import java.awt.*; 
import javax.swing.*;
import java.awt.event.*;
public class MarkStudent{ 
    double totalMarks;
    String grade;

public void setTotalMarks(double totalMarks) {
    this.totalMarks = totalMarks;
    }

public double getTotalMarks() {
    return totalMarks;
 }

public void setGrade(String grade) {
    this.grade = grade;
    }

public String getGrade(){
    return grade;
}

public static void main(String[] args) {
    JLabel lab=new JLabel("Enter number of students in class: ");
    final JTextField text=new JTextField(20);

    JButton b=new JButton("Find");
    lab.setBounds(10,10,150,20);
    text.setBounds(180,10,100,20);
    b.setBounds(180,40,100,20);

    JFrame f=new JFrame();
    f.setLayout(null);
    f.add(lab);
    f.add(text);
    f.add(b);
    f.setSize(300,100);
    f.setVisible(true);
    b.addActionListener(new ActionListener(){
        public void actionPerformed(ActionEvent e){

    int num = Integer.parseInt(text.getText());
    MarkStudent data[] = new MarkStudent[num];
    for(int a = 0; a < data.length; a++) {
       String input=JOptionPane.showInputDialog(null,"Please insert student marks: ");

        double marks = Double.parseDouble(input);
        while(marks>100||marks<0){
        String  st=JOptionPane.showInputDialog(null,"Invalid marks! Marks should be in between 0 to 100!");
            marks=Double.parseDouble(st);
        }
        data[a] = new MarkStudent();
        data[a].setTotalMarks(marks);

        if(marks < 40){
            data[a].setGrade("E");
        }
        if(marks >= 40 && marks <= 44) {
            data[a].setGrade("D");
        }
        if (marks >= 45 && marks <= 49) {
            data[a].setGrade("D+");
        }
        if (marks >= 50 && marks <= 54) {
            data[a].setGrade("C-");
        }
        if (marks >= 55 && marks <= 59) {
            data[a].setGrade("C");
        }
        if (marks >= 60 && marks <= 64) {
            data[a].setGrade("C+");
        }
        if (marks >= 65 && marks <= 69) {
            data[a].setGrade("B-");
        }
        if (marks >= 70 && marks <= 74) {
            data[a].setGrade("B");
        }
        if (marks >= 75 && marks <= 79) {
            data[a].setGrade("B+");
        }
        if (marks >= 80 && marks <= 84) {
            data[a].setGrade("A");
        }
        if (marks >= 85 && marks <= 100) {
            data[a].setGrade("A+");
        }
        if (marks < -100 && marks >= 100) {
           data[a].setGrade("Not in the list of the table");
       }
    }

continue.....

int count1 = 0; int count2 = 0; int count3 = 0; int count4 = 0; int count5 = 0; int count6 = 0; int count7 = 0; int count8 = 0; int count9 = 0; int count10 = 0; int count11 = 0; int count12 = 0;

        for (int i = 0; i < num; i++) {
            MarkStudent show = data[i];
            String g = show.getGrade();
            if (g.equals("A+")) {
                count1++;
            }
            if (g.equals("A")) {
                count2++;
            }
            if (g.equals("B+")) {
                count3++;
            }
            if (g.equals("B")) {
                count4++;
            }
            if (g.equals("B-")){
                count5++;
            }
            if (g.equals("C+")){
                count6++;
            }
            if (g.equals("C")){
                count7++;
            }
            if (g.equals("C-")){
                count8++;
            }
            if (g.equals("D+")){
                count9++;
            }
            if (g.equals("D")){
                count10++;
            }
            if (g.equals("E")){
                count11++;
            }
            if (g.equals("Not in the list of the table")){
                count12++;
            }

        }
        JOptionPane.showMessageDialog(null,"Number of student getting A+ and A grade: "
                + (count1 + count2));
        JOptionPane.showMessageDialog(null,"Number of student getting B+ and B grade: "
                + (count3 + count4));
        JOptionPane.showMessageDialog(null,"Number of student getting A and B+ grade: "
                + (count2 + count3));
        JOptionPane.showMessageDialog(null,"Number of student getting B and B- grade: "
                + (count4 + count5));
        JOptionPane.showMessageDialog(null,"Number of student getting B- and C+ grade: "
                + (count5 + count6));
        JOptionPane.showMessageDialog(null,"Number of student getting C+ and C grade: "
                + (count6 + count7));
        JOptionPane.showMessageDialog(null,"Number of student getting C and C- grade: "
                + (count7 + count8));
        JOptionPane.showMessageDialog(null,"Number of student getting C- and D grade: "
                + (count8 + count9));
        JOptionPane.showMessageDialog(null,"Number of student getting D and D- grade: "
                + (count9 + count10));
        JOptionPane.showMessageDialog(null,"Number of student getting D- and E grade: "
                + (count10 + count11));
        JOptionPane.showMessageDialog(null, "The result cannot be display because the marks is not in the list of the table."
                + (count12));


                }
        });
    }
}