cs1302-hw06 Generic Method Implementation

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-- Proven Fact

This homework assignment explores functional interfaces and lambda expressions in conjunction with generic methods and interfaces. In this exercise, only the generic method signatures will be provided. Implementation details are left to the student.

Course-Specific Learning Outcomes

• LO2.d: (Partial) Implement new generic methods, interfaces, and classes in a software solution.
• LO2.e: (Partial) Utilize existing generic methods, interfaces, and classes in a software solution.
• LO4.a: (Partial) Design, create and use interfaces in a software solution.
• LO4.b: (Partial) Utilize interface-based polymorphism in a software solution.

References and Prerequisites

• 1302 Textbook Chapter: Generics
• 1302 Textbook Chapter: Lambda Expressions
• java.util.function.Predicate Interface Documentation
• java.util.function.Function Interface Documentation

Questions

In your notes, answer the following questions. These instructions assume that you are logged into the Odin server.

NOTE: For each step, please provide in your notes the full command that you typed to make the related action happen along with an explanation of why that command worked. Some commands require multiple options. It is important to not only recall what you typed but also why you typed each of them. If context is necessary (e.g., the command depends on your present working directory), then please note that context as well. You won't need to submit your notes in your final submission. However, if done properly, your exercise notes will serve as a helpful study guide for the exam.

Introduction

1. In this assignment, you will be referring to multiple generic interfaces, some of which have multiple generic type parameters. You must keep parameter composition in mind when referring to the API documentation, especially in cases where inheritance and interfaces are involved. For example, consider the code snippets below. We've omitted most of the details so that you can focus on just what is presented.

   public interface Set<E> {
       public boolean add(E item);
       public boolean contains(E o);
       ...
   } // Set

   public class TreeSet<T> implements Set<T> { // <---------- LINE1 
       // The method bodies are omitted but would be needed in a real implementation
       public T first() { ... }
       public boolean add(T item) { ... }
       ...
   } // TreeSet

   The way it's written, the T in implements Set<T> will always be the same as the T in TreeSet<T>. This means all of the following are true:

   • TreeSet<String> implements Set<String>
   • TreeSet<Scanner> implements Set<Scanner>
   • TreeSet<ConnectFour> implements Set<ConnectFour>

   Here is a snippet that utilizes what has been shown so far:

   public static void main(String[] args) {
       Set<String> set = new TreeSet<String>(); // <--------- LINE2
       set.add("hello"); // <-------------------------------- LINE3
       System.out.println(set.first()); // <----------------- LINE4
   } // main

   The table below shows the impact of type composition for the indicated lines in the snippets. Anytime you see "⇨" in the table, you should read it as "is replaced by" or "becomes":

   LINE#	Result of Composition	Replacements
   LINE1	Set<E> ⇨ Set<T>	E ⇨ T
   LINE2	Set<E> ⇨ Set<String>	E ⇨ String
   LINE2	TreeSet<T> ⇨ TreeSet<String>	T ⇨ String
   LINE3	add(E) ⇨ add(String)	E ⇨ T, T ⇨ String
   LINE4	T first() ⇨ String first()	T ⇨ String

Getting Started

1. Use Git to clone the repository for this exercise onto Odin into a subdirectory called cs1302-hw06:

   $ git clone --depth 1 https://github.com/cs1302uga/cs1302-hw06.git
   

2. Change into the cs1302-hw06 directory that was just created and look around. There should be multiple Java files contained within the directory structure. To see a listing of all of the files under the src subdirectory, use the find command as follows:

   $ find src
   

Exercise Steps

Checkpoint 1 Steps

1. If you have not read and understood the prerequisite readings, you should do that before attempting to implement the methods in this homework assignment.

2. LambdaFun.java contains method signatures and documentation for three generic methods. We will implement and test these methods in the order that they appear in the Java program. Before getting to work on method implementation, create and checkout a branch called method1 to perform the work related to this checkpoint. You can do this using the following command:

   $ git checkout -b method1
   

   EXPLANATION: When you create a branch, it is as if Git makes a copy of the current branch without the cp command! If you checkout the branch (like we just did), then stage and commit changes, then those commits do not affect the main branch. In this way, you can work on adding new features or fixing bugs until you are confident that they work. Towards the end of this checkpoint, you will checkout the main branch, and it will appear as it did when you started the homework assignment. Then, you will merge changes from the method1 branch into the currently checked out branch. You can do the same kind of thing in your projects:

   1. branch to work on a new feature;
   2. stage and commit as you test that feature; then
   3. once confident, checkout main and merge your branch commits into main. This way, your main branch is always in a good state.

3. Confirm that you are on the desired branch using git status and/or git branch. You should be able to tell by the output that you are currently on the method1 branch. How can you tell this is the case?

4. You will start by implementing the printlnMatches method in LambdaFun.java. The exact signature for this method is:

   private static <T> void printlnMatches(T[] array, Predicate<T> condition)

   Answer the following questions about this method in your notes:

   1. What is the generic type parameter?
   2. Specifically, what reference types can replace T?
   3. In order to call this method, we need a reference to an object of a class that implements Predicate<T>. Predicate<T> is a functional interface. Once you identify the single, abstract method in the Predicate API documentation, write the full method signature in your notes for reference. Pay careful attention to the return type and the type of the formal parameter.

5. Implement the printlnMatches method in LambdaFun.java. You do not need to use a lambda for this step. You will only need to use the object of type Predicate<T> referred to by condition to call the appropriate method. If you did the previous step, then you know what method(s) can be called with condition.

6. Take a few minutes to read the in-line comments above the last two statements in the main method of LambdaFun.java.

   1. The containsA variable uses a lambda expression to provide an implementation for the single, abstract method of the Predicate<String> interface that you wrote down in your notes in a previous step. Please note the parameterization (String) that replaces T. See the Oracle tutorial if you're still unsure about the syntax. Use your answer for 3.iii. to help you write this lambda.
   2. This lambda returns true if the string argument contains the letter "a" (case sensitive). You may wish to refer to the documentation for java.lang.String.

7. Variable containsA references an object of type Predicate<String>. HOLD THE FRONT DOOR! That one line of code created a class that implemented an interface and created an object of that class! It must be the case since containsA is a reference variable of an interface type and the code compiles. This object contains a specific implementation of the single abstract method in the interface.

8. Compile and run your code. You will know if everything is working correctly if the method only prints strings containing the letter "a". There should be 4 total lines of output at this point.

9. Stage and commit all changes.

10. Using the last two statements in the main method as an example, make three additional calls to printlnMatches that output the information below. After implementing each call, we recommend compiling and running before moving on to the next output. Make sure to include a print statement in between each method call so you can distinguish between the output.

    1. Call 1: Outputs all strings in the array.
    2. Call 2: Outputs all strings that are less than 6 characters.
    3. Call 3: Tricky: Outputs all strings that contain two or more a's.

11. Once you are confident that your method calls are correct, check your output against the expected output. If your output does not match the expected output, revise your method calls or your lambda expressions.

12. Compile and make sure your code passes the check1302 style audit.

13. Now that everything on this branch compiled, ensure that all changes in the current branch have been staged and committed. Do not execute the next command without making sure all changes are staged and committed After that, checkout the main branch using the following command:

    $ git checkout main
    

14. Take a look at LambdaFun.java. Notice that you have reverted back to the original code (before you made any changes!). The new, updated, code is on the other branch. To incorporate the changes (merge the changes) from the method1 branch into the current (main) branch, use the following command:

    $ git merge method1
    

    Did the merge work? How do you know?

15. View the condensed, graphical version of your Git log using git adog. You should see that the current (HEAD) branch is main and that main and method1 are in the same place! You've now merged the changes into the main branch. Now, we don't need the method1 branch anymore. You can safely delete it with:

    git branch -d method1
    

    Check the output of git adog to see that it is removed.

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Checkpoint 2 Steps

1. For the next method, create and checkout a branch called method2 using the same steps given in the previous checkpoint.

2. Take a close look at the printlnMappedMatches method and its associated Javadoc in LambdaFun.java. The exact signature for this method is:

   private static <T> void printlnMappedMatches(T[] array, Predicate<T> condition, Function<T, String> mapper)

   Answer the following questions about this method in your notes:

   • What is the generic type parameter?
   • Specifically, what reference types can replace T?
   • In order to call this method, we need a reference to an object of a class that implements Function<T, String> and a reference to an object of a class that implements Predicate<T>. Similar to Predicate<T>, Function<T, R> is a functional interface. Write the full method signature of the single, abstract method of Function<T, R> in your notes. Pay careful attention to the return type and the type of the formal parameter. Note: The method can (but doesn't have to) return a datatype that is different from the datatype of the parameter.

3. Implement the printlnMappedMatches method in LambdaFun.java. You do not need to use a lambda for this step. You will only need to use the Function<T, R> and Predicate<T> references to call the appropriate methods.

4. At the end of the main method of LambdaFun class:

   • Declare a variable of type Predicate<Email> and assign to it, using a lambda expression, a reference to an object that tests if the sender of the email does not have a "gatech.edu" email address (we'll "pretend" that they go to the spam filter). Remember, you can always refer to the API documentation for the associated interface when trying to determine the layout of your lambda.
   • Declare a variable of type Function<Email, String> and assign to it, using a lambda expression, a reference to an object that takes an Email object as a parameter and returns only the contents of the email (don't include sender, recipient, date, etc.) as some nicely formatted String. Note: contents is an instance variable of the Email class. Also, remember that you can refer to the API documentation for the associated interface when trying to determine the layout of your lambda.
   • Note: If the code you add causes your main method to exceed the check1302 line limit, you can move some of your code into test methods and have main call the test methods. Just remember to fully comment your test methods.

5. Call the printlnMappedMatches using your newly created variables to print out emails in the array referred to by inbox that do not come from GA Tech. In other words, we don't want to receive any emails from a "gatech.edu" address. :)

   Make sure to provide sufficient output so that an instructor/TA will be able to easily tell that everything is working properly. See the previous checkpoint for an example of appropriate output.

6. Stage and commit all changes.

7. Make sure your code passes the check1302 style audit then stage and commit all changes.

8. When you are convinced that everything is working properly, merge your method2 branch into main as you did in the previous checkpoint. Do not delete the method2 branch so we can see that you created a branch.

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Submission Steps

Each student needs to individually submit their own work.

1. Create a plain text file called SUBMISSION.md directly inside the cs1302-hw06 directory with the following information:

   1. Your name and UGA ID number

   Here is an example of the contents of SUBMISSION.md.

   Sally Smith (811-000-999)
   

2. Change directories to the parent of cs1302-hw06 (e.g., cd .. from cs1302-hw06). If you would like to make a backup tar file, the instructions are in the submissions steps for hw01. We won't repeat those steps here and you can view them as optional.

3. Use the submit command to submit this exercise to csci-1302:

   $ submit cs1302-hw06 csci-1302
   

   Read the output of the submit command very carefully. If there is an error while submitting, then it will displayed in that output. Additionally, if successful, the submit command creates a new receipt file in the directory you submitted. The receipt file begins with rec and contains a detailed list of all files that were successfully submitted. Look through the contents of the rec file and always remember to keep that file in case there is an issue with your submission.

   Note: You must be on Odin to submit.

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Copyright © Michael E. Cotterell, Bradley J. Barnes, and the University of Georgia. This work is licensed under a Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License to students and the public and licensed under a Creative Commons Attribution-NonCommercial 4.0 International License to instructors at institutions of higher education. The content and opinions expressed on this Web page do not necessarily reflect the views of nor are they endorsed by the University of Georgia or the University System of Georgia.