Simple Interest Calculation Java Applet

import java.applet.*;
import java.awt.*;
import java.awt.event.*;
import java.io.*;

public final class InterestCalcApplet extends Applet implements ActionListener {

    TextField principalField, rateField, numYearsField;
    double simpleInterest = 0, amount = 0, principal = 0, numYears = 0, rate = 0;
    Button calcButton = new Button("Calculate");
    boolean error, error2, error3, error4;
    int er;
    File file;
    byte b[];
    String cmd = "";

    public InterestCalcApplet() {
        super();
        add("South", calcButton);
    }

    @Override
    public void init() {
        try {
            principalField = new TextField(9);
            rateField = new TextField(1);
            numYearsField = new TextField(1);
            Label principalLabel = new Label("Principal: ");
            Label rateLabel = new Label("Rate (%): ");
            Label numYearsLabel = new Label("Time (in years): ");
            addStyle();
            add(principalLabel);
            add(principalField);
            add(rateLabel);
            add(rateField);
            add(numYearsLabel);
            add(numYearsField);
            add("South", calcButton);
            calcButton.addActionListener(this);
        } catch (RuntimeException err) {
            System.exit(0);
        }
    }

    @Override
    public void actionPerformed(ActionEvent e) {
        try {
            error = error2 = error3 = error4 = false;
            setValue();
            simpleInterest = (principal * rate * numYears) / 100;
            amount = principal + simpleInterest;
            repaint();
        } catch (NullPointerException | NumberFormatException err) {
            error = true;
        } finally {
            if (error == true) {
                calcButton.setBackground(Color.red);
            } else {
                calcButton.setBackground(Color.decode("#294a8f"));
            }
        }
    }

    @Override
    public void paint(Graphics g) {
        if (error == false && error2 == false && error3 == false && error4 == false && er != 0) {
            g.drawString("The Simple Interest & Amount after " + numYears + " years", 60, 100);
            g.drawString("Simple Interest: " + simpleInterest, 60, 120);
            g.drawString("Total Amount: " + amount, 60, 140);
            g.setColor(Color.black);
        }
        if (error2 == true) {
            g.setColor(Color.red);
            g.drawString("Invalid Principal", 60, 100);
        }
        if (error3 == true) {
            g.setColor(Color.red);
            g.drawString("Invalid Time", 60, 120);
        }
        if (error4 == true) {
            g.setColor(Color.red);
            g.drawString("Invalid Rate", 60, 140);
        }
        g.drawString(cmd, 60, 160);
        er++;
    }

    public void setValue() {
        try {
            principal = new Double(principalField.getText().trim()).doubleValue();
        } catch (NumberFormatException | NullPointerException err) {
            error2 = true;
        } finally {
            if (error2 == true) {
                principalField.setBackground(Color.red);
            } else {
                principalField.setBackground(Color.decode("#294a8f"));
            }
        }
        try {
            numYears = new Double(numYearsField.getText().trim()).doubleValue();
        } catch (NumberFormatException | NullPointerException err) {
            error3 = true;
        } finally {
            if (error3 == true) {
                numYearsField.setBackground(Color.red);
            } else {
                numYearsField.setBackground(Color.decode("#294a8f"));
            }
        }
        try {
            rate = new Double(rateField.getText().trim()).doubleValue();
        } catch (NumberFormatException | NullPointerException err) {
            error4 = true;
        } finally {
            if (error4 == true) {
                rateField.setBackground(Color.red);
            } else {
                rateField.setBackground(Color.decode("#294a8f"));
            }
        }
    }

    public void addStyle() {
        calcButton.setBackground(Color.decode("#294a8f"));
        calcButton.setForeground(Color.white);
        numYearsField.setBackground(Color.decode("#3B5998"));
        principalField.setBackground(Color.decode("#3B5998"));
        rateField.setBackground(Color.decode("#3B5998"));
        numYearsField.setForeground(Color.white);
        principalField.setForeground(Color.white);
        rateField.setForeground(Color.white);
        calcButton.setFocusable(false);
    }
}