Java Swing : why must resize frame, so that can show components have added

• Do not add components to JFrame after the JFrame is visible (setVisible(true))

• Not really good practice to call setSize() on frame rather call pack() (Causes JFrame to be sized to fit the preferred size and layouts of its subcomponents) and let LayoutManager handle the size.

• Use EDT (Event-Dispatch-Thread)

• call JFrame#setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE) as said by @Gilbert Le Blanc (+1 to him) or else your EDT/Initial thread will remain active even after JFrame has been closed

Like so:

public static void main(String[] args) {
        //Create GUI on EDT Thread
        SwingUtilities.invokeLater(new Runnable() {
            @Override
            public void run() {

                  JFrame frame = new JFrame("JScroll Pane Test");
                  frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);  

                  JTextArea txtNotes = new JTextArea();
                  txtNotes.setText("Hello World");
                  JScrollPane scrollPane = new JScrollPane(txtNotes);
                  frame.add(scrollPane);//add components

                  frame.pack();
                  frame.setVisible(true);//show (after adding components)
            }
        });
}