Swing

package exemplos;

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;

public class TesteLookAndFeel extends JFrame {
	
	private String strings[] = {"Metal", "Motif", "Windows"};
	private UIManager.LookAndFeelInfo aparencia[];
	private JRadioButton radio[];
	private ButtonGroup grupo;
	private JButton botao;
	private JLabel rotulo;
	private JComboBox comboBox;
	private JTextField campo;
	private JTextArea texto;
	
	//configura a GUI
	public TesteLookAndFeel() {
		super("Testando a aparência e o comportamento");
		
		Container container = getContentPane();
		
		// configura painel para a região NORTH de BorderLayout
		JPanel painelNorte = new JPanel();
		painelNorte.setLayout(new GridLayout( 2, 2, 5, 5));
		
		//configura rótulo para o painel NORTH
		rotulo = new JLabel("Esta é a aparência Metal");
		rotulo.setVerticalTextPosition(SwingConstants.CENTER);
		container.add(rotulo);
		
		//configura o botão para o painel NORTH
		botao = new JButton("Eu sou um Botão");
		painelNorte.add( botao );
		
		campo = new JTextField("Qualquer texto");
		painelNorte.add(campo);
		
		//configura caixa de combinação para o painel NORTH
		comboBox = new JComboBox(strings);
		painelNorte.add( comboBox );
		
		//anexa o painelNorte à região NORTH do painel de conteúdo
		container.add( painelNorte, BorderLayout.NORTH );
		
		//cria array para os botões de opção
		radio = new JRadioButton[3];
		
		//configura painel para a região SOUTH de BorderLayout
		JPanel painelSul = new JPanel();
		painelSul.setLayout( new GridLayout( 1, 3 ) );
		
		//configura botões de opção para o painelSul
		radio = new JRadioButton[ 3 ];
				radio[ 0 ] = new JRadioButton( "Metal" );
				radio[ 1 ] = new JRadioButton( "Motif" );
				radio[ 2 ] = new JRadioButton( "Windows" );
				
		grupo = new ButtonGroup(); //implementa exclusão mútua
		TratadorDeItens trat = new TratadorDeItens();
		
		for (int count = 0; count < radio.length; count++) {
			radio[ count ].addItemListener( trat );
			grupo.add( radio[ count ] );
			painelSul.add( radio[ count ] );
		}
		
		//anexa o painelSul à região SOUTH do painel de conteúdo
		container.add( painelSul, BorderLayout.SOUTH );
		
		//obtém informações sobre a aparência e
		//comportamento instalado
		
		aparencia = UIManager.getInstalledLookAndFeels();
		
		setSize( 400, 300 );
		setVisible( true );
		radio[ 0 ].setSelected( true );
	}
	
	//usa UIManager para mudar a aparência e comportamento da GUI
	private void mudeTheLookAndFeel( int valor ) 
	{
		// muda aparência e comportamento
		try{
			UIManager.setLookAndFeel(
			aparencia[ valor ].getClassName());
			SwingUtilities.updateComponentTreeUI( this );
		}
		// processa problemas com a mudança da aparência e
		// do comportamento
		catch ( Exception exception ) {
			exception.printStackTrace();
		}
	}
	
	// executa a aplicação
	
	public static void main( String args[] ){
		TesteLookAndFeel aplicacao = new TesteLookAndFeel();
		aplicacao.setDefaultCloseOperation(
		JFrame.EXIT_ON_CLOSE );
	}
	
	// classe interna privativa para tratar eventos dos botões de opção
	private class TratadorDeItens implements ItemListener {
		
		// processa a seleção de aparência e comportamento
		// feita pelo usuário
		public void itemStateChanged( ItemEvent evento ){
			for( int count = 0; count < radio.length; count++ ){
				if( radio[ count ].isSelected() ){
					rotulo.setText( "Esta é a aparência " +  
					strings[ count ]);
					comboBox.setSelectedIndex( count );
					mudeTheLookAndFeel( count );
				}
			}
		}
	}

}

[list]Exception in thread “AWT-EventQueue-0” java.lang.NullPointerException

at javax.swing.plaf.basic.BasicComboBoxUI.setPopupVisible(Unknown Source)

at javax.swing.JComboBox.setPopupVisible(Unknown Source)

at javax.swing.JComboBox.hidePopup(Unknown Source)

at javax.swing.JComboBox$1.ancestorAdded(Unknown Source)

at javax.swing.AncestorNotifier.fireAncestorAdded(Unknown Source)

at javax.swing.AncestorNotifier.componentShown(Unknown Source)

at java.awt.AWTEventMulticaster.componentShown(Unknown Source)

at java.awt.Component.processComponentEvent(Unknown Source)

at java.awt.Component.processEvent(Unknown Source)

at java.awt.Container.processEvent(Unknown Source)

at java.awt.Window.processEvent(Unknown Source)

at java.awt.Component.dispatchEventImpl(Unknown Source)

at java.awt.Container.dispatchEventImpl(Unknown Source)

at java.awt.Window.dispatchEventImpl(Unknown Source)

at java.awt.Component.dispatchEvent(Unknown Source)

at java.awt.EventQueue.dispatchEvent(Unknown Source)

at java.awt.EventDispatchThread.pumpOneEventForFilters(Unknown Source)

at java.awt.EventDispatchThread.pumpEventsForFilter(Unknown Source)

at java.awt.EventDispatchThread.pumpEventsForHierarchy(Unknown Source)

at java.awt.EventDispatchThread.pumpEvents(Unknown Source)

at java.awt.EventDispatchThread.pumpEvents(Unknown Source)

at java.awt.EventDispatchThread.run(Unknown Source)

[/list]

SwingUtilities.updateComponentTreeUI( this );