1. Compsci 201, Compare+Analysis
Owen Astrachan
Jeff Forbes
September 20, 2017
9/20/17
Compsci 201, Fall 2017, Compare+Analysis

2. Compsci 201, Fall 2017, Compare+Analysis
G is for …
Garbage Collection
Nice to call new and not call delete!
Git
Version control that's so au courant
GPL
First open source license
Google
How to find Stack Overflow
9/20/17
Compsci 201, Fall 2017, Compare+Analysis

3. Review Pre-class Thinking
Think of this as reading, but work to answer questions rather than simply reading code
Post questions to Piazza as needed
9/20/17
Compsci 201, Fall 2017, Compare+Analysis

4. String and Object .hashCode
Every object has a .hashCode method
Default version? Why does it work for Objects?
x.equals(y) x.hashCode() == y.hashCode()
Why do most classes override both .equals and .hashCode?
Correctness and Performance
9/20/17
Compsci 201, Fall 2017, Compare+Analysis

5. Compsci 201, Fall 2017, Compare+Analysis
What are instance variables? Initialized?
Index used so hash of “cat” != “act”
public int hashCode() {
int h = hash;
if (h == 0 && value.length > 0) {
char val[] = value;
for (int i = 0; i < value.length; i++) {
h = 31 * h + val[i]; }
hash = h;
return h;
9/20/17
Compsci 201, Fall 2017, Compare+Analysis

6. Compsci 201, Fall 2017, Compare+Analysis
How do we tell if two strings are equal?
What about a String and an Object?
Examine characters at index k in s1 and s2
If not equals, done, return false
How many chars to examine?
How do we make code easier to read than what’s given in String.java?
9/20/17
Compsci 201, Fall 2017, Compare+Analysis

7. Compsci 201, Fall 2017, Compare+Analysis
Is this code the same?
See while loop in
public boolean equals(Object o) {
if (this == o) return true;
if (! (o instanceof String)) return false;
if (value.length != o.value.length) return false;
for(int k=0; k < value.length; k++){
if (value[k] != o.value[k]) return false; }
return true;
9/20/17
Compsci 201, Fall 2017, Compare+Analysis

8. Compsci 201, Fall 2017, Compare+Analysis
When Strings Collide
9/20/17
Compsci 201, Fall 2017, Compare+Analysis

9. Compsci 201, Fall 2017, Compare+Analysis
When Strings Collide
Generate strings that will collide
Find such strings in the wild
String
hashCode
ayay
ayBZ
bZay
bZbZ
String
hashCode
buzzards
righto
snitz
unprecludible
9/20/17
Compsci 201, Fall 2017, Compare+Analysis

10. Compsci 201, Fall 2017, Compare+Analysis
WOTO
9/20/17
Compsci 201, Fall 2017, Compare+Analysis

11. Compsci 201, Fall 2017, Compare+Analysis
Finishing Last Week
DIY SimpleHashSet using an internal hash table
ArrayList of buckets/lockers, indexable
Find the “right” bucket using .hashCode
Internally the bucket stores ArrayList
How did we analyze what happens?
Create a StringWrapper class that we can instrument
Why can’t we instrument String.java?
9/20/17
Compsci 201, Fall 2017, Compare+Analysis

12. Interplay of .hashCode/.equals
Use StringWrapper class to help understand
Static variables that count calls of methods
When is .hashCode called?
When is .equals called?
What is interplay between these?
Change size of hash table and see what happens!
Which calls change, why do they change?
9/20/17
Compsci 201, Fall 2017, Compare+Analysis

13. Thinking about Hashing
What does StringWrapper help us understand?
9/20/17
Compsci 201, Fall 2017, Compare+Analysis

14. Compsci 201, Fall 2017, Compare+Analysis
Comparing and Sorting
Arrays.sort, Collections.sort
{“ant”, “bat”, “cat”, “dog”}
What algorithm is used in sorting?
How to change to sort-in-reverse or other order
9/20/17
Compsci 201, Fall 2017, Compare+Analysis

15. Strings are Comparable
Compare strings lexicographically, natural ordering, dictionary order
“zebra” > “aardvark” but “Zebra” < “aardvark”
Conceptual, cannot use < or > or ==
“yak”.compareTo(s) returns < 0, == 0, > 0
s is “zebra”, “yak”, and “toad”, respectively
The int convention also used in C++, C, others
9/20/17
Compsci 201, Fall 2017, Compare+Analysis

16. Not Everything is Comparable
9/20/17
Compsci 201, Fall 2017, Compare+Analysis

17. Compsci 201, Fall 2017, Compare+Analysis
(x,y) < (z,w)
Can we compare Point objects?
To be “comparable”, implement the Comparable interface, supply .compareTo(..) method
9/20/17
Compsci 201, Fall 2017, Compare+Analysis

18. Compsci 201, Fall 2017, Compare+Analysis
Build on What You Know
How does .equals work?
Make sure you have the correct type
Cast, compare
public boolean equals(Object o) {
if (o == null || ! (o instanceof Point)) {
return false; }
Point p = (Point) o;
return p.x == x && p.y == y;
9/20/17
Compsci 201, Fall 2017, Compare+Analysis

19. Compsci 201, Fall 2017, Compare+Analysis
Extend what you know
This is method in Point class
Point implements Comparable<Point>
public int compareTo(Point p) {
if (this.x < p.x) return -1;
if (this.x > p.x) return 1;
if (this.y < p.y) return -1;
if (this.y > p.y) return 1
return 0; }
9/20/17
Compsci 201, Fall 2017, Compare+Analysis

20. What is a Java Interface?
An enforceable abstraction: methods required
Set and Map interfaces
Comparable interface
If Set<String> is parameter then can pass …
HashSet<String> or TreeSet<String>
Can sort String or anything that’s Comparable
Call .compareTo(..) method
9/20/17
Compsci 201, Fall 2017, Compare+Analysis

21. Compsci 201, Fall 2017, Compare+Analysis
Shafi Goldwasser
2012 Turing Award Winner
RCS professor of computer science at MIT
Twice Godel Prize winner
Grace Murray Hopper Award
National Academy
Co-inventor of zero-knowledge proof protocols
Work on what you like, what feels right, I now of no other way to end up doing creative work
9/20/17
Compsci 201, Fall 2017, Compare+Analysis

22. Compsci 201, Fall 2017, Compare+Analysis
Why use an interface?
9/20/17
Compsci 201, Fall 2017, Compare+Analysis

23. Compsci 201, Fall 2017, Compare+Analysis
Why use an Interface?
Work with frameworks, e.g., java.util.Collection
Iterable, Serializable, and more – use with Java
ArrayList, LinkedList, TreeSet, HashSet all …
.clear(), .contains(o), .addAll(..), .size(), … .toArray()
9/20/17
Compsci 201, Fall 2017, Compare+Analysis

24. Compsci 201, Fall 2017, Compare+Analysis
Developer Time
Making changes in timeSet and timeUtilSet
Change the call, don’t change method: util
TreeSet, HashSet implement same interface
Change the call, change method signature
Same methods: ArraySet, SimpleHashSet, but no common interface explicit
9/20/17
Compsci 201, Fall 2017, Compare+Analysis

25. Redesign for Efficiency
Consider APT anonymous
Green solution
for each message:
If count(message,’a’) <= count(headline,’a’) OK
Repeat for ‘b’, ‘c’, …, ‘z’
Rescan the headline text for ‘a’, … ‘z’ AND for every message
Is this a concern?
9/20/17
Compsci 201, Fall 2017, Compare+Analysis

26. Compsci 201, Fall 2017, Compare+Analysis
Quantifying Concerns
Counting # occurrences of ‘a’ in headlines requires looking at all Hsize characters
We do this 26 times, that’s 26* Hsize (why)
We do this for M messages, that’s 26*Hsize*M
Also 26*Msize to count chars in each message
If we scan headline once to create map: Hsize
Process M messages is then 26*M + 26Msize
Is 26M + Hsize better than 26*Hsize*M ?
9/20/17
Compsci 201, Fall 2017, Compare+Analysis

27. Using Numbers to Understand
Is 26M + Hsize better than 26*Hsize*M ?
Suppose we have 100 messages with 1,000 characters in headlines
____ 2,600,000
Change with 10,000 characters in headlines
,000 ___ 26,000,000
9/20/17
Compsci 201, Fall 2017, Compare+Analysis

28. Compsci 201, Fall 2017, Compare+Analysis
Making Maps
In discussion saw int[] counters[256]
Can use Map<Character,Integer> to
for(char ch : str.toCharArray()){
counters[ch] += 1; }
for(char ch : str.toCharArray()){
if (! map.containsKey(ch)) map.put(ch,0);
map.put(ch, map.get(ch) + 1); }
9/20/17
Compsci 201, Fall 2017, Compare+Analysis