone with ssh

1/17/2020 Compsci 201, Spring 2020 15

Using a shell

• Place t

ace to type s shell c ell command nds

• On

n Mac ac us use T Ter erminal, W Wind indows us use B e Bash G Git it

• Wha

hat is is thi his?

• It’s a

a pat ath: h:

• Folder named IdeaProjects
• Folder named spring20

– May have other folders inside it

• $

$ is is a p a prompt, rea eady for a s a shel hell co command

1/17/2020 Compsci 201, Spring 2020 16

A few shell commands

• pw

pwd – di displ play c current pa path

• cd

d – cha hang nge i e into m main f n folder er/dir irect ectory

• cd nam

name

• - cha

hange int into f fold lder nam named nam name

• cd ..

.. – cha hang nge e back i into p parent nt f folder

• ls

ls

• sho

how f file iles in in cur current f fold lder

• Let’s s

see s some o

• f those…

e….

1/17/2020 Compsci 201, Spring 2020 18

Back to Work-Flow for Assignments

• Clone

ne wit ith h ssh sh

• Go
• to
• you
• ur s

shell

• cd (to folder you want to put your P0 in)
• git clone (SSH URL you copied)
• ls (will show your files)
• Using

ng I IntelliJ elliJ c comple lete t the as assig ignm nment nt

• Save code often to gitlab!

1/17/2020 Compsci 201, Spring 2020 19

Work-Flow for Assignments (cont)

• Send

d code de b back k to to gitl itlab (DO O OFTEN)

• cd ( into project folder)
• 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
• No

Now t w to Gradesc scop

• pe and s

d submit bmit project ect

• Don’t like results - fix code, push code, run on

Gradescope again

1/17/2020 Compsci 201, Spring 2020 20

Classes and P0

• How many

any P Per erson o

• bjec

ects c created?

• Each has a name and an age, different for each
• instance. Thus: instance variables

1/17/2020 Compsci 201, Spring 2020 22

Classes and P0

• How many Person o
• bjects cr

created?

• Each has a name and an age, different for each
• instance. Thus: instance variables
• To cr

create? Call ne new w whi hich inv nvokes a a co constructor

• No return type, initialize instance variables
• Acces

ccess l level els: p private o e only w withi hin c n class, p public f c from

• ther

her c classes es

• Technically there is a package access, we ignore

1/17/2020 Compsci 201, Spring 2020 23

1/17/2020 Compsci 201, Spring 2020 24

Constructor

• Sam

ame nam name a as c cla lass

• No return type
• Overlo

load ad with d differ erent ent p parameters

• Each should initialize all instance variables
• Fac

actor o

• ut

ut c common co code int into help helper m method if if leng lengthy

• Can call another constructor using this(…)

1/17/2020 Compsci 201, Spring 2020 25

What is this?

• An object

ct i instanc ance r ref efer ers t to itself elf

• Method or constructor: object references itself
• Every reference to an instance variable myVar

could be written as this.myVar

• Code f

e for a an object ct t to pass i itself elf:

• callMethod(this,"hello");
• Construct

uctor c can c n call o ll other er construc uctor

• this("hello");

1/17/2020 Compsci 201, Spring 2020 26

Running a Java Program

• On

n lap laptop/desktop la laun unch/run p point int is is t the m he main ain method in in any any c cla lass

• Driver programs in P0, runs/drives the code
• Method signature requir

uired to run program

1/17/2020 Compsci 201, Spring 2020 27

WOTO (3 minutes)

http:// //bi bit.ly/2 /201spr pring20-01 0117 17-1

1/17/2020 Compsci 201, Spring 2020 28

Luis von Ahn

• Duke 2000, Math
• Duke Honorary Degree 2017
• CEO Duolingo
• Macarthur Award, 2006
• MIT-Lemelson Prize, 2018

1/17/2020 Compsci 201, Spring 2020 29

“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.”

Arrays, APTs, and APIs

• Why

hy is is alli alliteration im important in in writ iting?

• Why are t

thes ese i e important nt i in program amming ing?

• APIs c

crea eate p e possib ibilit ilitie ies

1/17/2020 Compsci 201, Spring 2020 30

Array Details

• Once a

ce array c created, it' t's siz ize i e is fixed, c can' n't g grow!

• Indexable elements can be changed
• Using

g a[k] we can r n read/w /writ ite va values ues

• Instance variable a.length is size of array
• No parentheses, hence not a method
• Notice dot notation: object dot name

1/17/2020 Compsci 201, Spring 2020 31

Indexing for loops and arrays

• Constr

tructin ting and i d initi itiali lizing … … int[] a = new int[100]; for(int k=0; k < a.length; k += 1){ a[k] = 99; }

• Let an A

n API-call ll f fill i ll in array: y: java.util.Arrays

• 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 32

For each loops and arrays

• For each l

loop: no no ind ndex, no no cha changing w wha hat’s s stored int[] a = {1,2,3,4,5,6,7,8,9,10}; int sum1 = 0; int sum2 = 0; for(int k=0; k < a.length; k += 1){ sum1 += a[k]; } for(int value : a){ sum2 += value; } System.out.println(sum1 == sum2);

1/17/2020 Compsci 201, Spring 2020 33

For Loop Summary

• for(init; boolean guard; update) {…}
• for(int k=0; k < a.length; k+=1) {…}
• Initialization happens once, bef

efore g e guard check ecked ed f for t the f first t time, n , never er ag again ain

• Initialization can introduce variables: loop
• op s

scop

• pe
• Guard checked, if true loop body executes
• After loop body, update executes, guard

checked

1/17/2020 Compsci 201, Spring 2020 34

Control Construct Summary

• if (boolean) {…}
• 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

From Control to APIs

• Lis

ist and and Arra rrayList simila ilar t to arr rray, b but …. ….

• Grow as needed, can't use [k] to access
• Powerful APIs, e.g., as follows

1/17/2020 Compsci 201, Spring 2020 36

Solving an APT Together

• Totalit

ality ( (see A APT pag age o e on course s e site) http:/ ://www.cs.d .duk uke.e .edu/ u/cs csed ed/ne newapt/t /totalit ality.ht html

• Solve

e by h hand nd: : a = = { {20,30,40, ,30,40,50,60} 50,60} stype pe=“odd dd”

• Use w

e wha hat y you u kno now, b but ut im implement in in Java

• Che

heck id idea eas us using jshell ll (Java 9 9 and and la later)

• Command line is your friend!

1/17/2020 Compsci 201, Spring 2020 38

Think Before You Code

• Solve b

e by hand nd … … Che heck ck y your understand anding ing o

• f

ex exam amples … thi hink ab about ut s solution you’ u’ll w l writ ite ... ...

• Then think before fingers on keys

1/17/2020 Compsci 201, Spring 2020 39

Coding Interlude

• Working

ing on Totalit ality APT i in I IntelliJ lliJ

• Odd? Even?
• Control: if, if-else, …

1/17/2020 Compsci 201, Spring 2020 40

WOTO (3 minutes)

http:/ ://b /bit.l .ly/ y/20 201s 1spring20 20-01 0117 17-2

1/17/2020 Compsci 201, Spring 2020 41

Josh Bloch

• Led desig

ign o n of Java C a Collec lectio ions ns Frame mework rk

• Former

erly J Java C a Chie ief A Archit chitec ect at at G Google le

• Professor o
• f the P

he Prac actice C CMU

1/17/2020 Compsci 201, Spring 2020 42

APIs should be easy to use and hard to misuse. It should be easy to do simple things; possible to do complex things; and impossible, or at least difficult, to do wrong things.

Visualizations Help Understanding?

• Javatutor to visua

uali lize c e code: : http:/ ://pytho hont ntut utor.co com/java. a.ht html

• Using the java.awt.Color class
• Both String and Color are immut

utab able le

• Once created, cannot every change

1/17/2020 Compsci 201, Spring 2020 43

Summary of Java-isms

• Loop us

usin ing ind indexes o

• ver

er an an ar array

• The for-loop: initialize; guard/check; update
• Totality: lo

loop over er o

• dd ind

index exes o

• nl

nly?

• In some cases, …
• How do we check

eck f for String ing e equalit lity?

• .equals compared to ==
• How

w do do we subm bmit a an APT APT?

• Test, Grade, REFLECT
• APTS – one grace day, NO LATE AFTER THAT

1/17/2020 Compsci 201, Spring 2020 44