more java basics - duke university · more java basics jeff forbes owen astrachan september 8, 2017...

More Java Basics

Jeff ForbesOwen Astrachan

September 8, 2017

9/8/17 Compsci 201, Fall 2017, Java Basics 1

D is for …• Debugging• A key skill in making your programs run

• Data-first• Data dictates the tradeoffs

• Digital• All about the 1s and 0s

9/1/17 Compsci 201, Fall 2017, Java Basics 2

Review: What is a class?• Encapsulates state and behavior• State is: instance variables, specific to each

object• Behavior is methods: update state, possibly

use or report on state• Class is a blueprint or template: object creation• Call new, invokes constructor, initialize object

• Objects communicate via methods, parameters• Object can pass itself: this refers to self

9/8/17 CompSci 201, Fall 2017, Java Basics 3

Access Control• public: accessible anywhere• private: accessible only within methods of this

class

• protected: accessible to this class or subclasses

• No modifier: accessible within class and packageMorelateronthese…

9/8/17 Compsci 201, Fall 2017, Java Basics 4

9/6/17 Compsci 201, Fall 2017, Simulation 5

Solving problems• See ClassScores APT• Prof. Drew Hilton’s 7 step process

9/8/17 Compsci 201, Fall 2017, Java Basics 6

Do it on paper!

9/8/17 Compsci 201, Fall 2017, Java Basics 7

• Find the maximally occurring number(s)• How do you count?• What data structures will you need?• Translate to code later

ArrayList• An array that does not have fixed length•No primitives! •Only Objects!

ArrayList<String> list = new ArrayList<String>();

// add to ArrayListlist.add("hello");

//check if element is in ArrayListboolean inList = list.contains("hello");

//get element at index fiveString word = list.get(5);

9/8/17 Compsci 201, Fall 2017, Java Basics 8

Java API• Application Programming Interface• Classes that are part of the Java development kit• Data structures• e.g., java.util.ArrayList, java.lang.String, java.util.Scanner

• Utility methods• e.g., java.lang.Math, java.util.Arrays

9/8/17 Compsci 201, Fall 2017, Java Basics 9

Useful methods for Arrays• Arrange elements in natural order

int[] example;Arrays.sort(example);

• Convert from List/Collection to Array• For arrays of objects (such as Strings) use the asList method in the Arrays class.• Pass this into the constructor of your Collection

• ExampleString[] words = String[N];. . .TreeSet<String> wordset = new

TreeSet<String>(Arrays.asList(words));

Writing & Testing ClassScores1. WOTO: http://bit.ly/201-f17-0908-12. How do you print all of the elements in an array

of ints?• Add a method printArray that will print the

elements in standard form[88, 70, 65, 70, 88]

3. How would printArrayList differ?

9/8/17 Compsci 201, Fall 2017, Java Basics 11

Charles Isbell.

http://www.pbs.org/newshour/bb/online-graduate-programs-offer-degrees-significant-savings/

For me, the differences are simple to state: Computationalists grok that models, languages and machines are equivalent.

• Context matters• Machine learning researcher• Systems that interact intelligently

with many other intelliggenceagents

• Exec. Assoc. Dean @ Georgia Tech • Rethinking education: Online

Masters in Computer Science

Sets• Set is an unordered collection of distinct objects

• Items are unique! Only one copy of each item in set!• We will use two different implementations of sets1. java.util.TreeSet

• A TreeSet is backed up by a tree structure (future topic)• Keeps items sorted (+)• Slower than HashSets ?? (-)

2. java.util.HashSet• A HashSet is backed up by a hashing scheme (future topic)• Items not sorted – should seem to be in random order (-)• Faster than TreeSets ?? (+)

CompSci 201

© Michael Frank

• The union of two sets A and B is the set containing elements that are either in A or in B.

• {a,b,c}È{2,3} = {a,b,c,2,3}• {2,3,5}È{3,5,7} = {2,3,5,3,5,7} ={2,3,5,7}

Union

Think “The United States of America includes every person who worked in any U.S. state last year.” (This is how the IRS sees it...)

© Michael Frank

• The intersection of two sets A and B is the set containing elements that are both A and B.

• {a,b,c}Ç{2,3} = ___• {2,4,6}Ç{3,4,5} = ______

Intersection

Think “The intersection of Main St. and 9th St. is just that part of the road surface that lies on both streets.”

© Michael Frank

Set Difference - Venn Diagram• The difference between two sets A and B is the

set containing elements that are in A but not B.• A−B is what’s left after B

“takes a bite out of A”

Set A Set B

© Michael Frank

Set Complements• The complement of a set A is the set of all

elements of the U, universal set (i.e. set containing all objects under consideration) that are not in A.

A = {x | x∉ A}

AU

WordCount Again!• WordCount3.java has two similar methods that count the

number of unique "words" in a file.1. Would you expect countWordsLoop or countWords

to be faster? Why?

2. What would happen if you used a TreeSet rather than a HashSet?

3. How would we write wordsInCommon and the necessary call in main to prints the number of Strings in common between two files.

CompSci 201

WOTO!http://bit.ly/201-f17-0908-2

Set Questions• Let• A is a Set of engineering undergraduate students at

Duke• B is a Set of students taking CompSci 201 at Duke.

• Express each of the following sets in terms of operations on sets A and B.

1. the set of engineers taking CompSci 2012. the set of engineers who are not taking CompSci 2013. the set of students who either are engineers or are taking

CompSci 2014. the set of students who either are not engineers or are

not taking CompSci 201

CompSci 201

ContinueQuiz

Converting from Collection to array

• Collections such as ArrayList and TreeSethave a toArray method• Syntax a bit awkward

• ExampleArrayList<String> words = new ArrayList<String>();

. . .String[] a =

(String[]) words.toArray(new String[0]);

orreturn (String[]) words.toArray(new String[0]);

SetOperations?• Implement set operations for two sets • Union, intersection, difference

• Implement set operations for array of sets• Union, intersection

• Refer to Java API• add, retain, remove• Variants with all

Conventions & Documentation• See Resources/Advice on Sakai• Variable name guidelines• Use nouns that describe what value is being

stored• Don’t reiterate the type involved

• Comments1. Abstraction: What does it do?• Comments for methods and classes

2. Implementation: How does it do it?• Inline comments as needed

9/8/17 Compsci 201, Fall 2017, Java Basics 22

Conventions & Documentation• Classes start with capital letter and then we have:• They’re public, except nested class? • camelCaseForMethods and ForClasses• Fields & instance variables: mySize, myMap, …• Constants (public static) are ALL_CAPS

• Standard identifiers• i, j, k: integer loop counters• n, len, length: integer number of elements in

collection• x, y: Cartesian coordinates (integer or real)• head, current, last: references used to iterate

over lists.

9/8/17 Compsci 201, Fall 2017, Java Basics 23

Summary• Write code to solve problem • Using arrays, files, etc.

• Introduce data structures and methods from Java API

• Reflect –• What’s clear? What’s still muddy?• http://bit.ly/201-f17-reflect

• Discussion warmup:• Bring a written solution to discussion• Complete and submit ClassScores as warmup

9/8/17 Compsci 201, Fall 2017, Java Basics 24