1. Compsci 201, Collections, Hashing, Objects
Owen Astrachan
February 1, 2019
2/1/19
Compsci 201, Spring 2019: Collections, Hashing

2. Compsci 201, Spring 2019: Collections, Hashing
G is for …
Git
Version control that's so au courant
Garbage Collection
Java recycles
Google
How to find Stack Overflow
2/1/19
Compsci 201, Spring 2019: Collections, Hashing

3. Compsci 201, Spring 2019: Collections, Hashing
PFFDoSMoY
Generic classes: ArrayList to HashSet
From ArrayList to HashSet to Collections to …
From Object.equals to Object.hashCode
Everything is an Object, what can an object do?
Using arrays and chars for APT problems
Toward understanding maps
2/1/19
Compsci 201, Spring 2019: Collections, Hashing

4. Compsci 201, Spring 2019: Collections, Hashing
WOTO Review
Did this very very quickly in class
2/1/19
Compsci 201, Spring 2019: Collections, Hashing

5. Compsci 201, Spring 2019: Collections, Hashing
Diyad ArrayList Review
ArrayList is a wrapper-class, array-like behavior but better, e.g., growth when needed
Internal state provides array characteristics
Methods allow for better functionality
2/1/19
Compsci 201, Spring 2019: Collections, Hashing

6. Diyad ArrayList Growth
When internal array full? Create new, copy, use
Efficient add, get, set when done repeatedly
Not possible if resize with +1, +100, +1000
Is possible if resize with *2 or *1.25
2/1/19
Compsci 201, Spring 2019: Collections, Hashing

7. Compsci 201, Spring 2019: Collections, Hashing
Generic ArrayList
Rather than String, use generic type parameter
Can use E, T, Type, any string in <E>
Similar to code for GrowableStringArrayList
java.util.List
Interface
2/1/19
Compsci 201, Spring 2019: Collections, Hashing

8. Can E be anything? String, Point, …
Method .equals that works as expected for E !
Internal array myStorage contains Objects
ConformingArrayList<String>
What .equals is called? Object or String?
Runtime decision, not compile time decision
What does elt actually reference?
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Compsci 201, Spring 2019: Collections, Hashing

9. Compsci 201, Spring 2019: Collections, Hashing
Why Diyad?
Traditionally use ArrayList<E> -- client code
Understand methods via API
Problem solving in many contexts
Efficiency: a.get(1)as fast as a.get(1000)
Why efficient? Understanding by analysis
From the internal array which is efficient
From doubling on resize rather than adding one
2/1/19
Compsci 201, Spring 2019: Collections, Hashing

10. Toward Understanding HashSet
Adding objects to HashSet<..>, avoid duplicates
We’ll see with Point class, doesn’t work
We’ll see with String class, does work
Just as we needed to add .equals() …
Need some knowledge of Object and internals of HashSet<..>, how does set.add(X) work?
Every object can convert itself to a number
Ask not what you can do to an object …
2/1/19
Compsci 201, Spring 2019: Collections, Hashing

11. Compsci 201, Spring 2019: Collections, Hashing
Modified Point Class
Why isn't output as expected?
Initially why didn't ArrayList work with Point?
2/1/19
Compsci 201, Spring 2019: Collections, Hashing

12. Hollywood Principle: .equals
Why program designers write Point.equals()?
To interact well with Java classes
We don't call .equals, other classes do
We called ArrayList.contains, it called .equals
Object has .equals, and .toString
Overload these for Point class
2/1/19
Compsci 201, Spring 2019: Collections, Hashing

13. Making .contains efficient
Why is ArrayList.contains(..) slow?
Search through entire list to find something
If list is sorted can we do better?
Think of a number between 1 and 1,024, I'll tell you high, low, correct: how many guesses needed?
How do you search for a book in the stacks?
That's not what you do in the stacks?
What about in ancient times …
2/1/19
Compsci 201, Spring 2019: Collections, Hashing

14. Finding an Object's number ..
Every object has .hashCode() method
Returns int value, used as “locker number”
Could return 39, 2, 57, … even -321
Cannot guarantee different for every Object!
Use .equals in locker
Search items in same locker
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Compsci 201, Spring 2019: Collections, Hashing

15. Compsci 201, Spring 2019: Collections, Hashing
Ideal world? Real world!
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Compsci 201, Spring 2019: Collections, Hashing

16. Compsci 201, Spring 2019: Collections, Hashing
Point.hashCode
Convert a Point to a number
Try to make every point a different number
That's not possible!!
For method below, what non-equal points have same .hashCode()?
2/1/19
Compsci 201, Spring 2019: Collections, Hashing

17. Inefficient but Correct .hashCode
Suppose .hashCode()simply returns 5
Every Point goes in the same locker
There are always collisions, but we try to minimize them. How are collisions resolved?
Can we modify PointDriver.java to stress-test?
How many different points can be made?
2/1/19
Compsci 201, Spring 2019: Collections, Hashing

18. Compsci 201, Spring 2019: Collections, Hashing
The hashCode contract
Every object has .hashCode() method
Inherited from Object, but typically overridden
and read online
Must respect .equals(): If a.equals(b) ?
a.hashCode() == b.hashCode()
Converse not true! There will be collisions
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Compsci 201, Spring 2019: Collections, Hashing

19. WOTO (correctness counts)
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Compsci 201, Spring 2019: Collections, Hashing

20. Compsci 201, Spring 2019: Collections, Hashing
Maria Klawe
President of Harvey Mudd
Dean of Engineering at Princeton, ACM Fellow, College Dropout (and re-enroller)
I personally believe that the most important thing we have to do today is use technology to address societal problems, especially in developing regions
Coding is today's language of creativity. All our children deserve a chance to become creators instead consumers of computer science.
2/1/19
Compsci 201, Spring 2019: Collections, Hashing

21. Default: Object.equals, .hashCode
For Objects p and q:
p.equals(q) is the same as p == q
Do p and q reference/point to same object
For Object p
p.hashCode() is location in memory of object
Thus: if p == q then
p.hashCode() == q.hashCode()
2/1/19
Compsci 201, Spring 2019: Collections, Hashing

22. Compsci 201, Spring 2019: Collections, Hashing
ArrayList and HashSet
Both have .add, .addAll, and more
Both iterable:for(Elt e : collection)
Both have .contains leveraging .equals
HashSet also uses .hashCode to reduce the collection iterated over: locker collisions
Object hygiene when developing your classes
.toString(), .equals(), .hashCode()
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Compsci 201, Spring 2019: Collections, Hashing

23. Compsci 201, Spring 2019: Collections, Hashing
When Strings Collide
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Compsci 201, Spring 2019: Collections, Hashing

24. Compsci 201, Spring 2019: Collections, Hashing
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
2/1/19
Compsci 201, Spring 2019: Collections, Hashing

25. Compsci 201, Spring 2019: Collections, Hashing
Concept: Inheritance
In Java, every class extends Object
Gets methods by default: .toString, .hashCode, .equals, and more
Inherit method + implementation
Subclass can override base class methods
Make .equals work for Point class
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Compsci 201, Spring 2019: Collections, Hashing

26. Compsci 201, Spring 2019: Collections, Hashing
Work in 201
How important are APTs?
How important are APT quizzes?
How important are assignments?
Earlier assignments, later assignments?
How important: reading and WOTO in-class
How important are reading quizzes?
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Compsci 201, Spring 2019: Collections, Hashing

27. Compsci 201, Spring 2019: Collections, Hashing
Alphabetical Order
Encryption? Maybe not
Think about high-level algorithm
Apply your algorithm to: "pop", "array", "deeds"
What do we need to do to code algorithm?
Recall: 'b' + 1 == 'c'
Recall: array['h'] is allowed, 'h' can be index
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Compsci 201, Spring 2019: Collections, Hashing

28. Compsci 201, Spring 2019: Collections, Hashing
Ransomware
Anonymous headline?
High level ideas …
Can we create "help me"?
Create counts['e'] -- # of e's
To use, what's needed
2/1/19
Compsci 201, Spring 2019: Collections, Hashing

29. How often does a string occur?
Strings stored in ArrayList?
Call Collections.frequency(list)
If in array a rather than ArrayList?
Collections.frequency(Arrays.asList(a))
Is this efficient? Does it matter?
Can create parallel arrays or use HashMap
Keep count[k] # occurrences of word[k]
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Compsci 201, Spring 2019: Collections, Hashing

30. WOTO (correctness counts)
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Compsci 201, Spring 2019: Collections, Hashing

31. 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
Shafi is a serious theoretical computer scientist. Not much to do here but read the bullets and the quote
9/20/17
Compsci 201, Fall 2017, Compare+Analysis