Lab 4 CS 455

CS 455 Lab 4: Array Lists

Spring 2025 [Bono]

Goals and Background

In this lab you're going to get practice programming with array lists by implementing a class, Nums, that encapsulates one (i.e., there's an ArrayList inside every Nums object) and that has a few methods to examine and manipulate it. The resulting class isn't that useful as is, but part of the point of this lab is to also practice implementing classes using incremental development, testing each subset as you create it.

Note: in this lab where we say implement so-and-so method, it means to implement it so its functionality matches the method comment already written for it (i.e., you will have to read the method comment to know exactly what the method is supposed to do), and it would also mean you are not changing the method header.

Remember to switch roles with your partner from what you were last time. For more information on lab partnerships, see Lab 2.

Reading and reference material

Horstmann, Sections 7.1.4, 7.3, 7.7 on arrays and array lists
Horstmann, Section 5.6 on test cases
Horstmann, Section Special Topic 4.4 on static methods; for more about how we are using static methods here see below
Horstmann, Appendix D: The Java Library.
An easier-to-understand, abbreviated version of Java API Documentation. This lab uses: java.util.ArrayList, java.util.Scanner, and java.lang.Integer,

Exercise 1 (1 checkoff point)

The starter code available on Vocareum is two files, Nums.java, and NumsTester.java. There is also a small text data file there called nums.in.

The class that encapsulates our array list is called Nums. It stores a sequence of ints and allows us to do some computations with the sequence. When you implement a non-trivial class, you want to write and test the minimal amount of code to make sure the data was put in it correctly, before implementing any other methods. That normally means implementing (1) at least one of the constructors and (2) an accessor method or print method that would allow us to see that it got created correctly.

Question 1.1 Why can doing this save you time in the long run, over implementing the whole class at once?

For the Nums class you're going to implement the constructor and the printVals method initially, although you'll will be doing more than that by the end of the exercise. Since the constructor just creates an empty Nums object, the results of this won't be too interesting. Read on through Step 3 before you start this task.

Step 1. Read the code and comments in the following files. First, here's a summary of what they contain, and some changes you will be making to them:

Nums.java contains the specification for the complete Nums class (i.e., method comments plus method headers). All the methods are currently empty, except for minimal code so the class can be compiled as is. These are called "stub" versions of the methods. Some of the stubs have non-empty bodies because they have a non-void return type, so require a return statement to compile. Those return statements don't necessarily return the correct value for the completed version of the methods: i.e., you will probably have to change them in the course of doing this lab.
NumsTester.java contains most of the code for a class that tests the Nums class. In particular, you are going to add code to test the add method as part of exercise 1, but the code to test the rest of the methods is already present. Note: unlike the usual tester programs, this one is interactive, so you can work a little more with a Scanner.

Step 2. Go ahead and compile and run NumsTester so you can see what the output looks like with the stubs version of the Nums class.

Step 3. Implement the Nums constructor and printVals method. This, of course, also involves creating the instance variable(s) that are private to the class. You can run NumsTester with this code, to make sure an empty Nums object gets created and printed correctly. Look at the first few lines of main; that's where this code gets tested.

Step 4. Write the code necessary to to test building and printing a non-empty Nums object. To do that, you need to implement the readNums method in NumsTester, then the Nums add method itself (more about that process in the next few paragraphs).

More about the static method, readNums in NumsTester: (for more information about static methods, see the sidebar below). You're going to test our Nums class with input from a file (more about that below), so readNums should not print out any prompts for the user. We wrote a similar program, ScoreCounts.java in a recent lecture (available in the Vocareum Lecture Code area under 01-30). We'll review what's involved here:

You can test if you've reached the end-of-file (eof) by using the Scanner method hasNextInt, which returns a boolean. (Hint: you can find some documentation for selected Java classes in Appendix D of the text.) When running the program with input from the keyboard (i.e., not from a file), you can generate an end-of-file by typing newline then Ctrl-d after you have entered all the desired input.
To test your code with data from a file (rather than from keyboard input), you can use Unix input redirection so that System.in is linked to a file instead of the keyboard. Look at the contents of the file nums.in. Run your program on it using the following command.
```
java NumsTester < nums.in 
```
Note: running the program this way it will no longer wait for input from the keyboard.

Once you finish readNums and add will be the first time you'll be able to test that a non-empty Nums object gets created and printed correctly. Once you've completed the implementation your program should print out the list of numbers entered in the exact format described in the printVals comments (that includes no space after the last number printed). Note: NumsTester will still not get correct results for the tests of the other two Nums methods: minVal and valuesGT, since you haven't implemented those methods yet.

Step 5. Create at least two other test files to test boundary cases of the code you have written. Call them test1.in and test2.in. (And, of course, fix your program as necessary if it doesn't work properly on the boundary cases.)

To get checked-off, show the TA the answer to question 1.1, your working NumsTester subset program, running on regular and boundary cases; also show him or her the source code you wrote for this exercise, and your test cases.

Exercise 2 (1 checkoff point)

Save your results from exercise 1 by creating a subdirectory ex1 and making a copy of Nums.java and NumsTester.java there. (Hint: You can make the copy in one command using a wildcard.)

For Exercise 2 keep working with the the version of the files that's in your work (i.e., home) directory.

Read the comments for the minVal method of the Nums class so you know what it's supposed to do.

Question 2.1 Write down the expected results of calling minVal on each of the Nums objects that would result from the input in: nums.in, test1.in, and test2.in.

Complete the minVal method of the Nums class. Once you test your code, answer the question below.

Question 2.2 From just looking at the results of a call to minVal, you can't always tell whether the Nums object you called it on was empty or non-empty. Write down two examples of non-empty Nums objects such that they return the same value as for an empty Nums object.

To get checked-off, show the TA your working minVal code, including the source code; show it running on your various test cases from exercise 1, plus additional ones if necessary; and show him or her the answers to your questions.

Exercise 3 (1 checkoff point)

Back up what you have so far by making a copy of your source files in a new ex2 directory (like you did at the start of the previous exercise). Continue working with the code in your work directory this part of the lab.

Read the comments for valuesGT method of the Nums class so you know what it's supposed to do. Note: valuesGT is short for "values greater than".

Then look at the hard-coded test cases for valuesGT in NumsTester. The method is called from the testFilter method in NumsTester. (testFilter just packages up the computation and output for a particular test of valuesGT, so you don't have to keep repeating that code to do multiple tests.) These test cases are designed to go with the data in nums.in.

Question 3.1 Write down the expected results of the hard-coded tests of valuesGT from NumsTester when run on: nums.in, test1.in, and test2.in.

Question 3.2 Create an additional test case in a new file test3.in that explicitly tests a boundary case of valuesGT when run with the hard-coded tests of valuesGT from NumsTester. Add the expected results of of this new test case to the list of expected results you gave in Question 3.1

Implement the valuesGT method of the Nums class. Check that your actual results match the expected results when you run it on each of the given test files. Remember, sometimes a mismatch is because of a mistake in the expected results.

To get checked-off, show the TA your working valuesGT code, including the source code; show it running on all four of the test input files plus additional ones if necessary; and show him or her the answers to the questions.

What are static methods? You've been introduced to static methods in context of calling them in the Java library, such as in the Math class (described in Special Topic 4.4 of the textbook), and you've been using static in your main headers even though you may not have really known why. In this lab we have a few more static method definitions, namely readNums and testFilter in NumsTester, so this is a good opportunity to look at this in more detail.
A better name for a static method would be a class method; it's is inside the scope of a particular class, but it is not associated with an individual instance of the class (i.e., object). As you have seen, from outside the enclosing class they are called with the class name instead of an object name (e.g., Math.round(...)). When you call a static (or non-static) method from another method in the same class, you don't have to use the dot notation (e.g., calling testFilter(...) from main). They are also analogous to regular functions in languages like C/C++, Python, and Matlab; in contrast, in Java all functions must appear inside classes: when we want a function that's does not operate on an object instance we use a static method.
The methods in NumsTester are also static because they are helper methods for a static method (namely, main). In Java, main is always static because it is called (by the run-time system) with no associated instance of its enclosing class (i.e., no NumsTester object, in this case). In virtually all the Java programs you've seen and will write for this course all the variables used by main will be local variables (or the args parameter, which we'll get to later in the semester). Generally, static methods cannot access instance variables of their class, because there is no instance involved (i.e., no this), and thus classes with only static methods, such as NumsTester, don't have instance variables at all. The two other methods in NumsTester are helper functions for main to reduce the complexity of main and the amount of code you have to write overall. Any data they use are passed via explicit parameters.
To summarize, to make a procedure-oriented design in Java you make a main class that has a static main method, plus helper methods that are static methods in that same class using parameter passing and return values to communicate information between these functions. Our test programs, including NumsTester, use such a design.

Exercise 4 (1 checkoff point)

In this exercise you are just going to answer some more questions about the code you wrote.

You added coded that used hasNextInt() as part of the readNums method you implemented as part of Exercise 1. hasNextInt will return false on more than just end-of-file.

Question 4.1 Give two examples of other possible things you could type in to cause this program to terminate (i.e., besides Ctrl-d). Try each of these on your program.

Look at the second line of code in the testFilter method in NumsTester which contains two "." (dot) operators:

       nums.valuesGT(threshold).printVals();

Question 4.2 Write down an equivalent statement with more parentheses to show the grouping (i.e., associativity) of the two dot operators, that is, to show what part of the expression gets evaluated first. Hint: if you think about it, only one grouping makes sense.

Question 4.3 What is the type of the innermost sub-expression you put parentheses around (i.e., the one that gets evaluated first)?

Question 4.4 Write a sequence of statements equivalent to the whole statement in question, but that uses a local variable to store intermediate results.

Our Nums class uses an array list. You could have used an array instead. (Note: Sections 7.7.4 and 7.7.8 of the text may be helpful in answering the the following two questions related to this.)

Question 4.5 What's the disadvantage of using an ArrayList compared to an array in the Nums class?

Question 4.6 What's the advantage of using an ArrayList compared to an array in the Nums class?

Question 4.7 Many programs involve arrays (or equivalently, array lists), but you don't always need to store all the values when you are processing a sequence of numbers. For each of the following tasks related to this lab say whether an array is necessary to complete the task, and briefly describe why or why not. We're interested in this because we could use a lot less memory if we don't have to save all the numbers to process large data sets.

To put it another way, do you need to save all the numbers to do the task? (why or why not)

Reading in a sequence of numbers and printing them all out.
Reading in a sequence of numbers and printing out the minimum value in the sequence.
Reading in a sequence of numbers, printing them out, and printing out the minimum value in the sequence.
Reading in a threshold value and a sequence of numbers, and printing out the numbers from the sequence that are above the threshold value.
Same task, as the previous one, but with the specification that you you have to read in the sequence of numbers before you read in the threshold.
Reading in a sequence of numbers, and generating the exact output produced by NumsTester.java about that data.

Checkoff for DEN students

Make sure you put your name and NetID in all the files you submit.

When you click the Submit button, it will be looking for and compiling (for source code) the files
README, test1.in, test2.in, test3.in, Nums.java, NumsTester.java, ex1/Nums.java, ex1/NumsTester.java, ex2/Nums.java, and ex2/NumsTester.java. As usual, the lab is due by 11:59pm on Sunday Pacific Time at the end of the week for this lab.