Swing

Focus

Review the focus specification and focus tutorial.

When needing to control focus on window/dialog open, if possible, avoid the use of requestFocus completely and use a custom FocusTraversalPolicy with an overridden getDefaultComponent. Otherwise, use requestFocusInWindow instead of requestFocus or grabFocus:

• requestFocus(): Note that the use of this method is discouraged because its behavior is platform dependent. Instead we recommend the use of requestFocusInWindow().

• grabFocus(): Client code should not use this method; instead, it should use requestFocusInWindow().

In general, avoid performing cross-window focus requests:

There is no foolproof way, across all platforms, to ensure that a window gains the focus. [...] If you want to ensure that a particular component gains the focus the first time a window is activated, you can call the requestFocusInWindow method on the component after the component has been realized, but before the frame is displayed. Alternatively, you can apply a custom FocusTraversalPolicy to the frame and call the getDefaultComponent method to determine which component will gain the focus.

Hello World

import java.awt.Component;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import javax.swing.BoxLayout;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JPanel;
import javax.swing.SwingUtilities;
import javax.swing.border.EmptyBorder;

public class SwingHelloWorld {
    public static void main(String[] args) {
        SwingUtilities.invokeLater(new Runnable() {
            @Override
            public void run() {
                new SwingHelloWorld().createAndShowMainFrame();
            }
        });
    }
    
    private int clicked = 0;

    private void createAndShowMainFrame() {
        JFrame frame = new JFrame("Frame");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

        JPanel panel = new JPanel();
        panel.setLayout(new BoxLayout(panel, BoxLayout.Y_AXIS));

        JLabel label1 = new JLabel("Hello World");
        label1.setBorder(new EmptyBorder(10, 10, 10, 10));
        label1.setAlignmentX(Component.CENTER_ALIGNMENT);
        
        JLabel label2 = new JLabel("Clicked: " + clicked);
        label2.setBorder(new EmptyBorder(10, 10, 10, 10));
        label2.setAlignmentX(Component.CENTER_ALIGNMENT);

        JButton button = new JButton("Button");
        button.setAlignmentX(Component.CENTER_ALIGNMENT);
        button.addActionListener(new ActionListener() {
            @Override
            public void actionPerformed(ActionEvent e) {
                label2.setText("Clicked: " + (++clicked));
            }
        });
        
        panel.add(label1);
        panel.add(button);
        panel.add(label2);

        frame.getContentPane().add(panel);

        frame.pack();
        frame.setVisible(true);
    }
}

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