Ho provato a modificare la gestione dell'evento nella seguente maniera:

codice:
		//crea pulsante
		JButton generaButton = new JButton("Genera");

		ggpanel = new ggPanel(HEIGHT, WIDTH, nNode, v);
		ggpanel.setVisible(true);
		contentPane.add(ggpanel, BorderLayout.CENTER);
		rngpanel = new rngPanel(HEIGHT, WIDTH, nNode, v);
		rngpanel.setVisible(true);
		contentPane.add(rngpanel, BorderLayout.CENTER);
		clumppanel = new clumpPanel(HEIGHT, WIDTH, nNode, v);
		clumppanel.setVisible(true);
		contentPane.add(clumppanel, BorderLayout.CENTER);
		mstapanel = new mstaPanel(HEIGHT, WIDTH, nNode, v);
		mstapanel.setVisible(true);
		contentPane.add(mstapanel, BorderLayout.CENTER);

		//associa un evento al bottone
		generaButton.addActionListener(new
			ActionListener(){
				public void actionPerformed(ActionEvent event){
					System.out.println("***DEBUG hai scelto "+ faceCombo.getSelectedItem() );
					if(faceCombo.getSelectedItem()=="Gabriel Network"){
                         			rngpanel.setVisible(false);
                         			clumppanel.setVisible(false);
                         			mstapanel.setVisible(false);
                         			ggpanel.setVisible(true);
                         		}
                         		if(faceCombo.getSelectedItem()=="Relative Neighborhood Network"){
                         			ggpanel.setVisible(false);
                         			clumppanel.setVisible(false);
                         			mstapanel.setVisible(false);
                         			rngpanel.setVisible(true);
                         		}
                         		if(faceCombo.getSelectedItem()=="Clump Network"){
                         			ggpanel.setVisible(false);
                         			rngpanel.setVisible(false);
                         			mstapanel.setVisible(false);
                         			clumppanel.setVisible(true);
                         		}
                         		if(faceCombo.getSelectedItem()=="Minimun Spanning Tree"){
                         			ggpanel.setVisible(false);
                         			rngpanel.setVisible(false);
                         			clumppanel.setVisible(false);
                         			mstapanel.setVisible(true);
                        		}
					System.out.println("***DEBUG hai inserito "+ nNodeField.getValue() +" nodi.");
				}//actionPerformed
			});//ActionListener
Ma mi visualizza solo il grafo del Minimum Spanning Tree mentre se seleziono un altro grafo dall'elenco non mi visualizza nulla.