Como troco o Look?

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

public class Calc extends JFrame
{
	
	private JButton b[];
	private String numbers[] = { "7", "8", "9", "/", "4", "5", "6", "*", "1", "2", "3", "-", "0", ".", "=", "+" };
	private JTextField t;
	private JPanel p;	

	public Calc()
	{
		super( "Calculadora Bacaninha" );	//titulo da janela
		p = new JPanel();
		p.setLayout( new GridLayout( 4, 4 ) );
		
		t = new JTextField(10);
		
		Container c = getContentPane();//container p/ jogar objetos
		c.setLayout ( new BorderLayout() );
		
		//instancia os objetos de botão
		b = new JButton[ numbers.length ];

		for( int i = 0; i < numbers.length; i++ ){
			b[ i ] = new JButton( numbers[ i ] );
		    p.add( b[ i ] );	//ordem dos botoes
		}

		c.add( t, BorderLayout.NORTH );
		c.add( p, BorderLayout.CENTER );
		
	}
	public static void main( String x[] )
	{
		Calc cal = new Calc();

		cal.setSize( 300, 200 );
		cal.show();
		cal.addWindowListener( new WindowAdapter() {
			public void windowClosing( WindowEvent e )
			{
				System.exit( 0 );
			}
		}
	);	
	}
}