Set Upper Left Corner with JPanel

JPanel panel = new JPanel();
panel.setBackground(Color.PINK);
jScrollPane1.setCorner(ScrollPaneConstants.UPPER_LEFT_CORNER, panel);

Set Upper Left Corner with JButton

JButton button = new JButton();
button.addActionListener(new ActionListener(){
    @Override
    public void actionPerformed(ActionEvent e) {
        addTextToTextArea("Upper Left Corner is a Button\n\n");
    }
});
button.setBackground(Color.PINK);
jScrollPane1.setCorner(ScrollPaneConstants.UPPER_LEFT_CORNER, button);

Size of Upper Left Corner

Component corner = jScrollPane.getCorner(ScrollPaneConstants.UPPER_LEFT_CORNER);
jLabel10.setText("Upper Left Corner: "+corner.getSize().width+"w "+corner.getSize().height+"h");

Set Upper Right Corner with JPanel

JPanel panel = new JPanel();
panel.setBackground(Color.ORANGE);
jScrollPane1.setCorner(ScrollPaneConstants.UPPER_RIGHT_CORNER, panel);

Set Upper Right Corner with JTable Corner Component

if (jScrollPane1.getViewport() != null && jScrollPane1.getViewport().getView() != null) {
    if (jScrollPane1.getViewport().getView() instanceof JTable) {
        Component corner = (Component) UIManager.get("Table.scrollPaneCornerComponent");
        jScrollPane1.setCorner(ScrollPaneConstants.UPPER_RIGHT_CORNER, corner);
    }
}

Set Upper Right Corner with JButton

JButton button = new JButton();
button.addActionListener(new ActionListener(){
    @Override
    public void actionPerformed(ActionEvent e) {
        addTextToTextArea("Upper Right Corner Button\n\n");
    }
});
button.setBackground(Color.ORANGE);
jScrollPane1.setCorner(ScrollPaneConstants.UPPER_RIGHT_CORNER, button);

Size of Upper Right Corner

Component corner = jScrollPane.getCorner(ScrollPaneConstants.UPPER_RIGHT_CORNER);
jLabel22.setText("Upper Right Corner: "+corner.getSize().width+"w "+corner.getSize().height+"h");

Set Lower Left Corner with JPanel

JPanel panel = new JPanel();
panel.setBackground(Color.CYAN);
jScrollPane1.setCorner(ScrollPaneConstants.LOWER_LEFT_CORNER, panel);

Set Lower Left Corner with JButton

JButton button = new JButton();
button.addActionListener(new ActionListener(){
    @Override
    public void actionPerformed(ActionEvent e) {
        addTextToTextArea("Lower Left Corner Button\n\n");
    }
});
button.setBackground(Color.CYAN);
jScrollPane1.setCorner(ScrollPaneConstants.LOWER_LEFT_CORNER, button);

Size of Lower Left Corner

Component corner = jScrollPane1.getCorner(ScrollPaneConstants.LOWER_LEFT_CORNER);
jLabel28.setText("Lower Left Corner: "+corner.getSize().width+"w "+corner.getSize().height+"h");

Set Lower Right Corner with JPanel

JPanel panel = new JPanel();
panel.setBackground(Color.MAGENTA);
jScrollPane1.setCorner(ScrollPaneConstants.LOWER_RIGHT_CORNER, panel);

Set Lower Right Corner with JButton

JButton button = new JButton();
button.addActionListener(new ActionListener(){
    @Override
    public void actionPerformed(ActionEvent e) {
        addTextToTextArea("Lower Right Corner Button\n\n");
    }
});
button.setBackground(Color.MAGENTA);
jScrollPane1.setCorner(ScrollPaneConstants.LOWER_RIGHT_CORNER, button);

Size of Lower Left Corner

Component corner = jScrollPane1.getCorner(ScrollPaneConstants.LOWER_RIGHT_CORNER);
jLabel25.setText("Lower Left Corner: "+corner.getSize().width+"w "+corner.getSize().height+"h");