SwingCurrencyConverter.java

// school work
import java.awt.*; //using awt's layouts
import java.awt.event.*; //using awt's event classes and listener interfaces
import javax.swing.*; //using swing's components and container

//a swing application extends from javax.swing.JFrame
public class SwingCurrencyConverter extends JFrame {
    private JTextField tfSGD, tfUSD, tfEuro;
    private float singapore, usa, euro;

    public SwingCurrencyConverter() {
        //swing components should be added to the content-pane of the JFrame
        Container cp = getContentPane();
        //set this container to grid layout of 3 rows and 2 columns
        cp.setLayout(new GridLayout(3, 2, 10, 3));

        //components are added left-to-right, top-to-bottom
        cp.add(new JLabel("Singapore Dollars "));
        tfSGD = new JTextField(10);
        tfSGD.setHorizontalAlignment(JTextField.RIGHT);
        cp.add(tfSGD);

        tfSGD.addActionListener(new ActionListener() {
            @Override
            public void actionPerformed(ActionEvent evt) {
                singapore = Float.parseFloat(tfSGD.getText());

                usa = (float) (singapore * 0.74);
                euro = (float) (singapore * 0.65);

                tfUSD.setText(String.format("%.2f", usa));
                tfEuro.setText(String.format("%.2f", euro));
            }
        });

        cp.add(new JLabel("US Dollars "));
        tfUSD = new JTextField(10);
        tfUSD.setHorizontalAlignment(JTextField.RIGHT);
        cp.add(tfUSD);

        tfUSD.addActionListener(new ActionListener() {
            @Override
            public void actionPerformed(ActionEvent evt) {
                usa = Float.parseFloat(tfUSD.getText());

                singapore = (float) (usa * 1.41);
                euro = (float) (usa * 0.92);

                tfSGD.setText(String.format("%.2f", singapore));
                tfEuro.setText(String.format("%.2f", euro));
            }
        });

        cp.add(new JLabel("Euros "));
        tfEuro = new JTextField(10);
        tfEuro.setHorizontalAlignment(JTextField.RIGHT);
        cp.add(tfEuro);

        tfEuro.addActionListener(new ActionListener() {
            @Override
            public void actionPerformed(ActionEvent evt) {
                euro = Float.parseFloat(tfEuro.getText());

                singapore = (float) (euro * 1.44);
                usa = (float) (euro * 1.07);

                tfUSD.setText(String.format("%.2f", usa));
                tfSGD.setText(String.format("%.2f", singapore));
            }
        });

        setDefaultCloseOperation(EXIT_ON_CLOSE); // for the "window-close" button
        setTitle("Swing Currency Converter");
        setSize(300, 170);
        setVisible(true);
    }

    // The entry main() method
    public static void main(String[] args) {
        // For thread safety, use the event-dispatching thread to construct UI
        javax.swing.SwingUtilities.invokeLater(new Runnable() {
            @Override
            public void run() {
                new SwingCurrencyConverter(); // Let the constructor does the job
            }
        });
    }
}