Calcular area confuso demais

package br.com.swing;

import java.awt.GridLayout;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JPanel;
import javax.swing.JTextField;

public class FrameRetanguloTeste extends FrameRetangulo implements
		ActionListener {

	/**
	 * 
	 */
	private static final long serialVersionUID = 1L;

	private JLabel lTitulo;
	private JLabel lBase;
	private JLabel lAltura;
	private JLabel lArea;

	private JTextField tBase;
	private JTextField tAltura;
	private JTextField tArea;

	private JButton bCalcular;

	private JPanel painelTitulo;
	private JPanel painelLabels;
	private JPanel painelTextField;
	private JPanel painelBotao;

	public void mostraGUI() {
		setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

		lTitulo = new JLabel("Avaliação N2 - A");
		lBase = new JLabel("Base (CM)   ");
		lAltura = new JLabel("Altura (CM)");
		lArea = new JLabel("Area");

		tBase = new JTextField(10);
		tAltura = new JTextField(10);
		tArea = new JTextField(10);

		bCalcular = new JButton("Calcular Area");

		// Titulo no Frame
		painelTitulo = new JPanel();
		painelTitulo.add(lTitulo);
		add(painelTitulo);

		// Adicionando os Labels a esquerda
		painelLabels = new JPanel();
		GridLayout lLabels = new GridLayout(3, 0);
		painelLabels.setLayout(lLabels);
		painelLabels.add(lBase);
		painelLabels.add(lAltura);
		painelLabels.add(lArea);
		add(painelLabels, "West");

		// Adicionando o botão ao sul
		painelBotao = new JPanel();
		painelBotao.add(bCalcular, "Center");
		add(painelBotao, "South");

		// Adicionando os TextField
		painelTextField = new JPanel();
		painelTextField.setLayout(new GridLayout(3, 0));
		painelTextField.add(tBase);
		painelTextField.add(tAltura);
		painelTextField.add(tArea);
		add(painelTextField, "Center");

		bCalcular.addActionListener(this);

		setSize(300, 150);
		setLocation(10, 10);
		setVisible(true);

	}

	// Action Listener dos botões
	public void actionPerformed(ActionEvent e) {
		// Verifica se o botão calcular foi pressionado
		if (e.getSource() == bCalcular) {
			setAltura(Integer.parseInt(tAltura.getText()));
			setBase(Integer.parseInt(tBase.getText()));
			tArea.setText(Double.toString((area())));

		}

	}

	public static void main(String[] args) {
		FrameRetanguloTeste frt = new FrameRetanguloTeste();
		frt.mostraGUI();
	}
}
//**************************************************************************************************************************************//

package br.com.swing;

import javax.swing.JFrame;

public class FrameRetangulo extends JFrame{   
  
    /**
	 * 
	 */
	private static final long serialVersionUID = 1L;
	
	private int base;   
    private int altura; 
    
	public double area(){  
    	double area  = (double)base*altura;
        return area;
    }   
	
    public int getBase() {
		return base;
	}
	public void setBase(int base) {
		this.base = base;
	}
	public int getAltura() {
		return altura;
	}
	public void setAltura(int altura) {
		this.altura = altura;
	}

       
}