DocumentListener Java, How do I prevent empty string in JTextBox?

enter image description here I have been working on a personal project to get better with programming. My goal is to make it much more robust, I am just starting. I am a current computer science student. Anyway, I am working on making a portion of the program as shown. I calculates the hourly wage and provides some outputs I havent implemented yet. I'm using DocumentListener so it it will automatically calculate. I am getting an error when the the text is removed completely from a box.I tried to fix it with the if statement:

 if (tipMon.equals("") || tipMon == null) {
 tipMon.setText("0");
 }

Here is what I have so far. It's not done yet and I apologize for the noob code. I started 2 months ago with actual coding.

 import java.awt.GridLayout;
 import java.awt.event.ActionListener;
 import java.awt.event.ActionEvent;
 import java.awt.event.KeyListener;

 import javax.swing.JFrame;
 import javax.swing.JLabel;
 import javax.swing.JTextField;
 import javax.swing.JOptionPane;
 import javax.swing.event.DocumentEvent;
 import javax.swing.event.DocumentListener;
 import javax.swing.text.Document;
 import javax.swing.text.FieldView;

 public class deliveryDocListener extends JFrame implements ActionListener, 
DocumentListener{

private JLabel mon, tues, wed, thurs, fri, sat, sun, hourlyWage, blank, row2, monWage,
    tuesWage,wedWage,thursWage, friWage, satWage, sunWage, total, totalTips, totalHours, 
    totalHourlyEarnings, totalPay, weekPay;

private JTextField hourlyWageInput, tipMon, tipTues, tipWed, tipThurs, tipFri, tipSat, tipSun,
    hourMon, hourTues, hourWed, hourThurs, hourFri, hourSat, hourSun;


public deliveryDocListener(){
    super("Delivery Helper v0.1 Alpha");
    setLayout(new GridLayout(0,4));

    hourlyWage = new JLabel("Hourly Wage: ");
    add(hourlyWage);
    hourlyWageInput = new JTextField("7.25", 5);

    add(hourlyWageInput);
    blank = new JLabel();
    add(blank);
    blank = new JLabel();
    add(blank);

    row2 = new JLabel("Day of the Week");
    add(row2);
    row2 = new JLabel("Tips");
    add(row2);
    row2 = new JLabel("Hours Worked");
    add(row2);
    row2 = new JLabel("Hourly Earnings");
    add(row2);

    mon = new JLabel("Monday");
    add(mon);



    tipMon = new JTextField("0");

    Document tipMonListener = tipMon.getDocument();
    //Document class doc variable stores what happens in the getDocument()
    //method, getDocument() i think is what checked it real time we shall see
    tipMonListener.addDocumentListener(this);
    //add listener to he text field, this refers to most recent object (tipMon = new JTextField("0");"
    //notice how its purple is the same as new where the object got made?
    add(tipMon);

    hourMon = new JTextField("0");
    Document hourMonListener = hourMon.getDocument();
    hourMonListener.addDocumentListener(this);
    add(hourMon);


    monWage = new JLabel("0");
    add(monWage);

    tues = new JLabel("Tuesday");
    add(tues);
    tipTues = new JTextField("0");
    add(tipTues);
    hourTues = new JTextField("0");
    add(hourTues);
    tuesWage = new JLabel("0");
    add(tuesWage);

    wed = new JLabel("Wednesday");
    add(wed);
    tipWed = new JTextField("0");
    add(tipWed);
    hourWed = new JTextField("0");
    add(hourWed);
    wedWage = new JLabel("0");
    add(wedWage);

    thurs = new JLabel("Thursday");
    add(thurs);
    tipThurs = new JTextField("0");
    add(tipThurs);
    hourThurs = new JTextField("0");
    add(hourThurs);
    thursWage = new JLabel("0");
    add(thursWage);

    fri = new JLabel("Friday");
    add(fri);
    tipFri = new JTextField("0");
    add(tipFri);
    hourFri = new JTextField("0");
    add(hourFri);
    friWage = new JLabel("0");
    add(friWage);

    sat = new JLabel("Saturday");
    add(sat);
    tipSat = new JTextField("0");
    add(tipSat);
    hourSat = new JTextField("0");
    add(hourSat);
    satWage = new JLabel("0");
    add(satWage);

    sun = new JLabel("Sunday");
    add(sun);
    tipSun = new JTextField("0");
    add(tipSun);
    hourSun = new JTextField("0");
    add(hourSun);
    sunWage = new JLabel("0");
    add(sunWage);

    blank = new JLabel();
    add(blank);
    blank = new JLabel();
    add(blank);
    blank = new JLabel();
    add(blank);
    blank = new JLabel();
    add(blank);

    total = new JLabel("Total: ");
    add(total);
    totalTips = new JLabel("totalTipsOutput");
    add(totalTips);
    totalHours = new JLabel("totalHoursOutput");
    add(totalHours);
    totalHourlyEarnings = new JLabel("totalHourlyEarningsOutput");
    add(totalHourlyEarnings);

    blank = new JLabel();
    add(blank);
    blank = new JLabel();
    add(blank);
    blank = new JLabel();
    add(blank);
    blank = new JLabel();
    add(blank);

    blank = new JLabel();
    add(blank);
    blank = new JLabel();
    add(blank);
    totalPay = new JLabel("Gross Income: ");
    add(totalPay);
    weekPay = new JLabel("totalPayOutput");
    add(weekPay);

}

@Override
public void changedUpdate(DocumentEvent e) { 
    // TODO Auto-generated method stub

}

@Override
public void insertUpdate(DocumentEvent e) {
    //executes when someone enters text into input
    String tipMonStr = tipMon.getText();
    //monWage.setText(tipMonStr);
    String hourMonStr = hourMon.getText();

    double x = Double.parseDouble(tipMonStr);
    double y = Double.parseDouble(hourMonStr);
    double z = Double.parseDouble(hourlyWageInput.getText());

    if (tipMonStr.length() == 0) {
        tipMon.setText("0");
    }

    if (hourMonStr.length() == 0) {
        y = 0;
        hourMonStr = "0";
    }

    if (hourlyWageInput.getText().length() == 0) {
        z = 0;
        //String z = "0";
    }

    monWage.setText(Double.toString((z*y+x)/y));

    //bug when nothing in cell because no number (0) to use in math
}

@Override
public void removeUpdate(DocumentEvent e) {
    //executes when someone enters text into input
    String tipMonStr = tipMon.getText();
    //monWage.setText(tipMonStr);
    String hourMonStr = hourMon.getText();

    double x = Double.parseDouble(tipMonStr);
    double y = Double.parseDouble(hourMonStr);
    double z = Double.parseDouble(hourlyWageInput.getText());
    monWage.setText(Double.toString((z*y+x)/y));

    if (tipMon.equals("") || tipMon == null) {
        tipMon.setText("0");
    }

}


public void updateLog(DocumentEvent e, String action) {
    monWage.setText(Double.toString(5));
}

@Override
public void actionPerformed(ActionEvent arg0) {
    monWage.setText(Double.toString(5));

}

}

Solution 1:

As @HFOE suggests, InputVerifier is the right choice, but verify() "should have no side effects." Instead, invoke calcProduct() in shouldYieldFocus().

 /**
 * @see http://stackoverflow.com/a/11818946/230513
 */
private class MyInputVerifier extends InputVerifier {

    private JTextField field;
    private double value;

    public MyInputVerifier(JTextField field) {
        this.field = field;
    }

    @Override
    public boolean shouldYieldFocus(JComponent input) {
        if (verify(input)) {
            field.setText(String.valueOf(value));
            calcProduct();
            return true;
        } else {
            field.setText(ZERO);
            field.selectAll();
            return false;
        }

    }

    @Override
    public boolean verify(JComponent input) {
        try {
            value = Double.parseDouble(field.getText());
            return true;
        } catch (NumberFormatException e) {
            return false;
        }
    }
}

Solution 2:

I'll make this an answer: I wouldn't use a DocumentListener for this purpose as it seems to me the wrong tool for the job. For one, it is continually listening and updating the results while the user is still entering data, data that is as yet incomplete, into the JTextField. Much better would be to use an ActionListener added to a JButton or to your JTextFields.

I suppose you could use a FocusListener, but even that concerns me since it is quite low-level.

Also: consider using an InputVerifier to validate your input.

Also: consider displaying your tabular data in a JTable where the 1st and 2nd columns are editable but the others are not.

Edit
I'm not sure if this is kosher, but it could work if you do your calculation from within the verifier. For example, updated for generality:

import javax.swing.*;

/**
* @see http://stackoverflow.com/a/11818183/522444
*/
public class VerifierEg {

    private static final String ZERO = "0.0";
    private JTextField field1 = new JTextField(ZERO, 5);
    private JTextField field2 = new JTextField(ZERO, 5);
    private JTextField resultField = new JTextField(ZERO, 10);

    private void createAndShowGui() {
        resultField.setEditable(false);
        resultField.setFocusable(false);

        JPanel mainPanel = new JPanel();
        final JTextField[] fields = {field1, field2};

        mainPanel.add(field1);
        mainPanel.add(new JLabel(" x "));
        mainPanel.add(field2);
        mainPanel.add(new JLabel(" = "));
        mainPanel.add(resultField);

        for (JTextField field : fields) {
            field.setInputVerifier(new MyInputVerifier(field));
        }

        JFrame frame = new JFrame("VerifierEg");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.getContentPane().add(mainPanel);
        frame.pack();
        frame.setLocationByPlatform(true);
        frame.setVisible(true);
    }

    private void calcProduct() {
        double d1 = Double.parseDouble(field1.getText());
        double d2 = Double.parseDouble(field2.getText());
        double prod = d1 * d2;
        resultField.setText(String.valueOf(prod));
    }

    public static void main(String[] args) {
        SwingUtilities.invokeLater(new Runnable() {

            @Override
            public void run() {
                VerifierEg eg = new VerifierEg();
                eg.createAndShowGui();
            }
        });
    }

    /**
    * @see http://stackoverflow.com/a/11818946/230513
    */
    private class MyInputVerifier extends InputVerifier {

        private JTextField field;
        private double value;

        public MyInputVerifier(JTextField field) {
            this.field = field;
        }

        @Override
        public boolean shouldYieldFocus(JComponent input) {
            if (verify(input)) {
                field.setText(String.valueOf(value));
                calcProduct();
                return true;
            } else {
                field.setText(ZERO);
                field.selectAll();
                return false;
            }

        }

        @Override
        public boolean verify(JComponent input) {
            try {
                value = Double.parseDouble(field.getText());
                return true;
            } catch (NumberFormatException e) {
                return false;
            }
        }
    }
}

Solution 3:

use JSpinner or JFormattedTextField with Number instance, then DocumentListener should be works correctly, no needed to parse String to Number instance
otherwise you have to use DocumentFilter for JTextField for filtering non numeric chars, rest (counting) stays unchanged, stil required robust parsing String to the Number instance

DocumentListener Java, How do I prevent empty string in JTextBox?

Solution 1:

Solution 2:

Solution 3:

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