Let’s pause before moving on to get more practice with Java’s collections—the lists, maps, and sets that are so useful for solving problems. We’ll also learn how we can combine these collections together to build more interesting data structures. Let’s get started!
In the past lessons we’ve seen how to create and use several standard Java collections:
These collections are quite useful on their own! However, they can also be combined to great effect. Let’s see an example.
You can combine
Sets in many interesting ways to build data structures to solve problems.
You can create
But generally, it’s more common for the top-level data structure to be a
We’ll get some practice working with these on this lesson’s practice and homework problems.
We’ll spend the rest of the lesson working on some problems that test our understanding of how to nest collections.
First, we’re asked to parse a
List<String> into a
Let’s do an example of that together, which you can use as a starting point for the practice problem that follows.
Write a method called
sectionListsToMap that, given a
Strings, parses it into a
Map<String, Set<String>> as follows.
String in the passed list contains a comma-separated list of names of people in a discussion section.
The first name is the section leader, and the rest are students.
Your map should map each section leader to the set of students in their section.
No section leader or student will appear twice in the data set.
For example, given the
Strings "challen,student1", "ruisong4,student2, student3" and "friendly,student4, student5",
your map would have keys "challen", "ruisong4", and "friendly".
"challen" would map to a set containing "student1", "ruisong4" would map to a set containing "student2" and
"student3", and so on.
You should assert that the passed
String is not
null, but if it is not
null it will have the format
A few hints for approaching this problem.
First, consider how to use
.trim appropriately to parse the input
You should get this part to work before proceeding.
Then consider when you need to create the map and each set, and how to populate them.
java.util.HashSet are already provided
You should not need additional
import statements to complete this problem.
Today’s homework problem is a challenge! Your goal is to complete the implementation of the Hawaiian Word translator you began earlier this semester. Since this problem is more difficult than other homework problems, we’re giving you an extra day to complete it. And, at the end of the day, please remember that this is just one problem, and you have homework drops.
We’ll also get you started with a walkthrough to help you think about how to approach this problem.
Note that the problem asks you to throw an exception in certain cases, something that we have not yet covered. The walkthrough describes how to do that.
Words from languages that we are unfamiliar with can be difficult to pronounce correctly. Phonetic pronunciation guides can help make them more accessible to us. For this problem, you will write a program that produces phonetic pronunciations for Hawaiian words.
Write a method
getPronunciation that accepts a single
String containing a potential Hawaiian word and returns
String containing the pronunciation guide.
If the passed
null or invalid, throw an
There are 12 valid characters in the Hawaiian language: a, e, i, o, u, p, k, h, l, m, n, and w.
Each Hawaiian word passed into our program must be inspected to ensure it contains only these characters, because
if it does not, then we don’t have a valid Hawaiian word.
If the passed word is not valid, throw an
Note that you do not need to handle spaces or any other characters.
You should ignore case when examining the input
String, but your pronunciation guide output
String should be
The consonants in the Hawaiian language are pronounced similarly to the English versions. The only exception is 'w', which is pronounced as a "v" if it follows an 'i' or 'e' and pronounced as "w" otherwise.
The vowels in the Hawaiian language are a, e, i, o, and u:
Vowel groups are also present in the Hawaiian language. More complex words can have many vowels that when grouped together require additional rules. This means we can't simply replace all 'a's with "ah", and so on.
We will consider the following simplification of the Hawaiian vowel groups for this problem:
Here are a few examples used by the test suite:
Note that the testing process will start by testing "w", then move on to single vowels, then test vowel groups, and then proceed to more complicated inputs, some of which will be invalid. So you can design your solution incrementally following the progression of the test suites.
A few hints to help you get started.
One way to proceed is to examine each character in the input
String and build up the pronunciation guide from an
String as you go.
However, for this to work, some rules need access to the previous character (like 'w') and others to the next
character (vowel groups), so it may be helpful to record the current character as well as the previous and next
characters inside your loop.
Doing this safely requires some care, given that the previous and next character are not always valid depending on
where you are in the
Usually you'll want to consume one character at a time. But when you find a vowel group, you'll need to make sure you skip the next character. For example, given the input "ai", you'll need to make sure you output only "eye" and not "eye-ee" or "ah-ee".
Finally, take care to insert dashes in the correct places. A pronunciation should never end in a dash. You can avoid this by keeping track of where you are inside the loop, but it may be simpler to simply detect if your pronunciation ends with a dash and remove it before returning.
Very few people can make a legitimate claim to the label “genius”. Dina Katabi is one of them. A full professor at MIT, her groundbreaking work on wireless networking and other topics has also earned her a MacArthur Fellowship, the substantial financial award unofficially known as the “Genius Grant”.
In this video she discusses some of her work, including the ability to use wireless signals is a way that you may find quite surprising:
Need more practice? Head over to the practice page.