CS 445 Recitation 1: Java from CLI, Git

Introduction

In this recitation exercise, you will practice compiling and executing Java programs from the command line interface (CLI), also known as the terminal or command prompt. It is important that you test your assignment submissions from the CLI, as this is the standard environment in which your code will be evaluated. You will also see examples of the differences between objects and the references that point to them. This is an important distinction in data structures.

In some assignments, you will be required to organize your code in packages. This is a standard way of organizing large collections of code that comprise different components. Packages also allow you more fine-grained control over access of member variables and methods in your classes, as we discussed in lecture. This technique of modularization is very important for large-scale programming projects, and is ubiquitous in both industry and open source development.

Java CLI

1. Download the provided code by cloning this Recitation 1 repository. You will find the following Java files:

   • Rec1A.java demonstrates basic CLI input and output, plus objects and references.
   • Rec1B.java demonstrates command line arguments.
   • Rec1C.java demonstrates a class in a package.

2. Read through Rec1A.java and consider what will happen when it is run. Predict what you'll see output from each of the print statements.

3. Use cd to navigate to your local directory that contains these files. To ensure you are able to compile and run Java programs from the CLI, compile and run Rec1A. Check your predictions regarding each print statement.

4. Change line 69 of Rec1A.java to make custom point to a separate object from matilda. Compile and run the code again and observe the changes.

5. Check which files have changed using git status. Commit your changes to your local clone of the git repository using git add Rec1A.java and git commit. Note that, even after doing so, your changes are not reflected in the online version on GitHub. Your TA will demonstrate what happens when changes are pushed (they will use their own fork of the repository where they have write access).

6. Read through Rec1B.java and consider what will happen when it is run.

7. To ensure you understand command line arguments, compile and run Rec1B. Try running it with different numbers of command line arguments. Try wrapping multiple words in quotes, and note the difference in the output of the program.

8. Read through Rec1C.java and consider the first line; note that this class is labeled as belonging to package cs445.rec1.

9. Try compiling and running Rec1C in the same way you did Rec1A and Rec1B. Note that it doesn't work! This is because Java expects to find the code inside a package folder.

10. Create nested package folders cs445/rec1/ using mkdir. Then, move Rec1C.java into cs445/rec1/ using git mv.

11. Remaining outside of the package folders (that is, without using cd to navigate into cs445/rec1/), compile and run Rec1C.java.

12. Commit your change of location for Rec1C.java using git commit.

13. Use git log to observe the changes that have been made since this repository was created. If you want to undo a commit, try git revert followed by the first few characters of the commit id. If you want to see details of what changed in a commit, use git show (again followed by the beginning of the id).

Conclusion

More importantly than just following these steps, the goal of these exercises is to prepare you for your programming assignments. If you have trouble completing these steps, or you do not understand them, ask your TA or come to office hours! Understanding these concepts will be crucial to success in your assignments!