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Lab #5: Class Roster Solution
Task
Your task is to read in a roster of students, the modules they take, the assessments they have completed, and the grade for each assessment. Then, given a query consisting of a triplet: a student, a module, and an assessment, retrieve the corresponding grade.
For instance, if the input is:
Steve CS1010 Lab3 A
Steve CS1231 Test A+
Bruce CS2030 Lab1 C
and the query is Steve CS1231 Test, the program should print A+.
In our scenario, a roster has zero or more students; a student takes zero or more modules, a module has zero or more assessments, and each assessment has exactly one grade. Each of these entities can be uniquely identifed by a string.
This lab also involves using the HashMap class from Java Collections.
Preamble
Map
Map<K,V> is a generic interface from the Java Collection Framework, the implementation of which is useful for storing a collection of items and retrieving an item. It maintains a map (aka dictionary) between keys (of type K) and values (of type V). The two core methods are (i) put, which stores a (key, value) pair into the map, and (ii) get, which returns the value associated with a given key if the key is found or returns null otherwise.
The following examples show how the Map<K,V> interface and its implementation HashMap<K,V> can be used.
jshell> Map<String,Integer> map = new HashMap<String,Integer>(); jshell> map.put("one", 1);
$.. ==> null
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jshell>
map.put("two", 2);
$.. ==>
null
jshell>
map.put("three", 3);
$.. ==>
null
jshell>
map.get("one")
$.. ==>
1
jshell>
map.get("four")
$.. ==>
null
jshell>
map.entrySet()
$.. ==>
[one=1, two=2, three=3]
jshell>
for (Map.Entry<String,Integer> e: map.entrySet()) {
...>
System.out.println(e.getKey() + ":" + e.getValue());
...>
}
one:1
two:2
three:3
Level 1
Assessment class and Keyable interface
We shall start by writing the Assessment class that implements the following Keyable interface.
interface Keyable {
String getKey();
}
Include a getGrade method that returns the grade of an assessment.
jshell>
new Assessment("Lab1", "B")
$.. ==>
{Lab1: B}
jshell>
new Assessment("Lab1", "B").getGrade()
$.. ==>
"B"
jshell>
new Assessment("Lab1", "B").getKey()
$.. ==>
"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 < test1.jsh
Check your styling by issuing the following
$ checkstyle *.java
Level 2
Module class
Write the Module class to store (via the put method) the assessments of a module in a map for easy retrieval as part
of answering queries. A module can have zero or more assessments, with each assessment having a title as a key — a
unique identifer.
jshell>
new Module("CS2040")
$.. ==>
CS2040: {}
jshell>
new Module("CS2040").getKey();
$.. ==>
"CS2040"
jshell>
new Module("CS2040").put(new Assessment("Lab1", "B"))
$.. ==>
CS2040: {{Lab1: B}}
jshell>
new Module("CS2040").put(new Assessment("Lab1", "B")).put(new Assessment("Lab2","A
$.. ==>
CS2040: {{Lab1: B}, {Lab2: A+}}
jshell>
new Module("CS2040").put(new Assessment("Lab1", "B")).put(new Assessment("Lab2","A
$.. ==>
{Lab1: B}
jshell>
new Module("CS2040").put(new Assessment("Lab1", "B")).put(new Assessment("Lab2","A
$.. ==>
{Lab2: A+}
jshell>
new Module("CS2040").put(new Assessment("Lab1", "B")).put(new Assessment("Lab2","A
$.. ==>
null
jshell>
/exit
Check the format correctness of the output by running the following on the command line:
2/6
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$
javac -Xlint:rawtypes *.java
$
jshell -q your_java_files_in_bottom-up_dependency_order < test2.jsh
Check your styling by issuing the following
$ checkstyle *.java
Level 3
Student class
Write a Student class that stores the modules he/she reads in a map via the put method. A student can read zero or more modules, with each module having a unique module code as its key.
jshell> new Module("CS2040").put(new Assessment("Lab1", "B")).get("Lab1")
$.. ==> {Lab1: B}
jshell> new Module("CS2040").put(new Assessment("Lab1", "B")).get("Lab1").getGrade() $.. ==> "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 $.. ==> CS2040: {{Lab1: B}}
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> /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 < test3.jsh
Check your styling by issuing the following
$ checkstyle *.java
Level 4
Generic class KeyableMap
You will notice that the implementations of the Student and Module classes are very similar. Hence, by applying the abstraction principle, write a generic class KeyableMap to reduce the duplication.
Hint: KeyableMap<V> is a generic class that wraps around a String key (i.e. implements Keyable) and a Map<String,V>. KeyableMap models an entity that contains a map, but is also itself contained in another container (e.g. a student contains a map of modules but could be contained in a roster). The parameter type V is the type of the value of items stored in the map; V must be a subtype of Keyable.
The class KeyableMap is a mutable class -- we made this decision since the Map implementation in Java Collection Framework is mutable. KeyableMap provides two core methods:
get(String key) which returns the item with the given key;
put(V item) which adds the key-value pair (item.getKey(),item) into the map. The put method returns a KeyableMap. To avoid type mismatch when chaining put method calls together, each sub-class of KeyableMap should override the put method from KeyableMap and change the return type to the corresponding sub-classes. E.g., Student should override put to return a Student through a type-cast (which is guaranteed to be safe). Moreover, how do we restrict the classes bound to type V to be able to invoke the getKey method? The trick is to defne the type parameter of Keyable as follows:
class KeyableMap<V extends Keyable> implements Keyable {
...
}
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jshell> new Module("CS2040").put(new Assessment("Lab1", "B")).get("Lab1")
$.. ==> {Lab1: B}
jshell> new Module("CS2040").put(new Assessment("Lab1", "B")).get("Lab1").getGrade()
$.. ==> "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
$.. ==> CS2040: {{Lab1: B}}
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> new Module("CS1231").put(new Assessment("Test", "A-")) instanceof KeyableMap
$.. ==> true
jshell> new Student("Tony").put(new Module("CS1231")) instanceof KeyableMap
$.. ==> true
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 < test4.jsh
Check your styling by issuing the following
$ checkstyle *.java
Level 5
Roster class
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.
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").get("CS1231").get("Test").getGrade()
$.. ==>
"A-"
jshell>
roster.get("Natasha").get("CS2040").get("Lab1").getGrade()
$.. ==>
"B"
jshell>
roster.get("Tony").get("CS1231").get("Exam")
$.. ==>
null
jshell>
roster.get("Steve")
$.. ==>
null
jshell>
roster.getGrade("Tony","CS1231","Test")
$.. ==>
"A-"
jshell>
roster.getGrade("Natasha","CS2040","Lab1")
$.. ==>
"B"
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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> new Roster("AY1920") instanceof Keyable
$.. ==> true
jshell> new Roster("AY1920").put(new Student("Tony")) instanceof Keyable
$.. ==> true
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.
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
Check the format correctness of the output by running the following on the command line:
https://codecrunch.comp.nus.edu.sg/task_view.php?tid=5032
5/6
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• javac -Xlint:rawtypes *.java
• java Main < test6.in
Check your styling by issuing the following
$ checkstyle *.java
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