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    • Compilation and Type Inference : 23

    • Practice with Collections : 22

    • Maps and Sets : 21

    • Lists and Type Parameters : 20

    • Imports and Libraries : 19

    • Multidimensional Arrays : 18

    • Practice with Strings : 17

    • null : 16

    • Algorithms and Strings : 15

    • Strings : 14

    • Functions and Algorithms : 13

    • Practice with Functions : 12

    • More About Functions : 11

    • Errors and Debugging : 10

    • Functions : 9

    • Practice with Loops and Algorithms : 8

    • Algorithms I : 7

    • Loops : 6

    • Arrays : 5

    • Compound Conditionals : 4

    • Conditional Expressions and Statements : 3

    • Operations on Variables : 2

    • Variables and Types : 1

    • Hello, world! : 0

    Practice with Collections

    import java.util.List;
    import java.util.Map;
    import java.util.ArrayList;
    import java.util.HashMap;
    Map<String, List<String>> petNames = new HashMap<>();
    petNames.put("Geoffrey", new ArrayList<>());

    We’ll 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!

    Nested Collections
    Nested Collections

    In the past lessons we’ve seen how to create and use several standard Java collections: Lists, Maps, and Sets:

    import java.util.List;
    import java.util.ArrayList;
    List<String> friends = new ArrayList<>();
    import java.util.Map;
    import java.util.HashMap;
    Map<String, String> favoriteIceCreams = new HashMap<>();
    favoriteIceCreams.put("me", "Peanut Butter Fudge");
    favoriteIceCreams.put("Gracie", "Human food yummy");
    favoriteIceCreams.put("Xyz", "Yick");

    These collections are quite useful on their own! However, they can also be combined to great effect. Let’s see an example.

    // Task list example

    Lists of Maps of Sets
    Lists of Maps of Sets

    You can combine Lists, Maps, and Sets in many interesting ways to build data structures to solve problems. You can create Lists of Maps:

    import java.util.List;
    import java.util.Map;
    import java.util.ArrayList;
    import java.util.HashMap;
    List<Map<String, String>> values = new ArrayList<>();
    Map<String, String> entry = new HashMap<>();
    entry.put("test", "me");

    Or Sets of Lists:

    import java.util.Set;
    import java.util.List;
    import java.util.HashSet;
    import java.util.ArrayList;
    List<String> first = new ArrayList<>();
    List<String> second = new ArrayList<>();
    Set<List<String>> set = new HashSet<>();
    // Because first and second have the same items, adding second does not modify the set

    But generally, it’s more common for the top-level data structure to be a Map: Maps of Maps, Maps of Lists, and Maps of Sets. We’ll get some practice working with these on this lesson’s practice and homework problems.

    Practice Problem Warm-Up
    Practice Problem Warm-Up

    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 Map<Set<String>>. Let’s do an example of that together, which you can use as a starting point for the practice problem that follows.

    // Warm-up for Section Lists to Map
    Created By: Geoffrey Challen
    / Version: 2022.2.0

    Write a method called sectionListsToMap that, given a List of Strings, parses it into a Map<String, Set<String>> as follows. Each 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 described above.

    A few hints for approaching this problem. First, consider how to use .split and .trim appropriately to parse the input String. 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.

    The imports java.util.Map, java.util.Set, java.util.HashMap, and java.util.HashSet are already provided for you. You should not need additional import statements to complete this problem.

    Homework Warm-Up
    Homework Warm-Up

    Next let’s discuss how to approach today’s homework problem. This problem is a bit trickier, since we need to determine when to properly insert entries into our Map, and do some String parsing. So let’s discuss how to get started.

    // Warm-up for Script Parser

    Solve: Script Parser

    Created By: Geoffrey Challen
    / Version: 2022.2.0

    Write a method called parseScript that accepts a single String and returns a Map<String, List<String>>. The passed String contains a script consisting of lines separated by newlines, each with the following format:

    Name: Line

    For example, here's a simple script:

    Geoffrey: What do you think of this homework problem?
    Ahmed: it's a bit sus
    Geoffrey: I bet they'll be able to figure it out!
    Maaheen: We'll be here to help if they need it.

    parseScript parses the script and returns a map mapping each character's name to their lines in order. So, for the script above, the map would contain three keys: "Geoffrey", "Ahmed", and "Maaheen". The List<String> for the key "Geoffrey" would contain the Strings "What do you think of this homework problem!" and "I bet they'll be able to figure it out!" The List<String> for the key "Amhed" would contain the String "it's a bit sus".

    A few hints for approaching this problem.

    You'll want to use .split to parse the passed String into individual lines. You should assert that the passed String is not null, but if it's not, it will have the format described above, and also not contain any blank lines.

    You'll also need to use .split to split each line into the name and their line of dialog. You can assume that the character ":" only appears to delimit the name of the rest of the line.

    The first time you encounter a character, there will not be an entry in your map for them. So you should check for this, and create the ArrayList when appropriate.

    There may be extra whitespace around the name or the line of dialogue, so use .trim appropriately.

    The following imports are provided for you: java.util.List, java.util.ArrayList, java.util.Map, and java.util.HashMap. You should not need to use other imports to solve this problem.

    CS People: Dina Katabi
    CS People: Dina Katabi

    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:

    More Practice

    Need more practice? Head over to the practice page.