Project: Student Gradebook

Last project: let's build a student gradebook! This one combines classes, HashMap, and ArrayList into a real-world application. Think of it like the stats screen in Vice City, but for school instead of crime

First, a quick HashMap refresher

A HashMap stores key-value pairs. Perfect for mapping a subject name to a grade

import java.util.HashMap;

public class Main {
    public static void main(String[] args) {
        HashMap<String, Integer> grades = new HashMap<>();
        grades.put("Math", 92);
        grades.put("English", 85);
        grades.put("Science", 78);

        System.out.println("Math: " + grades.get("Math"));
    }
}

Output

Math: 92

put adds a key-value pair, get retrieves the value for a key. Simple as that

Step 1: The Student class

Each student has a name and a HashMap that maps subject names to grades

import java.util.HashMap;

class Student {
    String name;
    HashMap<String, Integer> grades;

    Student(String name) {
        this.name = name;
        this.grades = new HashMap<>();
    }
}

We create an empty HashMap in the constructor. The student starts with no grades, we'll add them later with a method

Step 2: Adding grades

public class Main {
    void addGrade(String subject, int grade) {
        grades.put(subject, grade);
    }
}

Dead simple. grades.put("Math", 95) stores the grade 95 for Math. If the student already has a Math grade, it gets overwritten with the new one

Step 3: Calculating the average

To get the average, we need to sum all the grades and divide by how many there are. We loop through the HashMap's values

public class Main {
    double getAverage() {
        int sum = 0;
        for (int grade : grades.values()) {
            sum += grade;
        }
        return (double) sum / grades.size();
    }
}

grades.values() gives us all the grade numbers. We sum them up, then divide by grades.size() (the number of subjects). The (double) cast makes sure we get decimal division, not integer division

Step 4: Printing the report

Let's print a nice report showing the student's name, each subject with its grade, and the average. We'll loop through the HashMap's keySet to get both keys and values

public class Main {
    void printReport() {
        System.out.println("Student: " + name);
        for (String subject : grades.keySet()) {
            System.out.println("  " + subject + ": " + grades.get(subject));
        }
        System.out.println("  Average: " + String.format("%.1f", getAverage()));
    }
}

grades.keySet() gives us all the subject names. For each one, we print the subject and its grade. At the end, we print the average formatted to 1 decimal place

Important note about HashMap ordering

HashMaps do NOT guarantee order. If you add Math, English, Science, they might print in any order. That's just how HashMaps work internally. If you need a specific order, you'd use a LinkedHashMap instead (which preserves insertion order), but for now, regular HashMap is fine

For our exercise, we'll use a LinkedHashMap so the output is predictable

import java.util.LinkedHashMap;

class Student {
    String name;
    LinkedHashMap<String, Integer> grades;

    Student(String name) {
        this.name = name;
        this.grades = new LinkedHashMap<>();
    }
}

LinkedHashMap works exactly like HashMap but remembers the order you added things. Think of it like Vice City's stats screen: it always shows your stats in the same order

Full example

import java.util.LinkedHashMap;

class Student {
    String name;
    LinkedHashMap<String, Integer> grades;

    Student(String name) {
        this.name = name;
        this.grades = new LinkedHashMap<>();
    }

    void addGrade(String subject, int grade) {
        grades.put(subject, grade);
    }

    double getAverage() {
        int sum = 0;
        for (int grade : grades.values()) {
            sum += grade;
        }
        return (double) sum / grades.size();
    }

    void printReport() {
        System.out.println("Student: " + name);
        for (String subject : grades.keySet()) {
            System.out.println("  " + subject + ": " + grades.get(subject));
        }
        System.out.println("  Average: " + String.format("%.1f", getAverage()));
    }
}

public class Main {
    public static void main(String[] args) {
        Student s1 = new Student("Tommy");
        s1.addGrade("Math", 85);
        s1.addGrade("English", 92);
        s1.addGrade("Science", 78);
        s1.printReport();
    }
}

Output

Student: Tommy
  Math: 85
  English: 92
  Science: 78
  Average: 85.0

Notice how we used composition again: a Student has a HashMap of grades. This is the same pattern as the Inventory project where Inventory has an ArrayList of Items. Real-world Java is full of this: objects containing other objects, each with their own responsibilities

The Student class is self-contained: it knows how to add grades, calculate its own average, and print its own report. Each object manages its own data. That's good OOP design

Mission: Academy Transcript Generator

The station's training academy just finished exams. Each cadet has grades in multiple subjects, and the academy director needs a printed transcript for every student showing their scores and average.

1. Create a Student class with a name (String) and a LinkedHashMap<String, Integer> for grades
2. Add methods: addGrade(String subject, int grade), double getAverage(), printReport()
3. printReport() should print:
   • "Student: NAME" on the first line
   • " SUBJECT: GRADE" for each subject (two spaces before each)
   • " Average: X.X" at the end (one decimal place, two spaces before)
4. In main, store the data in variables: name1 plus math1/english1/science1 for the first student, name2 plus math2/english2/science2 for the second (start with Tommy: 90/85/92 and Lance: 78/82/88). Create the two Students from those variables and add their grades
5. Print both reports

Output

Student: Tommy
  Math: 90
  English: 85
  Science: 92
  Average: 89.0
Student: Lance
  Math: 78
  English: 82
  Science: 88
  Average: 82.7