FAQ for Homework 7: PennPals

Setup

Can I import other parts of the Java standard libraries?

Sure. Do note that our reference solution only imports things from java.util; if you think you need to import things outside of that, you should ask us beforehand to make sure you are on the right track.

Understanding the Problem

I’m confused about a certain command, where should I look?

There’s a table with a general description of what each command does under “Protocol” in the instructions. Start there. Afterwards, search for the command name on the instructions. For each command, we provide more information and a table describing error conditions.

I’m confused about a certain concept, where should I look?

We define every relevant concept on the instructions. Most of what you’ll need to understand the assignment can be found under “Task 1: understanding the problem” and “Protocol”.

I’m confused about a certain Error Condition, where should I look?

We specify every error condition in the javadocs for each method in ServerModel. For further reference, the javadocs for ErrorCode are here.

Model Design

What data structures should I use?

Consider the invariants that you need to maintain, and then choose the simplest possible data-structure that maintains them. Get comfortable with the Javadocs for each structure (see Lists, Sets, Maps, TreeMaps, TreeSets, and LinkedLists). Consider if the data is ordered vs. unordered, contains repeats or unique values, is a simple collection or a mapping from one type of data to another. According to the answer to those questions, choose a List, Set, or Map. You may also consider creating your own classes to group information rather than use a data structure with only primitive types.

What should I use for the static / dynamic type of my data structures?

The dynamic types you can use in this assignment are TreeMap, TreeSet, and LinkedList.

The static type should be the most generalized collection that still works for the code. For example:

Set<ClassName> s = new TreeSet<>();

Do I need to handle invalid inputs?

Yes! Throw the appropriate error based on where you are in the program.

What should the getters in ServerModel – that normally return a collection – return if there are no elements to be returned?

An empty collection.

One of my functions is very large, is that okay?

It is best to split large functions into helper functions. Not splitting up your code in this way will result in loss of style points. Remember to also comment the purpose of your helper functions. Try your best to avoid redundancy.

When you add elements to a list, do you add a clone of the object or a pointer to the original object?

Unless you’ve explicitly written code that makes a copy, a reference to the object will be passed to the list, so it will point to the same object on the heap.

Using the Collections library

What are some useful methods of the Map interface?

Besides the basic operations of containsKey, get, and put, you might want to also look at keySet and values. However, make sure that you use the map appropriately. For example, if you determine whether a key is present in the map by iterating over the keySet, instead of using containsKey, you will lose points in your manual grade.

How do I iterate over a Set or List?

You can use the iterator or the “foreach” syntax to access each element in the collection. See Lecture 29 or Chapter 25.4.

How do I iterate over all of the entries in a Map?

If you have a Map<A,B> m, you can use the iterator with the entrySet method. For example,

for (Map.Entry<A, B> entry: m.entrySet()){
    A k = entry.getKey();
    B v = entry.getValue();
    ... // do something with k and v
}

How do I implement equals and/or compareTo for my custom class?

Try to keep in mind what field (or fields) would make two instances of your class identical. Things like IDs are generally good to include since they should be different for different instances.

IntelliJ has a helper for writing equals if you go to Code > Generate… > equals() and hashCode(). (We don’t teach hashing in this class, so feel free to uncheck hashCode() if you do this). The helper will prompt you for the fields that will determine equality between two instances of the class.

For compareTo, IntelliJ should generate the method header if your class implements Comparable. From there, you should figure out which fields make one instance less than, greater than, or equal to another instance (similar to equals, but slightly more involved).

Running the Client

How do I run the client.jar? When I try to open the client.jar file I get a warning, what should I do?

If running locally, try right clicking on the file and selecting “Open” instead of double clicking on it. If you are using Codio, check the README. Make sure you start the server before running the clients.

How do I open two clients at the same time?

On Codio, click “Run Chat Client” multiple times to get multiple client instances. If you’re doing local development, read the Running the Client Locally section of the writeup.

Errors & Exceptions

Why am I getting a ConcurrentModificationException?

This happens when you try to change a data structure while still traversing it in a for-each loop. Look for cases where you are iterating through a collection and adding/removing from the same collection. We suggest you store what you need to remove in another collection and then iterate over that collection.

Why am I getting a compilation error when submitting?

If you get a compilation error in a file that you don’t recognize, such as SubmissionInviteOnlyTest.java, make sure that your zip file includes all of the correct files.

I’m getting a duplicate class error when I submit?

Make sure you are submitting your files to Gradescope without any extra files. You should only be uploading these files:

• src/main/java/org/cis120/ServerModel.java
• src/test/java/org/cis120/ServerModelTest.java
• Any additional classes you made

How do I interpret the Response Test Output?

If an assertEqual comparing an expected and an actual response object fails, it may be difficult to understand what happened.

Here’s an example of a test output:

response expected:<{User0=[:User1 JOIN java], User1=[:User1 JOIN java, :User1 NAMES java :@User0 User1]}> but was:<{User0=[:User1 JOIN java], User1=[:User1 NAMES java :@User0]}>

The test output displays who the Response is sent to and what messages they receive. The assert statement that produced the output shown above expected a Response would be sent to the users with nicknames “User0” and “User1”. The expected Response would tell User0 and User1 different things. User0 would be told that User1 joined the channel “java”: User0=[:User1 JOIN java]. User1 would be told two things. First, that User1 joined the channel “java”, and second, that the names of the users in the channel “java” are “User0” and “User1”: User1=[:User1 JOIN java, :User1 NAMES java :@User0 User1]. Note that the NAMES part of the message puts an @ before “User0”. The @ designates User0 as the owner of the channel “java”.

The actual Response was: {User0=[:User1 JOIN java], User1=[:User1 NAMES java :@User0]. How is this different from what the test expected? In this case, the NAMES part of the Response meant for User1 did not include User1. Now you know that the JoinCommand is producing a Response that doesn’t include User1 in its set of recipients, and you can use that information to start debugging your code!