Cvičenie 8 - Code Review Workshop

Úlohy

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Java Úlohy

Processing, filtering, and aggregating data with the Java Stream API

In this task you will build a student records system for a university administration office. The office keeps track of students, the courses they take, their year of study, and their grades. Your job is to implement a set of query methods that extract useful information from the raw data — all powered by Java Streams.

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Learning goals

By completing this task, you will practice:

• Creating and using Java Streams from collections
• Filtering elements with filter()
• Transforming data with map()
• Sorting with sorted() and custom comparators
• Collecting results with collect(), Collectors.toList(), Collectors.groupingBy(), and more
• Aggregating data with count(), max(), min(), and averaging collectors
• Limiting results with limit()
• Working with Optional as a stream terminal operation result
• Designing clean APIs with defensive copying

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Important note about implementation freedom

The tests in this task only verify the results, not how you achieve them. You are free to combine whichever stream operations you find most natural. There are often multiple valid ways to produce the same output — experiment with different approaches!

For example, to compute an average you could use Collectors.averagingDouble(), or mapToDouble().average(), or even reduce(). All are valid as long as the result is correct.

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What to implement

All classes must be in package org.jikvict.tasks.exposed.

1) StudentRecord

A data class representing one student's enrollment in a course.

Required class properties:

• Class name: StudentRecord
• Constructor:
  • StudentRecord(String studentId, String name, String course, int year, double grade)
  • Throws IllegalArgumentException on invalid input

Required behavior:

• studentId and name must be non-null and not blank
• course must be non-null and not blank
• year must be between 1 and 6 (inclusive)
• grade must be between 0.0 and 100.0 (inclusive)
• Methods:
  • String getStudentId()
  • String getName()
  • String getCourse()
  • int getYear()
  • double getGrade()

2) StudentDatabase

The main query engine. Holds a collection of student records and exposes stream-based query methods.

Required class properties:

• Class name: StudentDatabase
• Constructor:
  • StudentDatabase(java.util.List<StudentRecord> records)
  • Must defensively copy the input list (changes to the original list must not affect the database)
  • Throws IllegalArgumentException if records is null

Required methods:

List<String> getStudentNamesByCourse(String course)

Returns the names of all students enrolled in the given course, sorted alphabetically. Returns an empty list if no students are found.

Useful operations: filter, map, sorted, collect

Map<String, List<StudentRecord>> groupByCourse()

Returns a map where each key is a course name and each value is the list of student records for that course.

Useful operations: collect with Collectors.groupingBy

Map<Integer, Long> countStudentsByYear()

Returns a map where each key is a year and each value is the number of students in that year.

Useful operations: collect with Collectors.groupingBy and Collectors.counting

List<StudentRecord> getTopStudents(int n)

Returns the top n students by grade in descending order. If n is larger than the total number of students, return all students sorted by grade descending. If n is zero or negative, return an empty list.

Useful operations: sorted, limit, collect

Map<String, Double> getAverageGradePerCourse()

Returns a map where each key is a course name and each value is the average grade for that course.

Useful operations: collect with Collectors.groupingBy and Collectors.averagingDouble

List<StudentRecord> filterByGradeRange(double min, double max)

Returns all students whose grade is between min and max (inclusive).

Useful operations: filter, collect

Optional<StudentRecord> getHighestGradeStudent()

Returns the student with the highest grade. If multiple students share the highest grade, any one of them may be returned. Returns an empty Optional if the database is empty.

Useful operations: max

long countPassingStudents(double passingGrade)

Returns the number of students with a grade greater than or equal to passingGrade.

Useful operations: filter, count

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Constraints and expectations

• You are encouraged to use the Java Stream API for all query methods.
• Do not use external libraries — only the standard Java SDK.
• Avoid leaking mutable internal collections (defensive copying and unmodifiable views).

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Suggested implementation strategy

1. Implement StudentRecord first with full validation and getters.
2. Implement StudentDatabase with the constructor and defensive copying.
3. Start with simpler query methods (filterByGradeRange, countPassingStudents).
4. Move to grouping and aggregation methods (groupByCourse, countStudentsByYear, getAverageGradePerCourse).
5. Finish with getTopStudents and getHighestGradeStudent.
6. Experiment with alternative stream pipelines — try different ways to achieve the same result!