Managing Potluck Dinner Participants and Dishes in Java

Lesson 1

Introduction

Welcome to today's lesson, where we will tackle a common challenge in software engineering: the introduction of complex features while preserving backward compatibility. Our focus will be on a Potluck Dinner organization system, where we will manage participants and their respective dishes for each round. Get ready for an exciting journey through Java programming, step-by-step analysis, and strategic thinking. Let's dive into our adventure!

Starter Task Review

Initially, our Potluck Dinner organization system allows us to add and remove participants and manage their respective dishes for each round. There are three essential methods:

boolean addParticipant(String memberId): This method adds a participant. If a participant with the given memberId already exists, it won't create a new one but will return false. Otherwise, it will add the member and return true.
boolean removeParticipant(String memberId): This method removes a participant with the given memberId. If the participant exists, the system will remove them and return true. Otherwise, it will return false. When removing a participant, you need to remove their dish if they brought one.
boolean addDish(String memberId, String dishName): This method enables each participant to add their dishes for every round. If a participant has already added a dish for this round OR if the memberId isn't valid, the method will return false. Otherwise, it will add the dish for the respective participant's round and return true.

Let's write our Java code, which implements the functions as per our initial state:

Java
1import java.util.HashMap;
2import java.util.HashSet;
3import java.util.Map;
4import java.util.Set;
5
6public class Potluck {
7    private final Set<String> participants;
8    private final Map<String, String> dishes;
9
10    public Potluck() {
11        participants = new HashSet<>();
12        dishes = new HashMap<>();
13    }
14
15    public boolean addParticipant(String memberId) {
16        if (participants.contains(memberId)) {
17            return false;
18        } else {
19            participants.add(memberId);
20            return true;
21        }
22    }
23
24    public boolean removeParticipant(String memberId) {
25        if (!participants.contains(memberId)) {
26            return false;
27        } else {
28            participants.remove(memberId);
29            dishes.remove(memberId);
30            return true;
31        }
32    }
33
34    public boolean addDish(String memberId, String dishName) {
35        if (!participants.contains(memberId) || dishes.containsKey(memberId)) {
36            return false;
37        } else {
38            dishes.put(memberId, dishName);
39            return true;
40        }
41    }
42}

In this code, we employed a Java HashSet to store unique participant IDs and a HashMap to store the participant's ID and their respective dish name. With this foundation laid, let's introduce some advanced functionalities.

Introducing Advanced Functionalities

Our Potluck Dinner organization system is currently simple but practical. To make it even more exciting, we're going to introduce a "Dish of the Day" feature. This feature will enable participants to vote, and the dish receiving the most votes will be declared the "Dish of the Day."

To add this feature, we will define two new methods:

boolean vote(String memberId, String voteId): This method will allow a participant to cast a vote for a dish. Each participant can cast a vote only once per round. If a participant tries to vote again or if the memberId isn't valid, it should return false.
String dishOfTheDay(): This method will calculate and return the "Dish of the Day" based on the votes received. If multiple dishes tie for the highest number of votes, the dish brought by the participant who joined first acquires precedence. If there are no votes, the function returns null.

Building Advanced Features. Step 1: Constructor Change

Next, we will extend our existing Potluck class to accommodate these new features:

Java
1import java.util.HashMap;
2import java.util.LinkedList;
3import java.util.Map;
4import java.util.Queue;
5
6public class Potluck {
7    private Map<String, Long> participants;
8    private Map<String, String> dishes;
9    private Map<String, String> votes;
10
11    public Potluck() {
12        participants = new HashMap<>();
13        dishes = new HashMap<>();
14        votes = new HashMap<>();
15    }
16}

Firstly, we altered our participants data structure into a HashMap, where the key is the participant's ID and the value is the time of joining (we use the time of adding to the system as the join time).

We also added a votes HashMap to store the votes, where the key is the participant ID and the value is the participant ID for whom they have cast the vote.

Step 2: Implementing the 'vote' method

Java
1public boolean vote(String memberId, String voteId) {
2    if (participants.containsKey(memberId) && !votes.containsKey(memberId)) {
3        votes.put(memberId, voteId);
4        return true;
5    } else {
6        return false;
7    }
8}

In the vote function, we simply check whether the memberId exists and whether the vote hasn't been cast before. If both conditions are fulfilled, we add the vote to our votes and return true; otherwise, we return false.

Step 3: Implementing the 'dishOfTheDay' method

Java
1public String dishOfTheDay() {
2    if (votes.isEmpty()) {
3        return null;
4    }
5
6    // Count votes for each dish
7    Map<String, Integer> voteCount = new HashMap<>();
8    for (String vote : votes.values()) {
9        voteCount.put(vote, voteCount.getOrDefault(vote, 0) + 1);
10    }
11
12    // Determine maximum votes
13    int maxVotes = voteCount.values().stream().max(Integer::compare).orElse(0);
14
15    // Collect all dishes with maximum votes
16    Set<String> maxVoteDishes = new HashSet<>();
17    for (Map.Entry<String, Integer> entry : voteCount.entrySet()) {
18        if (entry.getValue() == maxVotes) {
19            maxVoteDishes.add(entry.getKey());
20        }
21    }
22
23    // Find the earliest joining time among the tied dishes
24    long earliestJoinTime = Long.MAX_VALUE;
25    String dishOfTheDayMember = null;
26    for (String memberId : maxVoteDishes) {
27        if (participants.get(memberId) < earliestJoinTime) {
28            earliestJoinTime = participants.get(memberId);
29            dishOfTheDayMember = memberId;
30        }
31    }
32
33    return "Participant: '" + dishOfTheDayMember + "', Dish: '" + dishes.get(dishOfTheDayMember) + "'";
34}

In the dishOfTheDay function, we start by ensuring we perform the calculation only if there are votes. Afterward, we calculate the count of votes for each dish, determine the dish or dishes that have received the maximum votes, and if there's a tie, we select the dish brought by the participant who joined the earliest. Finally, we return the selected "Dish of the Day."

Step 4: Updating Previous Methods for Compatibility and Integration

As we introduce the two advanced functionalities, vote and dishOfTheDay, our existing system needs to be adeptly updated to ensure seamless integration while maintaining backward compatibility. Here's how we've refined the previous methods:

The addParticipant method initially just added a participant to a set. With the introduction of the "Dish of the Day" feature, we now store the participant ID along with their join time in a HashMap. This timestamp is crucial for determining the precedence in case of a tie in votes.

Java
1public boolean addParticipant(String memberId) {
2    if (participants.containsKey(memberId)) {
3        return false;
4    } else {
5        participants.put(memberId, System.currentTimeMillis());
6        return true;
7    }
8}

This adjustment ensures that every new participant is tracked with a join time, allowing us to leverage this information for the "Dish of the Day" feature.

Step 5: Adjustments in 'removeParticipant' and 'addDish'

The original removeParticipant method did not consider the impact of removing a participant who might have already voted or added a dish. To maintain the integrity of our voting and dish tracking, we now clean up all related entries across dishes and votes HashMap's when a participant is removed.

Java
1public boolean removeParticipant(String memberId) {
2    if (!participants.containsKey(memberId)) {
3        return false;
4    } else {
5        participants.remove(memberId);
6        dishes.remove(memberId);
7        votes.remove(memberId);
8        return true;
9    }
10}

The addDish method's logic remained mostly unchanged since it primarily interacts with the dishes HashMap. However, ensuring that the method checks for the validity of the memberId against the updated participants' storage is essential.

Ensuring Backward Compatibility

Participants Data Structure Change: By transitioning from a HashSet to a HashMap for storing participant details, we've inherently kept the unique identification trait while adding the functionality for storing join times. This change is backward compatible as it requires no modification in how participant IDs are provided or handled externally.
Method Signature Consistency: All method signatures (addParticipant, removeParticipant, addDish) remain unchanged. This deliberate decision ensures that existing integrations with our system interface seamlessly without modifications from client code.

Final Implementation

Combining all our steps, this is the final implementation of our Potluck class with all the required methods:

Java
1import java.util.HashMap;
2import java.util.HashSet;
3import java.util.Map;
4import java.util.Set;
5
6public class Potluck {
7    private Map<String, Long> participants;
8    private Map<String, String> dishes;
9    private Map<String, String> votes;
10
11    public Potluck() {
12        participants = new HashMap<>();
13        dishes = new HashMap<>();
14        votes = new HashMap<>();
15    }
16
17    public boolean addParticipant(String memberId) {
18        if (participants.containsKey(memberId)) {
19            return false;
20        } else {
21            participants.put(memberId, System.currentTimeMillis());
22            return true;
23        }
24    }
25
26    public boolean removeParticipant(String memberId) {
27        if (!participants.containsKey(memberId)) {
28            return false;
29        } else {
30            participants.remove(memberId);
31            dishes.remove(memberId);
32            votes.remove(memberId);
33            return true;
34        }
35    }
36
37    public boolean addDish(String memberId, String dishName) {
38        if (!participants.containsKey(memberId) || dishes.containsKey(memberId)) {
39            return false;
40        } else {
41            dishes.put(memberId, dishName);
42            return true;
43        }
44    }
45
46    public boolean vote(String memberId, String voteId) {
47        if (participants.containsKey(memberId) && !votes.containsKey(memberId)) {
48            votes.put(memberId, voteId);
49            return true;
50        } else {
51            return false;
52        }
53    }
54
55    public String dishOfTheDay() {
56        if (votes.isEmpty()) {
57            return null;
58        }
59
60        // Count votes for each dish
61        Map<String, Integer> voteCount = new HashMap<>();
62        for (String vote : votes.values()) {
63            voteCount.put(vote, voteCount.getOrDefault(vote, 0) + 1);
64        }
65
66        // Determine maximum votes
67        int maxVotes = voteCount.values().stream().max(Integer::compare).orElse(0);
68
69        // Collect all dishes with maximum votes
70        Set<String> maxVoteDishes = new HashSet<>();
71        for (Map.Entry<String, Integer> entry : voteCount.entrySet()) {
72            if (entry.getValue() == maxVotes) {
73                maxVoteDishes.add(entry.getKey());
74            }
75        }
76
77        // Find the earliest joining time among the tied dishes
78        long earliestJoinTime = Long.MAX_VALUE;
79        String dishOfTheDayMember = null;
80        for (String memberId : maxVoteDishes) {
81            if (participants.get(memberId) < earliestJoinTime) {
82                earliestJoinTime = participants.get(memberId);
83                dishOfTheDayMember = memberId;
84            }
85        }
86
87        return "Participant: '" + dishOfTheDayMember + "', Dish: '" + dishes.get(dishOfTheDayMember) + "'";
88    }
89}

Lesson Summary

Bravo! You've successfully completed the task of introducing complex features while preserving backward compatibility. This skill is invaluable in real-life software engineering scenarios, where existing codebases can be thousands or even millions of lines long, and breaking changes can be catastrophic. Strengthen this ability with even more practice and explore similar challenges. I'll see you in the next lesson! Happy coding!

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