Adjusting JPanels to correct size

Question

I have my code in this format, but the jpanels are not listening the size i set. All the three panels are in the same size when i run the code. Is there any way to adjust them?

JPanel mainpanel = new JPanel(mainPanel,BoxLayout.Y_AXIS);
JPanel tablepanel1 = new JPanel();
tablepanel1.setSize(900,400);
JPanel selectionpanel = new JPanel(new GridBagLayout());
selectionpanel.setSize(900,100);
JPanel tablepanel2 = new JPanel();
tablepanel2.setSize(900,400);

mainpanel.add(tablepanel1);
mainpanel.add(selectionpanel);
mainpanel.add(tablepanel2);

frame.add(mainpanel);
frame.setSize(900,900);

Answer 1

Did you mean to use BoxLayout for mainpanel?:

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

This layout manager only listens to preferred sizes rather than the component size. The best way is to override getPreferredSize:

JPanel selectionpanel = new JPanel() {
    @Override
    public Dimension getPreferredSize() {
        return new Dimension(900, 400);
    }
};

Also better to use JFrame#pack rather than setting dimensions for the frame.