Calculadora.java

import javax.swing.JFrame;
import javax.swing.JTextField;
import javax.swing.JButton;
import java.awt.FlowLayout;
import java.awt.Dimension;
import java.awt.event.ActionListener;
import java.awt.event.ActionEvent;

public class Calculadora extends JFrame
{
    private JTextField ac;
    private JTextField input;
    private JButton div;
    private JButton mul;
    private JButton res;
    private JButton sum;

    public Calculadora()
    {
        setTitle("Calculadora");
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        setLayout(new FlowLayout());
        prepare();
        pack();
    }

    private void prepare(){
        Dimension size = new Dimension(45,30);
        ActionListener l = new ActionListener()
        {
            @Override
            public void actionPerformed(ActionEvent e)
            {
                double rsp = 0;
                String op = e.getActionCommand();
                switch(op){
                    case SUMA :
                        rsp = getAcumulador() +
                              getEntrada();
                        break;
                    case RESTA :
                        rsp = getAcumulador() -
                              getEntrada();
                        break;
                    case MULTI :
                        rsp = getAcumulador() *
                              getEntrada();
                        break;
                    case DIVI :
                        rsp = getAcumulador() /
                              getEntrada();
                        break;
                }
                ac.setText(Double.toString(rsp));
            }
        };

        ac = new JTextField("0.0");
        input = new JTextField();
        mul = new JButton(MULTI);
        div = new JButton(DIVI);
        res = new JButton(RESTA);
        sum = new JButton(SUMA);

        mul.setActionCommand(MULTI);
        div.setActionCommand(DIVI);
        res.setActionCommand(RESTA);
        sum.setActionCommand(SUMA);

        mul.addActionListener(l);
        div.addActionListener(l);
        res.addActionListener(l);
        sum.addActionListener(l);

        ac.setPreferredSize(size);
        input.setPreferredSize(size);
        mul.setPreferredSize(size);
        div.setPreferredSize(size);
        res.setPreferredSize(size);
        sum.setPreferredSize(size);

        ac.setEnabled(false);

        add(ac);
        add(input);
        add(sum);
        add(res);
        add(mul);
        add(div);
    }

    private double getAcumulador()
    {
        return Double.parseDouble(ac.getText());
    }

    private double getEntrada()
    {
        return Double.parseDouble(input.getText());
    }


    private static final String SUMA = "+";
    private static final String RESTA = "-";
    private static final String MULTI = "*";
    private static final String DIVI = "/";

    public static void main(String[] args)
    {
        (new Calculadora()).setVisible(true);
    }
}