/* funzione di custom */
void customBtn(JButton abtn){
abnt.setPreferredSize(DIMENSION_BUTTON);
abnt.setMaximumSize(DIMENSION_BUTTON);
abnt.setMinimumSize(DIMENSION_BUTTON);
}

btn1=new JButton();
customBtn(btn1);

btn2=new JButton();
customBtn(btn2);

btn3=new JButton();
customBtn(btn3);

JPanel pnlButtons=new JPanel();
pnlButtons.setLayout(new BoxLayout(pnlButtons,BoxLayout.Y_AXIS));
pnlButtons.add(btn1);
pnlButtons.add(btn2);
pnlButtons.add(btn3);