if (! frame2.isVisible()) {
  frame2.setVisible(true);
}

public static void main(String[] args) throws Exception {
    java.awt.EventQueue.invokeLater(new Runnable() {
        
        private javax.swing.JFrame f1 = new javax.swing.JFrame();
        private javax.swing.JFrame f2 = new javax.swing.JFrame();
        
        @Override
        public void run() {
            f1.setSize(300, 300);
            f1.setLocationRelativeTo(null);
            f1.setDefaultCloseOperation(javax.swing.JFrame.EXIT_ON_CLOSE);
            
            f2.setSize(150, 150);
            f2.setLocationRelativeTo(f1);
            f2.setDefaultCloseOperation(javax.swing.JFrame.DISPOSE_ON_CLOSE);
            f2.setVisible(false);
            
            javax.swing.JButton b = new javax.swing.JButton();
            b.setText("Button");
            b.addActionListener(new ActionListener() {
                @Override
                public void actionPerformed(ActionEvent e) {
                    f2.setLocationRelativeTo(f1);
                    if (f2.isVisible()) {
                        f2.toFront();
                        f2.repaint();
                    } else {
                        f2.setVisible(true);
                    }
                }
            });
            f1.add(b);
            f1.setVisible(true);
        }
    });
}