Lab #6: Making Things Optional Solution

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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");


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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 defned (although it does defne 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!

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As such, we use the flatMap method instead. You may think of flatMap as fattening 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. Defne 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.

Defne 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 fnd 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 frst 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.


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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 fnd Optional::ifPresentOrElse useful.

Compile and check your styling by issuing the following

    • javac -Xlint:rawtypes *.java

    • checkstyle *.java



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