CS 46B - Lecture 14

Cover page image

Pre-class reading

Sections 15.3-15.4

I don't want to strike, but...

... I will if we don't get a contract
Inflation from 2004 - 2015: 26%
Change in the average CSU presidential salary since 2004: 36%
Change in the average faculty salary since 2004: 10%

Sets

Unordered collection
Optimized for fast access
Two implementations: hash set and tree set
Hash set: Groups elements with same “hash code”
Tree set: Arranges elements by sort order

Constructing Sets

Choose between hash set and tree set
Use tree set if you want to visit in sorted order
Elements need to be comparable, or supply comparator
Otherwise use hash set

Store reference in variable of interface type Set

Set<String> names = new HashSet<>();
Set<String> names = new TreeSet<>();
Set<String> names = new TreeSet<>(myComparator);

Working with Sets

`Set<String> names;`	Use the interface type for variable declarations
`names = new HashSet<>();`	Use a `TreeSet` if you need to visit the elements in sorted order.
`names.add("Romeo");`	Now `names.size()` is 1
`names.add("Fred");`	Now `names.size()` is 2
`names.add("Romeo");`	`names.size()` is still 2. You can’t add duplicates.
`if (names.contains("Fred"))`	The `contains` method checks whether a value is contained in the set. In this case, the method returns `true`.
`System.out.println(names);`	Prints the set in the format `[Fred, Romeo]`. The elements need not be shown in the order in which they were inserted.
`for (String name : names) {...}`	Use this loop to visit all elements of a set. (You can also use an iterator.)
`names.remove("Romeo");`	Now `names.size()` is 1
`names.remove("Juliet");`	It is not an error to remove an element that is not present. The method call has no effect.
`names.removeIf(s -> s.startsWith("R"));`	Removes all elements that fulfill the predicate.

Lecture 14 Clicker Question 1

How many unique words are there in "Alice in Wonderland", if you ignore letter case?

Complete this program.

1994
5472
5909
Something else

Lecture 14 Clicker Question 2

Try to implement this method with the minimum amount of work.

What did you do?

I used one or more “for each” loops
I used one or more iterators
I used methods from the Set interface, without any loops
I couldn't get the test to pass

Lecture 14 Clicker Question 3

Try to implement this method with the minimum amount of work.

What did you do?

I used one or more “for each” loops
I used one or more iterators
I used methods from the Set interface, without any loops
I couldn't get the test to pass

Maps

Associates keys and values
Example: Name -> Color object is a Map<String, Color>
HashMap and TreeMap implementations
TreeMap visits the keys in sorted order

Working with maps

`Map<String, Integer> scores;`	Keys are strings, values are Integer wrappers. Use the interface type for variable declarations.
`scores = new TreeMap<>();`	Use a `HashMap` if you don’t need to visit the keys in sorted order.
`scores.put("Harry", 90); scores.put("Sally", 95);`	Adds keys and values to the map.
`scores.put("Sally", 100);`	Modifies the value of an existing key.
`int n = scores.get("Sally"); Integer n2 = scores.get("Diana");`	Gets the value associated with a key, or null if the key is not present. `n` is 100, `n2` is null.
`System.out.println(scores);`	Prints scores.toString(), a string of the form `{Harry=90, Sally=100}`
`for (String key : scores.keySet()) { Integer value = scores.get(key); . . . }`	Iterates through all map keys and values.
`scores.remove("Sally");`	Removes the key and value.

Lecture 14 Clicker Question 4

We want to count how many words have length 1, length 2, and so on.

Complete this program.

How many words have length 10?

386
970
I got a different number
I got a NullPointerException

Hash Functions

Object class has a hashCode method
Computes an integer for each object
Override in your own class to be compatible with equals
Hash code should depend on object contents
Hash code should “spread around” objects into different numbers
It's ok to have a few “collisions”—objects with the same hash code

Example: Hash code for String

final int HASH_MULTIPLIER = 31;
int h = 0;
for (int i = 0; i < s.length(); i++)
   h = HASH_MULTIPLIER * h + s.charAt(i);

eat	100184 = 31 * (31 * 'e' + 'a') + 't'
tea	114704
Juliet	–2065036585
Ugh	84982
VII	84982

Implementing Hash Functions

Form hash code of each instance variable that is compared in equals
Combine these hash codes

public class Country
{
   public int hashCode()
   {
      int h1 = name.hashCode();
      int h2 = new Double(area).hashCode();
      final int HASH_MULTIPLIER = 31;
      int h = HASH_MULTIPLIER * h1 + h2;
      return h;
   }
}

Lecture 14 Clicker Question 5

What is the hash code of the string "d¢"?

(Hint: Appendix 1 of your text book)

3266
970
I got a different number
\u0064\u00A2