Compsc sci 201 201 Wo Work, Nbody dy, , ArrayL yLists ts Susan Rodger January 29, 2020 1/29/2020 Compsci 201, Spring 2020 1
F is for … • Folder • aka Directory – where things are stored in Git • Function • Abstraction – a method in Java 1/29/2020 Compsci 201, Spring 2020 2
PFTW • Get etting ng t thing ngs done i ne in 201 • How to succeed and enjoy the effort • Mund ndan ane J e Java-is isms ms • From char to autoboxing: primitives • What is this ? • Gener neric classes es: H How Ar ArrayList st wo works • Design, create, test, measure 1/29/2020 Compsci 201, Spring 2020 3
Getting Things Done in 201 • What do these d dat ata m mean f n for y you, u, f for m me, f for t the communit unity o of 286 s stud udent ents i in Compsc sci 201? 201? 1/29/2020 Compsci 201, Spring 2020 7
From Last Time … Go over WOTO: Correctness Counts http://b ://bit.l .ly/20 /201s 1spring2 g20-012 124-2 1/24/2020 Compsci 201, Spring 2020 8
Object class, equals method • In In Java vaDoc • Signa natur ure o e of equals als met etho hod 1/29/2020 Compsci 201, Spring 2020 9
@Override .equals • Create a new P Point m met etho hod • Use annotation @Override, help with errors boolean equals(Object o) { … Must u use t e this is s signa natur ure, t to implem lement ent: • • Cast parameter appropriately • Compare instance fields 1/29/2020 Compsci 201, Spring 2020 10
Point inherits Object.equals • This d s doesn sn’t w t work f k for Point o t objects ts! • Def efault ult simply uses ==, no idea about points • a.equals(b) if a and b ref eference nce t the same o e object ect • Two different (0,0) points not the same 1/29/2020 Compsci 201, Spring 2020 11
Point equals fixed! • Must u use s e same s e signa natur ure • Must c st cast st O Object to t to Point 1/29/2020 Compsci 201, Spring 2020 12
Contract for Equality • Reflex lexiv ive e x.equals(y) then y.equals(x) • Trans ansit itivit ivity: : x.eq(y), y.eq(z) then n x.eq(z) • Check x.equals(x) as as a a special c ecial cas ase w e with = h == • Check this.getClass() == o.getClass() • Don’t want to have an apple == orange • Cas ast O Object ct p param ameter er a and u use i e instance ance va variab iables les • See Point as example 1/29/2020 Compsci 201, Spring 2020 16
Amanda Randles, Duke 2005 • ACM Grace M Murray H Hoppe pper Aw Award ( d (<= 35 yo yo) For developing HARVEY, a massively parallel circulatory simulation code capable of modeling the full human arterial system at subcellular resolution and fostering discoveries that will serve as a basis for improving the diagnosis, prevention, and treatment of human diseases. //XXX and Amanda Peters //Compsci 100: Huffman Coding //November 19, 2002 I felt like working in a pair was a really successful way to complete the program. It helped the most when it came to working out basic logic and finding errors. I found it really helpful because he often would see the basic logic to the code and I could help more with the implementation. I feel like it was a successful group and we both contributed a lot. 1/29/2020 Compsci 201, Spring 2020 17
Reading Points • We'll t ll typic ically lly u use a e a Scanne nner t to read value ues • Use .hasNext(),.hasNextDouble(), … • If/while there's more to read? Call .next() • Met etho hod .next() retur urns ns a a String ing • Method .nextDouble() returns a double … • See e PointRea eader er.j .java cla lass, us useful in in NB NBod ody 1/29/2020 Compsci 201, Spring 2020 18
Scanner Sources for Reading • Construct uct a a Scanne anner f from System em.in .in • Reads from keyboard/console • .hasNextX() true until end-of-file OR no X • Control-D on OS X, Control-Z on Windows • Cons nstruct a a Scanner f from a a File ile • Reads from file, ex exception c could uld hap happen • .hasNextX() true until all of file read OR no X • Each call of .nextX() returns the next X, internally the Scanner "remembers" where it last read 1/29/2020 Compsci 201, Spring 2020 19
Scanner hasNext and next • Think nk a about ut s scanner nner a as a a long r reel/s l/sour urce ce of data • If .hasNext() returns true, there is something to read by Scanner cursor/reader • Calling .next() returns and and ad advances cursor • Scanner object maintains cursor internally • Source: f : file, S , String ing, , terminal inal, … , … 1/29/2020 Compsci 201, Spring 2020 20
N-Body Simulation • Class Celes lestialB ialBody repres esent ents C Celes lestia ial B l Body • Planet, Sun, Asteroid • Models an object in 2D space, not 3D • Position, Velocity, Mass, Image for display • Class NB NBod ody drives es t the s simula ulation • Compute gravitational forces: physics • Time-step simulation • compute all forces, update ,display 1/29/2020 Compsci 201, Spring 2020 21
Class CelestialBody • Illus lustrates s stand andard J Java i idioms • Constructors, Methods, Instance Variables • State i is private: s : six i instanc nce v e variab iable les • myXPos , … using my my convention - this object • Initialized by constructors • Methods ar are p e pub ubli lic • Include access ssors aka getters for state • No setters , cannot change myXPos other than via the update method, a mutator 1/29/2020 Compsci 201, Spring 2020 22
The Object Concept • Every i instan ance ce v variab able a and ever ery non-sta tatic tic m meth thod d acces cessed ed/cal called ed a after Obj bject. t.Dot • b.getX(), b.calcForcExertedBy(other) • From w withi hin a n a class, e e.g., Celestia tialB lBody • myXPos , getX(),this.myXPos , • All are equivalent as is this.getX() • Some e prefer er a always using t this. – clear earer er? 1/29/2020 Compsci 201, Spring 2020 23
NBody numbers • Floating ing p point nt i issue ues, p , proble lems, q , quand ndaries ies • When is (a + b) + c != a + (b + c) • When is a/b * c != a*c / b • Watch for this in Gradescope tests!! 1/29/2020 Compsci 201, Spring 2020 25
Debugging Arithmetic • Order er o of operatio ions ns w with f h floating ing p point v values ues c can n result ult i in overflo low, u , under erflo low, m , more • Small number + Big number … 1/29/2020 Compsci 201, Spring 2020 26
Debugging double Arithmetic • Integer v values ues a are n not the s same as e as D Double v le values ues • 1/0 is … whereas 1.0/0 is … 1/29/2020 Compsci 201, Spring 2020 27
Completing NBody • Please r e read t the e TL;DR d docum cument nt • Test at each step, push constantly using Git • Afte ter usin ing suppli pplied T d Test… c classes, p proceed d to to simu mula lati tion • Must be able to read data file to simulate • Understand the basics, read carefully • Analysis is: comple mplete b before s submit mittin ting to to Gradesc scop ope for final s submissi ssion on 1/29/2020 Compsci 201, Spring 2020 28
Now look at DNAMaxNucleotide • Retur urn n the s strand and f from s strand ands a array w with m h most occur urrence nces o of nucleo cleotid ide e nuc nuc. R . Retur urn l n longes est such s ch strand nd • Exa Exampl ple 1/29/2020 Compsci 201, Spring 2020 29
Algorithm - DNAMaxNucleotide • Does es t thi his co code m e mak ake t the he alg algorithm c cle lear? • Why must count be a helper method? • Why can't max = 0 before loop? 1/29/2020 Compsci 201, Spring 2020 30
Two Versions of Helper Method • Iter erating ing o over each c ch char aract acter er o of a string ng • Note that nuc is a one-character string • How does s.substring(a,b) work? 1/29/2020 Compsci 201, Spring 2020 31
Critique of another implementation • Where d e does t this is s solut lutio ion c n come from? • Strings are immutable, s.replace(…) • Replace every "a" with "" (nothing) • Is this better? Different? More clever? More of a hack? … 1/29/2020 Compsci 201, Spring 2020 32
WOTO http:// //bi bit.ly/2 /201spr pring20-01 0129 29-1 1/29/2020 Compsci 201, Spring 2020 33
Donald Knuth • aka “The Donald” • Turing award (and others) • Author of “The Art of Computer Programming” • Arguably most important book written in Computer Science • First publication: Mad Magazine If you optimize everything you will always be unhappy. Everyday life is like programming, I guess. If you love something you can put beauty into it. https://www.youtube.com/watch?v=cK7yyjXfbc4 1/29/2020 Compsci 201, Spring 2020 34
From Array to ArrayList • Have int[], String[], CelestialBody[] • Array of any type, but d doesn sn't g 't grow • Can't use .contains with array, can't print • The The java.ut utils ils.Arrays cla lass ha has s some hel help 1/29/2020 Compsci 201, Spring 2020 35
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