Specification for javax.swing.JTabbedPane.getBoundsAt(int) says:

"If the tab at this index is not currently visible in the UI, then returns null."

This doesn't happen for all meanings of tab's visibility.

Please see the following code sample:

-----
import javax.swing.*;
import java.awt.*;

public class Tabs {

    public static void main(String[] args) {

        final JFrame jFrame = new JFrame();
        jFrame.setSize(100, 100);

        final JTabbedPane tabbedPane = new JTabbedPane();
        jFrame.add(tabbedPane);
        for (int i=0; i<50; i++) {
            final JPanel panel = new JPanel();
            panel.setBackground(new Color(i*5, i*5, i*5));
            tabbedPane.add("panel " + i, panel);
        }

        tabbedPane.setSelectedIndex(20);
        jFrame.setVisible(true);

        for (int i=0; i<50; i++) {
            System.out.println("i=" + i + ", getBoundsAt()=" + tabbedPane.getBoundsAt(i));
        }
    }
}
-----