convert to GUI

import java.awt.*; 
import javax.swing.*;
import java.awt.event.*;
public class Exam{ 
    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());
    Exam data[] = new Exam[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 Exam();
        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+");
        }
        }

continue..

int markAplus = 0, markA = 0, markBplus = 0, markB = 0, markBminus = 0;
int markCplus = 0, markC = 0, markCminus = 0, markDplus = 0, markD = 0, markE = 0;

        for (int i = 0; i < num; i++) {
            Exam show = data[i];
            String g = show.getGrade();
            if (g.equals("A+")) {
                markAplus++;
            }
            if (g.equals("A")) {
                markA++;
            }
            if (g.equals("B+")) {
                markBplus++;
            }
            if (g.equals("B")) {
                markB++;
            }
            if (g.equals("B-")){
                markBminus++;
            }
            if (g.equals("C+")){
                markCplus++;
            }
            if (g.equals("C")){
                markC++;
            }
            if (g.equals("C-")){
                markCminus++;
            }
            if (g.equals("D+")){
                markDplus++;
            }
            if (g.equals("D")){
                markD++;
            }
            if (g.equals("E")){
               markE++;
            }

        }
        JOptionPane.showMessageDialog(null,"Number of student getting A+: "+markAplus);
        JOptionPane.showMessageDialog(null,"Number of student getting A: "+markA);
        JOptionPane.showMessageDialog(null,"Number of student getting B+: "+markBplus);
        JOptionPane.showMessageDialog(null,"Number of student getting B: "+markB);
        JOptionPane.showMessageDialog(null,"Number of student getting B-:"+markBminus);
        JOptionPane.showMessageDialog(null,"Number of student getting C+: "+markCplus);
        JOptionPane.showMessageDialog(null,"Number of student getting C: "+markC);
        JOptionPane.showMessageDialog(null,"Number of student getting C-: "+markCminus);
        JOptionPane.showMessageDialog(null,"Number of student getting D+: "+markDplus);
        JOptionPane.showMessageDialog(null,"Number of student getting D: "+markD);
        JOptionPane.showMessageDialog(null,"Number of student getting E: "+ markE);


                }
        });
    }
}