Is it possible to calculate JTextFields into a JLabel as they are being typed?

Question

Is it possible to calculate a group of JTextFields as they are typed in and set the result to a JLabel?

For Example:

int iCalc = 0;
JTextField x = new JTextField();
JTextField y = new JTextField();
JTextField z = new JTextField();

JLabel lCalc = new JLabel("Your total is: " + calc);

As i am typing, calc gets calculated. I put in x "12" and 12 is shown. and in y i put "10" 22 is automatically typed in.. ect, ect.

To clarify "automatically" i don't want to use key listener (like pressing tab to for next TextField) (unless it is absolutely necessary) And I don't want to hit a button to do the calculations

Its much like typing Hello World into a String: as I Type the Label will show: "H", "e", "l" ect, ect.

Hope I clarified enough

Answer 1

You can use a DocumentListener for this purpose, add it to the Document of each JTextField in question and perform the addition operation in the insertUpdate(...) and removeUpdate(...) method of the DocumentListener interface.

Here is one working example, for further insight :

import java.awt.*;
import javax.swing.*;
import javax.swing.event.DocumentEvent;
import javax.swing.event.DocumentListener;

public class DocumentListenerExample {

    private JPanel contentPane;
    private JPanel compPanel;
    private JTextField firstField;
    private JTextField secondField;
    private JTextField thirdField;
    private JLabel resultLabel;

    private static final String RESULT = "Result : ";
    private int result;

    private void displayGUI() {
        JFrame frame = new JFrame("Document Listener Example");
        frame.setDefaultCloseOperation(JFrame.DISPOSE_ON_CLOSE);

        contentPane = new JPanel(new GridBagLayout());
        compPanel = new JPanel(new BorderLayout(5, 5));

        JPanel fieldPanel = new JPanel(new GridLayout(3, 3, 5, 5));
        JLabel firstLabel = new JLabel("First Number : ", JLabel.CENTER);
        firstField = new JTextField(10);
        firstField.getDocument().addDocumentListener(
                                    new MyDocumentListener());
        JLabel secondLabel = new JLabel("Second Number : ", JLabel.CENTER);
        secondField = new JTextField(10);
        secondField.getDocument().addDocumentListener(
                                    new MyDocumentListener());
        JLabel thirdLabel = new JLabel("Third Number : ", JLabel.CENTER);
        thirdField = new JTextField(10);
        thirdField.getDocument().addDocumentListener(
                                    new MyDocumentListener());
        fieldPanel.add(firstLabel);
        fieldPanel.add(firstField);
        fieldPanel.add(secondLabel);
        fieldPanel.add(secondField);
        fieldPanel.add(thirdLabel);
        fieldPanel.add(thirdField);

        JPanel footerPanel = new JPanel();
        resultLabel = new JLabel((RESULT + "0"), JLabel.CENTER);
        footerPanel.add(resultLabel);

        compPanel.add(fieldPanel, BorderLayout.CENTER);
        compPanel.add(footerPanel, BorderLayout.PAGE_END);
        contentPane.add(compPanel);

        frame.setContentPane(contentPane);
        frame.pack();
        frame.setLocationByPlatform(true);
        frame.setVisible(true);
    }

    private class MyDocumentListener implements DocumentListener {

        @Override
        public void changedUpdate(DocumentEvent de) {}

        @Override
        public void insertUpdate(DocumentEvent de) {
            calculateSum();
        }

        @Override
        public void removeUpdate(DocumentEvent de) {
            calculateSum();
        }

        private void calculateSum() {
            int first = 0, second = 0, third = 0;
            if (firstField.getDocument().getLength() > 0)
                first = Integer.parseInt(firstField.getText()); 
            if (secondField.getDocument().getLength() > 0)
                second = Integer.parseInt(secondField.getText());
            if (thirdField.getDocument().getLength() > 0)
                third = Integer.parseInt(thirdField.getText());
            result = first + second + third;
            resultLabel.setText(RESULT + result);
        }
    }

    public static void main(String[] args) {
        Runnable runnable = new Runnable() {
            @Override
            public void run() {
                new DocumentListenerExample().displayGUI();
            }
        };
        EventQueue.invokeLater(runnable);
    }
}

You might have to add a DocumentFilter too to the Document to only allow putting Digit inside the JTextField, instead of any random character. Here is a related example