how to get average value from hashmap?

Question

I have a problem with understanding the whole structure and I do not know how to get the average grades of students and how to calculate them

class Grades  {
    int grade1;
    int grade2;
    int grade3;
    int grade4;
    int grade5;
    int grade6;
    int grade7;

    public Grades(int grade1, int grade2, int grade3, int grade4, int grade5, int grade6, int grade7) {
        this.grade1 = grade1;
        this.grade2 = grade2;
        this.grade3 = grade3;
        this.grade4 = grade4;
        this.grade5 = grade5;
        this.grade6 = grade6;
        this.grade7 = grade7;
    }
    public int getGrade1() {
        return grade1;
    }
    @Override
    public String toString() {
        return grade1 + ", " + grade2 + ", " +  grade3 + ", " +  grade4 + ", " +  grade5 + ", " +  grade6 + ", " +  grade7;
    }
}
  public class ObjectsTestetON {
  public static void main(String[] args) {
  Student student1 = new Student("Ania", "Nowak", "00001");
  Student student2 = new Student("Marcin", "Kasprzak", "00002");
  Student student3 = new Student("Kasia", "Siodłak", "00003");
  Student student4 = new Student("Błażej", "Markowski", "00004");

  Grades grad1 = new Grades(5, 4, 5, 4, 5, 5, 4);
  Grades grad2 = new Grades(3, 4, 5, 4, 3, 4, 4);
  Grades grad3 = new Grades(4, 4, 5, 4, 4, 5, 4);
  Grades grad4 = new Grades(2, 4, 3, 4, 3, 2, 3);

  Map<Student, Grades> gradesParameter = new HashMap<>();

  gradesParameter.put(student1, grad1);
  gradesParameter.put(student2, grad1);
  gradesParameter.put(student3, grad1);
  gradesParameter.put(student4, grad1);


 //generally i don't know how to construct this :(
 // to get avearge value for every student grades
  for (Grades i : gradesParameter.keySet()) { // :(
  System.out.println("key: " + i + " value: " + gradesParameter.get(i)); // :(
   }
  }
}

I did it as it was in my course generally every grades1, 2 etc you could assign a specific name like test card homework or other but I did not do it I left it as an assessment 1 2 3 4 .. everything can be done in several ways and this is probably not very effective for this type of issue normally you could use different indicators in this example average grades without grades from homework etc but it was not important I modeled only like example

Answer 1

The Map is defined with key of type Student and value of type Grades.

• To get all keys from a map use Map.keySet() which returns a Set (no duplicates!) of keys.
• To get all values from a map use Map.values() which returns a Collection (may contain duplicates) of values.

Map<Student, Grades> gradesParameter = new HashMap<>();

// get all keys
Set<Student> students = gradesParameter.keySet();

// keys to print all students
for (Student student : gradesParameter.keySet()) { // here was your error
  System.out.println("key: " + student + " value: " + gradesParameter.get(student)); // works by using student as key
}

// get all values
Collection<Grades> grades = gradesParameter.values();
System.out.println("values: " + grades);
  
// values to calculate the average value of students' grades
double averageGradeOfCourse = calculateAverage(grades); // the overall average over all students' grades

The calculation depends on the definition of your value class Grades.

For example:

• the average of multiple students' grades (e.g. the course-average)

public static double calculateAverage(Collection<Grades> allGrades) {
    int total = 0;
    double count = 0; // typed double only to have result a double

    // accumulate total and count over all grades
 
    return count == 0 ? 0 : (total / count); // attention: div by zero!
}

Simple average getter in class Grades

Given your implementation of class Grades which holds separate int fields for 7 grades.

You could add a method getAverage() to your class Grades that returns a double as average of all 7 grades.

class Grades  {
    int grade1;
    int grade2;
    int grade3;
    int grade4;
    int grade5;
    int grade6;
    int grade7;

    public Grades(int grade1, int grade2, int grade3, int grade4, int grade5, int grade6, int grade7) {
        this.grade1 = grade1;
        this.grade2 = grade2;
        this.grade3 = grade3;
        this.grade4 = grade4;
        this.grade5 = grade5;
        this.grade6 = grade6;
        this.grade7 = grade7;
    }

    // field getter and toString methods omitted for brevity 

    // could also name it getAverage()
    public double asAverage() {
        long total = 0;  // suspect the sum of ints could be long
        // use a double here (although int would be sufficient) to have double as result of division 
        double count = 7;  // count of all occurrences accumulated, must not be zero!

        // calculation starts here
        total = grade1 + grade2 + grade3 +  grade4 +  grade5 + grade6 + grade7;

        return total / count;  // count must not be 0 to prevent division by zero error!
    }
}

Answer 2

I don't know what your Grades class looks like, but I assume it contains an int[]:

public class Grades {
    private final int[] grades;
    
    public Grades(int... grades) {
        this.grades = grades;
    }

    public int[] getGrades() {
        return grades;
    }
}

To find the average grade out of all grades:

gradesParameter.values().stream().mapToDouble(grades -> IntStream.of(grades.getGrades()).average().getAsDouble()).average().getAsDouble();

To find every average grade if all students have the same amount of them:

Collection<Grades> values = gradesParameter.values();
double[] result = new double[7];
int size = values.size();
for (Grades grades : values) {
    int[] grades = grades.getGrades();
    for (int i = 0; i < grades.length; i++) {
        result[i] += grades[i] / size;
    }
}