I prefer GridBagLayout:
final JPanel panel2 = new JPanel();
panel2.setLayout(new GridBagLayout());
panel2.setBackground(Color.WHITE);
final GridBagConstraints cons = new GridBagConstraints();
cons.weightx = 1D;
cons.weighty = 1D;
cons.anchor = GridBagConstraints.NORTHWEST;
panel2.add(box, cons);
tabbedPane.addTab("Balance", panel2);
note: box is not needed