Chegg cal

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
import java.io.*;
import java.util.*;

public class Main extends JFrame{
   private GridLayout glm = new GridLayout(3, 1, 5, 20);
   private JPanel jp1 = new JPanel();
   private JPanel jp2 = new JPanel();
   private JPanel jp3 = new JPanel();
   private GridLayout gl1 = new GridLayout (1, 2);
   private JLabel jl1 = new JLabel("Enter Infix Expression", JLabel.CENTER);
   private JTextField jtf1 = new JTextField();
   private GridLayout gl2 = new GridLayout(1, 3, 5, 9);
   private JButton jbEvaluate = new JButton("Evaluate");
   private GridLayout gl3 = new GridLayout(1, 2);
   private JLabel jl2 = new JLabel("Result", JLabel.CENTER);
   private JTextField jtf2 = new JTextField();
   private int result;

   public String userInput(){
       return jtf1.getText();
   }
   public Main(){
       setTitle("Infix Expression Evaluator");
       setSize(500, 200);
       setLayout(glm);
       jp1.setLayout(gl1);
       jp1.add(jl1);
       jp1.add(jtf1);
       add(jp1);
       jp2.setLayout(gl2);
       jp2.add(new JLabel(""));
       jp2.add(jbEvaluate);
       jp2.add(new JLabel(""));
       add(jp2);
       jp3.setLayout(gl3);
       jp3.add(jl2);
       jp3.add(jtf2);
       jtf2.setEditable(false);
       jtf2.setBackground(Color.lightGray);
       add(jp3);
       setLocationRelativeTo(null);
       setResizable(true);
       setVisible(true);
   }

   class EnterActionListener implements ActionListener{
       public void actionPerformed(ActionEvent ae){
           try {
               result = Evaluation.testPair(userInput());
               jtf2.setText(Integer.toString(result));
           } catch (DivisionException de) {
               new DivisionException();
           }
       }
   }

   public static void main (String[] args){
       Main main = new Main();
   }
}

===============

//Evaluation.java

import java.util.*;
import javax.swing.JOptionPane;

public class Evaluation {

   //infix to postfix
   private String expression;
   private int expSize;
   private char testChar, openPair;

   //testing () pair
   public boolean testPair(String exp){
       expression = exp;
       Stack stack = new Stack();
       expSize = expression.length();

       for(int i=0; i<expSize; i++){
           testChar = exp.charAt(i);
           switch(testChar){
           case '(' :
               stack.push(testChar);
               break;
           case ')' :
               if(stack.isEmpty()){
                   return false;
               }else{
                   openPair = (char) stack.pop();
                   if((openPair==')') && (testChar!='(')){
     JOptionPane.showMessageDialog(null, "Paranthesis does not match", "Paranthesis Error", JOptionPane.WARNING_MESSAGE);
     return false;

                   }else{
                       break;
                   }
               }
           }
       }
       if(stack.isEmpty()){
           return true;
       }else{
           return false;
       }
       Evaluation.InfixToPostfix(expression);
   }

   public static void InfixToPostfix(String infix) throws DivisionException{
       char testChar;
       String expression2 = infix;
       int expSize = expression2.length();
       Stack stack = new Stack();
       String postfix = "";
       for(int i=0; i<expSize; i++){
           testChar = expression2.charAt(i);
           switch(testChar){
           case '(':
               continue;
           case '0' :
           case '1' :
           case '2' :
           case '3' :
           case '4' :
           case '5' :
           case '6' :
           case '7' :
           case '8' :
           case '9' :
               postfix += Character.toString(testChar);
               break;
           case '+' :
           case '-' :
           case '/' :
           case '*' :
               stack.push(testChar);
               break;
           case ')' :
               postfix += stack.pop();
               break;
           case ' ':
               JOptionPane.showMessageDialog(null, "Blank space is unacceptable", "Spacing Error", JOptionPane.WARNING_MESSAGE);
           default:
           }
       }

       while(!stack.isEmpty()){
           postfix += stack.pop();
       }

       Evaluation eval = new Evaluation();
       eval.evaluate(postfix);
   }//infix to postfix end

   //calculate postfix expression
   public static int evaluate(String postfix) throws DivisionException{
       char testChar;
       int expSize = postfix.length();
       int num1=0, num2=0, result=0;
       Stack stack = new Stack();

       for(int i=0; i<expSize; i++){
           testChar = postfix.charAt(i);

           if(testChar=='+' || testChar=='-' || testChar=='*' || testChar=='/'){
               num2 = (int) stack.pop();
               num1 = (int) stack.pop();

               switch(testChar){
               case '+' :
                   result = (int) stack.push(num1+num2);
                   break;
               case '-' :
                   result = (int) stack.push(num1-num2);
                   break;
               case '*' :
                   result = (int) stack.push(num1*num2);
                   break;
               case '/' :
                   if(num2=='0'){
                       throw new DivisionException();
                   }
                   result = (int) stack.push(num1/num2);
                   break;
               }
           }else{
               result = (int) stack.push(testChar-'0');
           }
       }
       return result;
   }
}

=============

//DivisionException.java

import javax.swing.JOptionPane;

public class DivisionException extends Exception{
   public DivisionException(){
       JOptionPane.showMessageDialog(null, "Division Error", "Cannot divide a number by 0", JOptionPane.WARNING_MESSAGE);
   }
}