Java 简易计算器程序

    最近开始学习java，参考教科书写了一个简单的计算器程序，贴上来，以便有用的时候可以参考。
    呵呵，哈哈！！！
import java.awt.Container;
import java.awt.FlowLayout;
import java.awt.GridLayout;
import java.awt.Point;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.awt.event.ItemEvent;
import java.awt.event.ItemListener;
import java.text.DecimalFormat;
import javax.swing.*;
public class Calculator extends JFrame implements ActionListener, ItemListener {

private JButton buttons[];
private JButton btClear, btTransfer;
private JTextField resultArea;
private JPanel resultPanel, buttonsPanel, transferPanel;
int cmbIndex = 1;
private String string[] = { "7", "8", "9", "/", "6", "5", "4", "*", "3",
"2", "1", "-", "0", ".", "=", "+" };
private String s = "", shu1 = "", shu2 = "", shu3 = "";
private static int fo = 0;
JComboBox cmbox;
private boolean ff = true;
public Calculator() {
super("进制计算器");
Container container = getContentPane();
container.setLayout(new FlowLayout());

resultPanel = new JPanel();
resultArea = new JTextField(13);
resultPanel.add(resultArea);
btClear = new JButton("C");
resultPanel.add(btClear);

transferPanel = new JPanel();
cmbox = new JComboBox();
cmbox.addItem("10进制转换为2进制");
cmbox.addItem("10进制转换为8进制");
cmbox.addItem("10进制转换为16进制");
cmbox.addItem("2进制转换为8进制");
cmbox.addItem("2进制转换为10进制");
cmbox.addItem("2进制转换为16进制");
cmbox.addItem("8进制转换为2进制");
cmbox.addItem("8进制转换为10进制");
cmbox.addItem("8进制转换为16进制");
cmbox.addItem("16进制转换为2进制");
cmbox.addItem("16进制转换为8进制");
cmbox.addItem("16进制转换为10进制");
transferPanel.add(cmbox);
btTransfer = new JButton("转");
transferPanel.add(btTransfer);

buttonsPanel = new JPanel();
buttonsPanel.setLayout(new GridLayout(4, 4, 10, 10));
butto