2015-03-31_hashmap.md

Objectives

To continue learning about HashMap:

• How to add and get values by key (review)
• How to use more advanced methods (size, remove, clear, isEmpty)

Homework

Introducing this week's homework...

Twelve Days redux

Let's look at how much simpler "The Twelve Days of Christmas" assignment can be with a HashMap.

Review

Definition: A map is a collection that associates keys with values.

Exercise: Create a class WordTracker; import HashMap; and create a new HashMap named words. Its keys should be of type String and its values of type Integer.

Exercise: Prompt the user for some words; continue reading in words until the user inputs "stop". When calling put(), just pass 1 as the second parameter. Here's example program:

Please give me some words:
latte
coffee
mocha
stop
Thanks!

HashMap advanced methods

size()

Just like with an ArrayList, you can get the size of a HashMap by calling the method size.

Exercise: Modify your program to tell the user how many words they input.

Please give me some words:
latte
coffee
stop
Thanks! You have given me 2 word(s)!

remove()

Call remove(key) to remove the key parameter from the HashMap. For example, to remove "latte" from the previous exercise, we would write:

words.remove("latte");

Exercise: Modify your program so that if the user types rm, every subsequent word the user types removes it from the HashMap. If the user types add, the program switches back to adding. Use a boolean addMode to track which mode the program is in. Initialize it outside of your loop.

Please give me some words:
latte
coffee
rm
coffee
stop
Thanks! You have given me 1 word(s)!

Here's another program run:

Please give me some words:
foo
bar
rm
bar
add
qux
stop
Thanks! You have input 2 words.

clear()

Call clear() to remove all the elements from the HashMap.

Exercise: Modify your program so that if the user types "clr", the tracked words are cleared.

Please give me some words:
foo
bar
clr
stop
Thanks! You have input 0 words.

isEmpty()

Call isEmpty() to return a boolean indicating whether or not the HashMap is empty. Notice it is just syntactic sugar for...

words.size() == 0;

Exercise: Modify your program to check if words is empty at the end of the program.

Please give me some words:
stop
Thanks! You have input 0 words.
Words is empty.

keySet()

There are many ways to iterate over a HashMap. We're going to learn just one:

for (String word : words.keySet()) {
}

We'll look at the Set data structure on Thursday; for now, just know that calling keySet() returns an iterable collection and that you can iterate over it like above.

Exercise: Modify your program to print the counts of each word. For example:

Please give me some words:
banana
orange
stop
Thanks! You have input 2 words.
Here they are:
orange: 1
banana: 1

Final exercise!

Exercise: Modify your program to keep track of multiple inputs of the same word. For example:

Please give me some words:
banana
banana
banana
orange
stop
Thanks! You have input 2 words.
Here they are:
orange: 1
banana: 3

You've written a complex program! This kind of input should work for you now:

Please give me some words:
banana
orange
rm
orange
add
water
sugar
juice
juice
juice
juice
stop
Thanks! You have input 4 words.
Here they are:
water: 1
banana: 1
juice: 4
sugar: 1

In-class assignment: RestaurantFacts based on NYC Open Data

We'll be using NYC Open Data again to build a HashMap of restaurant data. Use FileParser to get an ArrayList representing the lines in the file and follow this stencil file.

Here's a simple program execution:

Name a restaurant:
MORRIS PARK BAKE SHOP
A

Bonus

Modify your HashMap of Strings to hold an ArrayList of Strings. When the user inputs a restaurant name, print all the relevant facts that you have.