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

public class EquationSolver extends JFrame
implements ActionListener
{
    private JTextField display;

    private static final String ARROW = "==> ";

    public EquationSolver()
    {
        super("Equation Solver");
        display = new JTextField(40);
        display.setFont(new Font("Monospaced", Font.BOLD, 14));
        display.setBackground(Color.white);
        display.setForeground(Color.blue);
        display.addActionListener(this);
        display.setText("Enter a math problem to solve");
        setBounds(50, 150, 450, 100);
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        Container c = getContentPane();
        c.setLayout(new FlowLayout());
        c.add(display);


        setVisible(true);
        // Select the whole display text:
        display.selectAll();

        // Prepare for typing:
        display.requestFocus();
    }

    public void actionPerformed(ActionEvent e)
    {
        // Skip if display has any selected text:
        if (display.getSelectedText() != null)
            return;

        String text = display.getText();
        display.setText(text + " " + process(text));
        display.selectAll();
    }

    private String process(String s)
    {
        String term1, op, term2;

        s = s.trim();


        for(int i=0;i<s.length();i++)
        {
           char x=s.charAt(i);
            if (x=='+' || x=='-' || x=='*' || x=='/' || x=='%')
            {
                int r=s.indexOf('x');
                String q=s.substring(0,r);
                    String y=s.substring(r+1,s.length());

                    s = q+" "+x+" "+y;
                break;
            }


        }


        StringTokenizer terms = new StringTokenizer(s);
        if (terms.countTokens() == 3)
        {
            term1 = terms.nextToken();
            op = terms.nextToken();
            term2 = terms.nextToken();
            return evaluate(term1, op, term2);
        }
        else
            return ARROW + "Syntax error";
    }

    /**
     *  Evaluates an arithmetic expression in the form
     *  "a +/- b" and returns the result
     *  in the form "= c" or an error message.
     */
    private String evaluate(String term1, String op, String term2)
    {
        String result;

        if (op.length() != 1)
            return ARROW + "Syntax error";

        char opSign = op.charAt(0);
        int a = Integer.parseInt(term1);
        int b = Integer.parseInt(term2);

        switch (opSign)
        {
            case '+':
            result = "= " + (a + b);
            break;

            case '-':
            result = "= " + (a - b);
            break;

            case '*':
            result = "= "+(a*b);
            break;

            case '/':
            if(b!=0 && a%b==0)
                result = "= "+(a/b);
            else if(b==0)
                return ARROW + " Divide by zero error!";
            else
                return ARROW + " "+a+" is not evenly divisible by "+b;
            break;

            case '%':
            if (b!=0)
                result = "= "+(a%b);
            else
                return ARROW + " Divide by zero error!";
            break;

            default:
            result = ARROW + "Invalid operation";
            break;
        }
        return result;
    }

    public static void main(String args[])
    {

        EquationSolver panel = new EquationSolver();

    }
}