How to change the value of the JTextField?

Question

I am trying to built a Java application using JFrame with Swing, and I have 5 JTextField instances in there. One of these is for Sum.

I need JTextField to change automatically as soon as I type some number in text fields.

How is that possible?

What I wrote is here.

private void displaytotalActionPerformed(java.awt.event.ActionEvent evt) {
// display total:
Float num1,num2,num3,num4,num5,num6,result;
num1 = display1b.getText().equals("") ? 0 : Float.parseFloat(display1b.getText());
num2 = display2b.getText().equals("") ? 0 : Float.parseFloat(display2b.getText());
num3 = display3b.getText().equals("") ? 0 : Float.parseFloat(display3b.getText());
num4 = display4b.getText().equals("") ? 0 : Float.parseFloat(display4b.getText());
num5 = display5b.getText().equals("") ? 0 : Float.parseFloat(display5b.getText());
num6 = display6b.getText().equals("") ? 0 : Float.parseFloat(display6b.getText());

result = num1+num2+num3+num4+num5+num6;

System.out.println(result);
}

I tried to get the sum and display it in this textfield using button and it worked. But i want it to be done automatically. But the code above doesnt display anything on textfield.

I am quite new to this and I appreciate if you kindly guide.

Answer 1

I think you're looking for

• the setText() method of a JTextField.
• Writing a DocumentListener to know when the text changes and apply sum to it
• Writing a DocumentFilter to make sure your program does not crash with input different to numbers.

An alternative to this is to use a JFormattedTextField and using listeners.

A couple of recommendations.

• Develop intuition, usually properties to java classes are changed by (get|set)Property. Use an IDE like Netbeans and it will help you finding stuff.
• Automatization is usually achieved by using a Listener, just learn when to use which (this is also part of intuition).
• When you find yourself writing repeated code, consider using a function. i.e.

Instead of having 6 times:

display1b.getText().equals("") ? 0 : Float.parseFloat(display1b.getText());

Consider having your fields in an ArrayList and write a function that iterates them and have a single line of the above to set all the values.

• Follow the JavaTutorials before additional efforts of hard-coding, or asking here. Will be more productive to you. Because you will learn how to learn for yourself and dig into documentation.

Answer 2

You need to use a DocumentListener attached to each of the number fields. This will alert you when the fields have changed.

From there you need to call some sort of sum method to automatically update the tally.

Something like...

public class AutoSum {

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

    public AutoSum() {
        EventQueue.invokeLater(new Runnable() {
            @Override
            public void run() {
                try {
                    UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());
                } catch (ClassNotFoundException | InstantiationException | IllegalAccessException | UnsupportedLookAndFeelException ex) {
                }

                JFrame frame = new JFrame("Testing");
                frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
                frame.setLayout(new BorderLayout());
                frame.add(new AutoSumPane());
                frame.pack();
                frame.setLocationRelativeTo(null);
                frame.setVisible(true);
            }
        });
    }

    public class AutoSumPane extends JPanel {

        private JTextField[] fields;
        private JTextField tally;

        public AutoSumPane() {

            fields = new JTextField[5];

            DocumentListener docHandler = new DocumentListener() {
                @Override
                public void insertUpdate(DocumentEvent e) {
                    autoSum();
                }

                @Override
                public void removeUpdate(DocumentEvent e) {
                    autoSum();
                }

                @Override
                public void changedUpdate(DocumentEvent e) {
                    autoSum();
                }
            };

            setLayout(new GridLayout(6, 1));
            for (int index = 0; index < 5; index++) {
                fields[index] = new JTextField(3);
                fields[index].getDocument().addDocumentListener(docHandler);
                fields[index].setHorizontalAlignment(JTextField.RIGHT);
                add(fields[index]);
            }

            tally = new JTextField(3);
            tally.setHorizontalAlignment(JTextField.RIGHT);
            tally.setEditable(false);

            add(tally);

        }

        protected float getValue(JTextField field) {
            float value = 0;
            String text = field.getText();
            if (text != null && text.trim().length() > 0) {
                try {
                    value = Float.parseFloat(text.trim());
                } catch (Exception e) {
                    e.printStackTrace();
                }
            }
            return value;
        }

        protected void autoSum() {
            float sum = 0;
            for (JTextField field : fields) {
                sum += getValue(field);
            }
            tally.setText(NumberFormat.getNumberInstance().format(sum));
        }
    }
}

for example.

Take a look at How to Write a DocumentListener for more information.

While you're at it, you might find Implementing a DocumentFilter of interest (and this for some examples)