CS2030 Lab #6: Making Things Optional

CS2030 Lab #6: Making Things Optional
Tags & Categories
Tags:
Categories:
Related Tutorials
Task Content
Making Things Optional
Topic Coverage
Optional
Functional Interfaces
Problem Description
This lab is a continuation of the class roster lab.
So far we have been dealing with invalid retrievals such as
new Module("CS2040").put(new Assessment("Lab1", "B")).get("Lab2")
which would result in a null value. Furthermore, chaining methods like
new Student("Tony").put(new Module("CS2040").put(new Assessment("Lab1", "B"))).get("CS2030
would result in a NullPointerException being thrown due to calling get("Lab1") on a null value.
Take a look at the Java documentation of Optional to familiarize yourself with the APIs available.
Note
On CodeCrunch, we will check for any use of null in your code. Any occurence of the string null would fail the
CodeCrunch test. Thus, avoid variable or method names that contain the substring null.
Further, we will disallow the use of methods Optional::get, Optional::isPresent and Optional::isEmpty,
as the former could cause NullPointerException, while the latter is essentially the same as checking for null.
Level 4+
Avoiding null with Optional
Modify the KeyableMap generic class such that each call to get returns an Optional where V is a subtype of
Keyable.
jshell> new Module("CS2040").put(new Assessment("Lab1", "B")).get("Lab1")
$.. ==> Optional[{Lab1: B}]
jshell> new Student("Tony").put(new Module("CS2040").put(new Assessment("Lab1", "B")))
$.. ==> Tony: {CS2040: {{Lab1: B}}}
jshell> new Student("Tony").put(new Module("CS2040").put(new Assessment("Lab1", "B"))).get
$.. ==> Optional[CS2040: {{Lab1: B}}]
jshell> Student natasha = new Student("Natasha");
Search search for...
in NUS Websites
NUS WebMail IVLE LIBRARY MAPS
26/11/2021, 03:14 CodeCrunch
https://codecrunch.comp.nus.edu.sg/task_view.php?tid=5034 2/4
jshell> natasha.put(new Module("CS2040").put(new Assessment("Lab1", "B")))
$.. ==> Natasha: {CS2040: {{Lab1: B}}}
jshell> natasha.put(new Module("CS2030").put(new Assessment("PE", "A+")).put(new Assessmen
$.. ==> Natasha: {CS2030: {{Lab2: C}, {PE: A+}}, CS2040: {{Lab1: B}}}
jshell> Student tony = new Student("Tony");
jshell> tony.put(new Module("CS1231").put(new Assessment("Test", "A-")))
$.. ==> Tony: {CS1231: {{Test: A-}}}
jshell> tony.put(new Module("CS2100").put(new Assessment("Test", "B")).put(new Assessment
$.. ==> Tony: {CS1231: {{Test: A-}}, CS2100: {{Lab1: F}, {Test: B}}}
jshell> /exit
As you can see, the only difference is that each value returned from an invocation of the get method is wrapped in an
Optional.
Check the format correctness of the output by running the following on the command line:
$ javac -Xlint:rawtypes *.java
$ jshell -q your_java_files_in_bottom-up_dependency_order < test4.jsh
Check your styling by issuing the following
$ checkstyle *.java
Level 5+
"Chaining" Optionals
Chaining Optionals together as such
new Student("Tony").put(new Module("CS2040").put(new Assessment("Lab1", "B"))).get("CS2040
will now result in a compilation error instead. This is because the Optional class does not have a get(String)
method defined (although it does define a get() method which, other than for debugging purposes, should typically be
avoided).
Rather than chaining in the usual way, we do it through a map or flatMap. Let's start with map.
jshell> new Module("CS2040").put(new Assessment("Lab1", "B")).get("Lab1")
$.. ==> Optional[{Lab1: B}]
jshell> new Module("CS2040").put(new Assessment("Lab1", "B")).get("Lab1").getGrade()
| Error:
| cannot find symbol
| symbol: method getGrade()
| new Module("CS2040").put(new Assessment("Lab1", "B")).get("Lab1").getGrade()
| ^------------------------------------------------------------------------^
jshell> new Module("CS2040").put(new Assessment("Lab1", "B")).get("Lab1").map(x -> x.getGr
$.. ==> Optional[B]
jshell> new Module("CS2040").put(new Assessment("Lab1", "B")).get("Lab1").map(Assessment:
$.. ==> Optional[B]
As expected, invoking getGrade() on an Optional results in a compilation error. However, we can perform a similar
chaining effect by passing in the functionality of getGrade (either in the form of a lambda or method reference) to
Optional's map method. Notice the return value is actually wrapped in another Optional. When using map, you can
think of the operation as "taking the value out of the Optional box, transforming it via the function passed to map, and
wrap the transformed value back in another Optional".
Now this is where things start to get interesting! Look at the following:
jshell> new Student("Tony").put(new Module("CS2040").put(new Assessment("Lab1", "B"))).get
$.. ==> Optional[Optional[{Lab1: B}]]
Observe that the return value is an Optional wrapped around another Optional that wraps around the desired
value! Why is this so? The difference lies in the return type of Assessment::getGrade (read getGrade method of
the Assessment class) and Module::get. The former returns a String, while the latter returns an Optional.
In x -> x.getGrade() (or Assessment::getGrade), the transformed value is simply the grade, and this is
wrapped in an Optional. However, passing x -> x.get("Lab1") in the above code snippet results in a transformed
value of Optional. And this transformed value is wrapped around another Optional via the map operation!
26/11/2021, 03:14 CodeCrunch
https://codecrunch.comp.nus.edu.sg/task_view.php?tid=5034 3/4
As such, we use the flatMap method instead. You may think of flatMap as flattening the Optionals into a single
one.
jshell> new Student("Tony").put(new Module("CS2040").put(new Assessment("Lab1", "B"))).get
$.. ==> Optional[{Lab1: B}]
jshell> new Student("Tony").put(new Module("CS2040").put(new Assessment("Lab1", "B"))).get
$.. ==> Optional[B]
Now you are ready to create a roster. Define a Roster class that stores the students in a map via the put method. A
roster can have zero or more students, with each student having a unique id as its key. Once again, notice the
similarities between Roster, Student and Module.
Define a method called getGrade in Roster to answer the query from the user. The method takes in three String
parameters, corresponds to the student id, the module code, and the assessment title, and returns the corresponding
grade.
In cases where there are no such student, or the student does not read the given module, or the module does not have
the corresponding assessment, then output No such record followed by details of the query. Here, you might find
Optional::orElse useful.
jshell> Student natasha = new Student("Natasha");
jshell> natasha.put(new Module("CS2040").put(new Assessment("Lab1", "B")))
$.. ==> Natasha: {CS2040: {{Lab1: B}}}
jshell> natasha.put(new Module("CS2030").put(new Assessment("PE", "A+")).put(new Assessmen
$.. ==> Natasha: {CS2030: {{Lab2: C}, {PE: A+}}, CS2040: {{Lab1: B}}}
jshell> Student tony = new Student("Tony");
jshell> tony.put(new Module("CS1231").put(new Assessment("Test", "A-")))
$.. ==> Tony: {CS1231: {{Test: A-}}}
jshell> tony.put(new Module("CS2100").put(new Assessment("Test", "B")).put(new Assessment
$.. ==> Tony: {CS1231: {{Test: A-}}, CS2100: {{Lab1: F}, {Test: B}}}
jshell> Roster roster = new Roster("AY1920").put(natasha).put(tony)
jshell> roster
roster ==> AY1920: {Natasha: {CS2030: {{Lab2: C}, {PE: A+}}, CS2040: {{Lab1: B}}}, Tony:
jshell> roster.get("Tony").flatMap(x -> x.get("CS1231")).flatMap(x -> x.get("Test")).map(A
$.. ==> Optional[A-]
jshell> roster.get("Natasha").flatMap(x -> x.get("CS2040")).flatMap(x -> x.get("Lab1")).ma
$.. ==> Optional[B]
jshell> roster.get("Tony").flatMap(x -> x.get("CS1231")).flatMap(x -> x.get("Exam")).map(A
$.. ==> Optional.empty
jshell> roster.get("Steve").flatMap(x -> x.get("CS1010")).flatMap(x -> x.get("Lab1")).map
$.. ==> Optional.empty
jshell> roster.getGrade("Tony","CS1231","Test")
$.. ==> "A-"
jshell> roster.getGrade("Natasha","CS2040","Lab1")
$.. ==> "B"
jshell> roster.getGrade("Tony","CS1231","Exam");
$.. ==> "No such record: Tony CS1231 Exam"
jshell> roster.getGrade("Steve","CS1010","Lab1");
$.. ==> "No such record: Steve CS1010 Lab1"
jshell> /exit
Check the format correctness of the output by running the following on the command line:
$ javac -Xlint:rawtypes *.java
$ jshell -q your_java_files_in_bottom-up_dependency_order < test5.jsh
Check your styling by issuing the following
$ checkstyle *.java
Level 6+
The Main class
Now use the classes that you have built and write a Main class to solve the following:
Read the following from the standard input:
The first token read is an integer N, indicating the number of records to be read.
The subsequent inputs consist of N records, each record consists of four words, separated by one or more
spaces. The four words correspond to the student id, the module code, the assessment title, and the grade,
respectively.
26/11/2021, 03:14 CodeCrunch
https://codecrunch.comp.nus.edu.sg/task_view.php?tid=5034 4/4
© Copyright 2009-2021 National University of Singapore. All
Rights Reserved.
Terms of Use | Privacy | Non-discrimination
MySoC | Computing Facilities | Search | Campus Map
School of Computing, National University of Singapore
The subsequent inputs consist of zero or more queries. Each query consists of three words, separated by one
or more spaces. The three words correspond to the student id, the module code, and the assessment title.
For each query, if a match in the input is found, print the corresponding grade to the standard output. Otherwise, print
"No Such Record:" followed by the three words given in the query, separated by exactly one space.
See sample input and output below. Inputs are underlined.
$ java Main
12
Jack CS2040 Lab4 B
Jack CS2040 Lab6 C
Jane CS1010 Lab1 A
Jane CS2030 Lab1 A+
Janice CS2040 Lab1 A+
Janice CS2040 Lab4 A+
Jim CS1010 Lab9 A+
Jim CS2010 Lab1 C
Jim CS2010 Lab2 B
Jim CS2010 Lab8 A+
Joel CS2030 Lab3 C
Joel CS2030 Midterm A
Jack CS2040 Lab4
Jack CS2040 Lab6
Janice CS2040 Lab1
Janice CS2040 Lab4
Joel CS2030 Midterm
Jason CS1010 Lab1
Jack CS2040 Lab5
Joel CS2040 Lab3
B
C
A+
A+
A
No such record: Jason CS1010 Lab1
No such record: Jack CS2040 Lab5
No such record: Joel CS2040 Lab3
Note:
You might find Optional::ifPresentOrElse useful.
Compile and check your styling by issuing the following
$ javac -Xlint:rawtypes *.java
$ checkstyle *.java