I wanna use a HashMap instead of an array..but how ??

Question

Here's the class I'm using...
public class Group {
     private Student[] list;
     public final int GROUPE_SIZE;
     private float[] finalGrades;
     public Group(int size) {
          GROUPE_SIZE = size;
          list = new Student[size];
     public void addStudents(Student[] list) {
               this.list = list;
     public Student[] getList(){
          return list;
     public void setFinalGrades() {
          finalGrades = new float[GROUPE_SIZE];
          for (int i = 0; i < GROUPE_SIZE; i++) {
          finalGrades[i] = list.getFinalGrade();
          Arrays.sort(finalGrades);
     public float getLowestGrade() {
          return finalGrades[0];
     public float getHighestGrade() {
          return finalGrades[GROUPE_SIZE-1];
     public float getAverage() {
          float total=0.0f;
          for (int i=0; i < GROUPE_SIZE; i++) {
               total+= finalGrades[i];
          return total/GROUPE_SIZE;

Answer 1

I have to replace all the arrays in this class by using the HashMap, but I don't know how
In order to know how, you need to know why.
It looks like your class provides grades for students. Now, a Map (of which HashMap is an implementation) provides for mapping from one object to another. In your case, you probably want to look up students' grades, so your mapping would be "student-->grade"
Read the collections examples linked above and you should be well on your way - but just remember that you need to know the "why" before the "how," otherwise you'll end up in a mess. Understand what it is you're trying to do, what the need is that your code satisfies.