JComboBox, ActionListener, How do I really use them?

Im currently learning java and stuck onto JComboBox. I have a feel things that I am trying out and hitting the wall for the pass 4 hours.

I am trying to let a user select 1-10 from a ComboBox. How do I get the value of the combobox? The value of the combo box is equivalent to quantity.

So I have another value which is maybe $10. If the user choose quantity 2.

I want to get the value of what the user choose, then take the value of $10 and times it by 2.

The result which is $20 will be displayed on the JTextField.

Please help :(

public class Panel extends JPanel {

    public Panel(){
        JPanel test = new JPanel(new GridBagLayout());

        String[] quantities1 = {"0","1","2","3","4","5","6","7","8","9","10"};
        JComboBox quantitiesCB = new JComboBox(quantities1);
        quantitiesCB.addActionListener(
                new ActionListener(){
                    public void actionPerformed(ActionEvent e){
                        JComboBox combo = (JComboBox)e.getSource();
                        String currentQuantity = (String)combo.getSelectedItem();
                    }
                }            
        );

        JTextField result = new JTextField();

        setLayout(new GridBagLayout());
        setPreferredSize(new Dimension(640,480));
        GridBagConstraints gbc = new GridBagConstraints();

        gbc.gridx = 0;
        gbc.gridy = 0;
        gbc.weightx = 0.1;
        gbc.weighty = 0.1;
        gbc.fill = GridBagConstraints.HORIZONTAL;
        gbc.anchor = GridBagConstraints.NORTH;
        add(quantitiesCB, gbc);
    } 
}

Answers

Just few changes:

public class Panel extends JPanel {

    public Panel(){
        JPanel test = new JPanel(new GridBagLayout());
        String value = "10";
        final JTextField result = new JTextField();

        String[] quantities1 = {"0","1","2","3","4","5","6","7","8","9","10"};
        JComboBox quantitiesCB = new JComboBox(quantities1);
        quantitiesCB.addActionListener(
                new ActionListener(){
                    public void actionPerformed(ActionEvent e){
                        JComboBox combo = (JComboBox)e.getSource();
                        String currentQuantity = (String)combo.getSelectedItem();
                        int value1 = Integer.valueOf(value);
                        int value2 = Integer.valueOf(currentQuantity);

                        String resultText = String.valueOf(value1*value2);
                        result.setText("$" + resultText);
                    }
                }            
        );

        setLayout(new GridBagLayout());
        setPreferredSize(new Dimension(640,480));
        GridBagConstraints gbc = new GridBagConstraints();

        gbc.gridx = 0;
        gbc.gridy = 0;
        gbc.weightx = 0.1;
        gbc.weighty = 0.1;
        gbc.fill = GridBagConstraints.HORIZONTAL;
        gbc.anchor = GridBagConstraints.NORTH;
        add(quantitiesCB, gbc);
    } 
}

I am guessing that you are looking for a way to convert a String to an Integer. That can be done with Integer.valueOf.

Below is a very small/basic demo code that works (but I don't know exactly what you are looking for):

import java.awt.GridBagConstraints;
import java.awt.GridBagLayout;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;

import javax.swing.JComboBox;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JPanel;
import javax.swing.JTextField;
import javax.swing.SwingUtilities;

public class Panel extends JPanel {

    private JTextField result;
    private JTextField amount;

    public Panel() {
        setLayout(new GridBagLayout());
        String[] quantities1 = { "0", "1", "2", "3", "4", "5", "6", "7", "8", "9", "10" };
        JLabel dollar = new JLabel("$");
        amount = new JTextField(3);
        JComboBox quantitiesCB = new JComboBox(quantities1);
        quantitiesCB.addActionListener(new ActionListener() {
            @Override
            public void actionPerformed(ActionEvent e) {
                JComboBox combo = (JComboBox) e.getSource();
                String currentQuantity = (String) combo.getSelectedItem();
                int a;
                int q;
                try {
                    a = Integer.valueOf(amount.getText());
                    q = Integer.valueOf(currentQuantity);
                    result.setText("$" + String.valueOf(a * q));
                } catch (NumberFormatException e1) {
                    e1.printStackTrace();
                    // Some invalid number
                }
            }
        });
        JLabel equal = new JLabel("=");
        result = new JTextField(5);
        JLabel quantity = new JLabel("Quantity:");
        GridBagConstraints gbc = new GridBagConstraints();
        add(dollar, gbc);
        add(amount, gbc);
        add(quantity, gbc);
        add(quantitiesCB, gbc);
        add(equal, gbc);
        add(result, gbc);
    }

    public static void main(String[] args) {
        SwingUtilities.invokeLater(new Runnable() {
            @Override
            public void run() {
                JFrame frame = new JFrame("Test");
                frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
                frame.add(new Panel());
                frame.pack();
                frame.setLocationRelativeTo(null);
                frame.setVisible(true);
            }
        });
    }
}

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