public class MyWinAppCreate extends JFrame implements ActionListener {

    public MyWinAppCreate() {
        super("My Frame");
	...
        Container con = getContentPane();
	JTabbedPane tab = new JTabbedPane();
        JPanel jpnl[] = new JPanel[5];
        for (int i = 0; i < 5; i++) {
            jpnl[i] = new JPanel();
            jpnl[i].setLayout(null);
            //и так пробовала
            //jpnl[i].setBounds(0, 0, con.getWidth(), con.getHeight()-50);
            //и так
	    //jpnl[i].setBounds(0, 0, con.getWidth(), 300);
            //и даже так
	    jpnl[i].setBounds(0, 0, this.getWidth(), this.getHeight()-50);
            tab.addTab(nametabs[i], jpnl[i]);
        }
	...
}