Can you help me with this java exercise

[Esi12]

Hi! I have An exercise with which i need some help. I already did this code but i have to find the frequency of each marks and i don"t know how to do it
this is the code:

import javax.swing.*;
import java.util.Collections;
import java.util.Vector;

public class VectorMarks {

/**
* Main method
* @param args
*/
public static void main(String[] args) {

    Vector<Integer> marks = getMarks();
    displayMarks(marks);
    displayAveragePassingMark(marks);
    displayHighestMarkAndStudentsWithIt(marks);
    displayStudentWithMark(8, marks);
    displayMarksInAscendingOrder(marks);
}

private static void displayMarksInAscendingOrder(Vector<Integer> marks) {
// sort the marks in ascending order and display them
    Collections.sort(marks);
    String output = "Marks in ascending order:\n";
    for(int i=0; i<marks.size(); i++) {
        output += marks.get(i) + "\n";
    }
    JOptionPane.showMessageDialog(null, output);
}

private static void displayStudentWithMark(int markValue, Vector<Integer> marks) {
// check how many students have this mark and display result
    int count = 0;
    for(int i=0; i<marks.size(); i++) {
        if(marks.get(i) == markValue) {
            count++;
        }
    }
    String output = "Students with mark " + markValue + ": " + count;
    JOptionPane.showMessageDialog(null, output);
}

private static void displayHighestMarkAndStudentsWithIt(Vector<Integer > marks) {
// find the highest mark and shows students with it
    int highestMark = findHighestMark(marks);
    String output = "Highest mark is " + highestMark + "\n";

    // count how many students have it
    int count = 0;
    for(int i=0; i<marks.size(); i++) {
        if(marks.get(i) == highestMark) {
            count++;
        }
    }
    output += count + " student(s) have it";
    JOptionPane.showMessageDialog(null, output);
}

private static int findHighestMark(Vector<Integer> marks) {
    int highest = 0;
    for(int i=0; i<marks.size(); i++) {
        if(marks.get(i) > highest) {
            highest = marks.get(i);
        }
    }
    return highest;
}

private static void displayAveragePassingMark(Vector<Integer> marks) {
// get the total and compute the average mark
    double total = 0;
    for(int i=0; i<marks.size(); i++) {
        total += marks.get(i);
    }
    double averageMark = total / marks.size();

    JOptionPane.showMessageDialog(null, "Average passing mark is " + String.format("%.2f", averageMark));
}

private static void displayMarks(Vector<Integer> marks) {
// display all the marks
    String output = "";
    for(int i=0; i<marks.size(); i++) {
        output += "Mark #" + (i+1) + ": " + marks.get(i) + "\n";
    }
    JOptionPane.showMessageDialog(null, output);
}

private static Vector<Integer> getMarks() {
// create vector for the marks
    Vector<Integer> marks = new Vector<>();

// get 20 marks
    for(int i=1; i<=20; i++) {
        String input = JOptionPane.showInputDialog(null, "Enter mark #" + i + ":");
        int mark = Integer.parseInt(input);
        marks.add(mark);
    }
    return marks;
}
}