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The purpose of this homework is to help you set up your development environment, get reacquainted with Java, and start getting familiar with tools you will use for the rest of the course. Although the homework description is long, we expect the step-by-step instructions will make doing the homework less overwhelming than reading it may be.
This homework links to (the middle of) some other handouts at many points. For convenience, here is the list of these handouts:
(It also links to various Java files and API documentation files that are not listed above.)
You are free to get help from anyone on any portion of this homework. That is, the standard collaboration policy does not apply to this homework. You will need to know this material for the remainder of the quarter, however, so make sure you understand thoroughly the tools and concepts in this homework.
Read the Project Software Setup document and decide where you will do your work.
Also choose what Java development tools you will use. We recommend using the IntelliJ IDE in CSE 331, but the choice is up to you. We also provide instructions for working from the Linux command line. If you choose a different IDE, you will need to figure out how to do the operations from it, or use the command line. IntelliJ is the most popular Java IDE. If IntelliJ is new to you, allocate some time to learn its features. That time will pay off — probably this quarter, and definitely in the future.
Once you have chosen and prepared a development environment, you should set it up for CSE 331 use. See Configuring IntelliJ for 331 and follow any instructions that are applicable. (Note: if you switch development environments later in the quarter, revisit these instructions.)
Your homework for this quarter will all be stored and tracked in a Git repository. Since this is the first time you're using this repository, you'll need to "clone" it to get a copy on your local computer. Follow the cloning instructions in the Project Software Setup handout. Also familiarize yourself with the other tools and commands listed in that document, and the concepts of Git in general. Note that, if you attended section, you've already done this with the group.
For this problem, you will fix some buggy code we provide.
See Editing and Compiling Source Files to help learn how to perform the following basic tasks: adding new files to your directory structure, compiling Java code, and reading the Java compiler's output (which may indicate errors).
Also read the Google Java Style Guide, which describes some standards for Java style used by Google. We expect you to have consistent, readable style throughout the quarter. This style guide may be a reasonable place to start if you're not sure how to achieve that, but we're not requiring that you exactly follow any specific part of that guide. The most important thing to have is consistent style, including style that is consistent with the starter code you may be modifying.
Try to compile the provided code in HolaWorld.java. You should see compilation errors for this
file. (And possibly in RandomHelloTest.java too; if so, ignore these for now since we will fix them
in the next part.) In particular, the lines:
System.out.println(world.);
and:
return SPANISH_GREE;
are problematic. (You might see the second error only after you've fixed the first one.) If you are using
IntelliJ, these errors will be marked with red squiggly lines in HolaWorld.java.
Fix these errors and run HolaWorld by running the setup:runHolaWorld gradle target.
See the Editing and Compiling Source Files handout for
more information on running gradle targets.
After you've fixed the errors and run the code, it would be a good time to commit your changes to your repository and push them to GitLab.
Create a Java class with a main method that will randomly choose, and then print to the console, one
of five possible greetings.
Create the file RandomHello.java, which will
define a Java class RandomHello that will reside in the Java package setup; that is,
its file name is
.../cse331-QUARTER-YourCSENetID/hw-setup/src/main/java/setup/RandomHello.java.
Java requires every runnable class to contain a main method whose signature is public static
void main(String[] args) (for example, see the main methods in HelloWorld and
in HolaWorld).
A code skeleton for the RandomHello class is shown below. IntelliJ will generate an empty class
skeleton for you if you use it to create the new RandomHello class. You're welcome to use that and
fill in the entire class yourself, or just copy/paste the skeleton we provide below and work from there.
RandomHello.java:
package setup;
/** RandomHello selects and prints a random greeting. */
public class RandomHello {
/**
* Prints a random greeting to the console.
*
* @param args command-line arguments (ignored)
*/
public static void main(String[] args) {
RandomHello randomHello = new RandomHello();
System.out.println(randomHello.getGreeting());
}
/** @return a greeting, randomly chosen from five possibilities */
public String getGreeting() {
// YOUR CODE GOES HERE
}
}
This skeleton is meant only to serve as a starting point; you are free to organize your code as you see fit.
Don't write your own random number generator to decide which greeting to select. Instead, take advantage of Java's Random class. (This is a good example of the adage “Know and Use the Libraries” as described in Chapter 9 of Joshua Bloch's Effective Java, 3rd edition. Learning the libraries will take some time, but it's worth it!)
Add the following to the body of your getGreeting() method:
Random randomGenerator = new Random();
This line creates a random number generator (not a random number, but a Java object that can
generate random numbers). In IntelliJ, your code may be marked as an error by a red underline. This is
because the Random class is defined in a setup that has not yet been imported
(java.lang and setup are the only packages that are implicitly imported). Java
libraries are organized as packages and you can only access Java classes in packages that are imported. To
import java.util.Random, add the following line under the package setup; declaration):
import java.util.Random;
This will import the class Random
into your file. In IntelliJ, to automatically add all necessary imports and remove unused imports, either choose
"Code | Optimize Imports" on the main menu, or type Ctrl+Alt+O to optimize your imports (documentation
for optimizing imports). Because there is only one class named Random, IntelliJ will figure
out that you mean to import java.util.Random and will add the above line of code automatically. (If
the name of the class that needs to be imported is ambiguous — for example, there is a java.util.List
as well as a java.awt.List
— then IntelliJ will prompt you to choose the one to import.)
Read the documentation for Random's nextInt(int n) method by going to the Java API and searching for
Random using the search bar in the top right-hand corner. In IntelliJ, you can hover over the class
or method name and press a hotkey to view documentation (check your IntelliJ settings/preferences, under "Keymap" to
find your specific hotkey for "Quick Documentation").
Use the nextInt(int n) method to choose your greeting. You don't have to understand all the details
of its behavior specification, only that it returns a random number from 0 to n-1.
One way to choose a random greeting is using an array. This approach might look something like:
String[] greetings = new String[5]; greetings[0] = "Hello World"; greetings[1] = "Hola Mundo"; greetings[2] = "Bonjour Monde"; greetings[3] = "Hallo Welt"; greetings[4] = "Ciao Mondo";
The main method in the skeleton code above prints the value returned by getGreeting.
So after you complete getGreeting, when the class is run the main method will print
that greeting.
When you are finished writing your code and it compiles, run it using the runRandomHello target
several times to ensure that all five greetings can be displayed.
Again, now would be a good idea to add your new class to version control, commit it, and push it.
Testing is essential for writing quality software. JUnit is a framework for creating unit tests in Java. A unit test checks that one specific, small piece (a "unit") of functionality is working correctly. (JUnit can be used to create other types of tests, too.) This problem provides a quick overview and simple example of how JUnit works. (Testing will be more significant in later assignments.)
Open both hw-setup/src/main/java/setup/Fibonacci.java and hw-setup/src/test/java/setup/FibonacciTest.java.
From the comments, you can see that FibonacciTest is a test of the Fibonacci class.
Now run the JUnit test
setup.FibonacciTest. You'll notice that it's failing its tests! Examine the test code and gradle's
test output to understand why the tests are failing, then use that information to find and fix the bugs in
Fibonacci so it's passing all the tests. The tests are correct, so you shouldn't modify FibonacciTest at all,
but it's probably useful to look at the code and see what it's doing.
Now look at the JUnit tests that we wrote for HolaWorld and RandomHello. They are
called HolaWorldTest and RandomHelloTest, respectively. Ensure that your modified code
passes these tests before you turn in your homework.
Locate the answers.txt file in your repo; you'll find it in hw-setup/src/main/java/setup/. In that
file, write your answers to the following questions using a few sentences max for each question.
Until now, we have only been introducing tools. This -part delves into a more interesting programming exercise. This part will likely be somewhat challenging for most of you. Don't be discouraged. We're here to help, and we expect that time spent now will pay off significantly during the rest of the course.
Look at Ball.java. A Ball is a simple object that has a volume.
Ball.java and fix all the problems with it.We have included a JUnit test BallTest to help you. Moreover, one of IntelliJ's warnings should help you find at least one of the bugs without even referring to the JUnit results.
Next, you'll be implementing the majority of a class called BallContainer. As before, skeleton code is
provided (see BallContainer.java). A BallContainer is a container for Balls. BallContainer
should support the following methods: your task is to fill in the code to implement these methods
correctly:
The specifications for these methods are found in the API documentation (javadoc comments) for BallContainer.
BallContainer uses a java.util.Set to keep track of the balls. Using a predefined Java data
structure saves significant work. Before implementing each method, read the documentation for
Set. Some of your methods will be as simple as calling the appropriate methods for Set.
To help you out, we have included a JUnit test called BallContainerTest.
Before you start coding, please take time to think about the following question.
There are two obvious approaches for implementing getVolume():
getVolume() is called, go through all the Balls in the Set and
add up the volumes. (Hint: one solution might use a for-each
loop to extract Balls from the Set.)
BallContainer whenever Balls are added
and removed. This eliminates the need to perform any computations when getVolume is
called.
Which approach do you think is the better one? Once you've determined which you think is better, or come
up with another solution that you think is better than either of the above, implement
getVolume() alongside the rest of BallContainer
In this problem, you will do a little more designing and thinking and a little less coding. You will
implement the Box class. A Box is also a container for Balls. The key difference between a
Box and a BallContainer is that a Box has only finite volume. Once a box is
full, we cannot put in more Balls. The size (volume) of a Box is defined when the constructor is called:
public Box(double maxVolume);
Since a Box is in many ways similar to a BallContainer, Box's
implementation delegates to a private BallContainer. This code reuse simplifies
Box, many of whose methods can simply
“delegate” to the corresponding method in BallContainer. This design of having
one class contain an object of another class and reusing many of the methods is called
composition or delegation.
(Optional Note: You may wonder why we did not make Box extend
BallContainer via inheritance. That is, why did we not make Box a subclass of
BallContainer? The reason is that Box is not a true subtype of BallContainer
because it is in fact more limited than BallContainer (a Box can only hold a
limited amount). A user who uses a BallContainer in their code cannot simply substitute
that BallContainer with a Box and assume the same behavior. (The code may
cause the Box to fill up, but he did not have this concern when using a BallContainer). For
this reason, it is unwise to make Box extend BallContainer. We will discuss
subtyping much more deeply later in the course.)
In addition to the constructor described above, you will need to implement the following new methods in
Box:
The specifications for these methods can be found in the API documentation (javadoc comments) for Box.
You shouldn't need to change your implementation of BallContainer or Ball for
this problem. In particular, you should not implement the Comparable
interface. If you are tempted to do so, consider using Comparator
instead. Comparator is a companion interface to Comparable and is used
throughout the Java libraries: check out the sort methods in java.util.Collections
as an example. In general, you are often presented with a choice between making an object "Comparable"
where the object itself "knows" how it is supposed to be compared, or implementing a separate
"Comparator" object where the comparison logic is separated from the object being compared. We'll study
the merits of either choice in greater detail later in this class, for now, consider the following when
trying to make a decision between the two: is the object you're working with likely to always
be compared in the same way, or are there multiple "comparisons" that make sense? (For example,
comparing Strings by length or alphabetically). This question will come up in later assignments (and
beyond!), so make sure you remember to consider it when you're working with comparisons of objects in
the future.
A few hints to consider before you start writing code:
BallContainer or Ball for this problem, then
explicitly document what changes you made and why in answers.txt.
TreeSet. Remember that TreeSet does not
store duplicates, and if you provide a TreeSet with a Comparator, it will
use that Comparator to determine duplication. See the TreeSet
API documentation for more details. (TreeSet is much used in CSE 142/143, but more
rarely used in practice.)
getBallsFromSmallest(), we recommend strongly that you
consider using Iterator.
At the end of each assignment, you must refer to the Assignment Submission Handout and closely follow the steps listed to submit your assignment. Do not forget to double check your submission as described in that handout - you are responsible for any issues if your code does not run when we try to grade it.
Use the tag name hw3-final for this assignment. To verify your assignment on attu, you can use
any of the gradle tasks described elsewhere in this assignment: hw-setup:runHelloWorld,
hw-setup:runHolaWorld, and hw-setup:runRandomHello.
Don't forget to write your answers to part 6 in the provided answers.txt file in your repo.
We should be able to find the following in the hw-setup/src directory of your repository:
HolaWorld.java that works as described in Part 5 with no compilation errorsRandomHello.java that prints out one of five random messages when its main
method is executed
Fibonacci.java that passes the three tests in FibonacciTest.java
(Note that you should not edit FibonacciTest.java to accomplish this task.)
Ball.java, BallContainer.java and Box.java that
pass their respective JUnit tests (Again, you should not modify the JUnit tests, though you are encouraged
to read the code to understand what they test.)
In this part, you will learn about IntelliJ's built-in debugger. A debugger can help you debug your program by allowing you to "watch" your program as it executes, one line at a time, and inspect the state of variables along the way. Using a debugger can be much more powerful and convenient than littering your program with statements that print output.
One of the most useful features of a debugger is the ability to set breakpoints in your program. When you run your program through the debugger, it will pause when it reaches a line with a breakpoint. You can inspect the current state of variables, then continue running your program normally or step through one line at a time to watch the variables change.
You should reference the IntelliJ Debugger Documentation throughout this section to learn how to use the debugging tools in IntelliJ. We won't be explaining every step of using the debugger in this section, so this is a good opportunity to practice reading documentation!
Adder.java. This simple program is supposed to print the sum of two user-provided
integers. Try running it a few times (or reading the source code) and you'll see that it doesn't behave as
expected.
return x - y; A red circle should
appear, indicating a breakpoint. (Clicking again removes the breakpoint.)
Adder.java and clicking “Debug...”.
As
before, enter two ints (say, 3 and 4) in the console when prompted. When your program hits the breakpoint,
IntellIJ will pause the program and a debugging window will show up at the bottom of your screen.
computeSum method). The left panel in the frames tab shows where the program is
currently paused, where the current method was called, and so on. (This is called the stack trace).
Double-click on a method name to see the corresponding line in your source code. Finally, the console tab
(next to “Debugger,” right above the “frames” area of the debugger) shows the
console window.
What are all the names and values listed in the “Variables” panel? What does the “frames” tab list as the current method and line number? (Write down the text that was highlighted when the Debug perspective first opened.)
return statement and
exit computeSum. Hit “Step Over” again to progress to the next line.
What are all the names and values listed in the “Variables” panel after each of the two step overs?
Being able to step through each piece of code and examine what's going on is a very powerful way to understand what your code is doing any why things may be going wrong. The IntelliJ debugger can to so much more than just what we've discussed here, so make sure to read the documentation linked above to learn more about what it can do to help you fix any problems you might have in your code.