利用AWT编写一个简易计算器

import java.applet.Applet;import java.awt.*;import java.awt.event.*;public class Caculator extends Applet implements ActionListener {// Label head=new Label("简易计算器");TextField input = new TextField(20);String str;int flag;double num1, num2, result;// public void itemStateChanged(ItemEvent e) {// }public void init() {// add(head);String buttons_name[] = { "1", "2", "3", "+", "4", "5", "6", "-", "7","8", "9", "0", "*", "/", "清空", "=" };str = "";Panel text = new Panel();text.setLayout(new FlowLayout());input.addActionListener(this);text.add(input);Panel buttons = new Panel();buttons.setLayout(new GridLayout(4, 4, 5, 5));Button btn[] = new Button[buttons_name.length];for (int i = 0; i < btn.length; i++) {btn[i] = new Button(buttons_name[i]);btn[i].setBackground(Color.pink);buttons.add(btn[i]);btn[i].addActionListener(this);}setLayout(new BorderLayout());// add("North",f);add("Center", text);add("South", buttons);// input.setText("0");setSize(250, 200);}public void actionPerformed(ActionEvent e) {if (e.getActionCommand().equals("0")) {if (str != "") {str = str + "0";input.setText(str);} else {input.setText(str);}}String s = e.getActionCommand();if (s.equals("1")) {str = str + "1";input.setText(str);}if (s.equals("2")) {str = str + "2";input.setText(str);}if (s.equals("3")) {str = str + "3";input.setText(str);}if (s.equals("4")) {str = str + "4";input.setText(str);}if (s.equals("5")) {str = str + "5";input.setText(str);}if (s.equals("6")) {str = str + "6";input.setText(str);}if (s.equals("7")) {str = str + "7";input.setText(str);}if (s.equals("8")) {str = str + "8";input.setText(str);}if (s.equals("9")) {str = str + "9";input.setText(str);}// 输入运算符号if (s.equals("+")) {num1 = Double.parseDouble(input.getText());str = "";flag = 1;}if (s.equals("-")) {num1 = Double.parseDouble(input.getText());str = "";flag = 2;}if (s.equals("*")) {num1 = Double.parseDouble(input.getText());str = "";flag = 3;}if (s.equals("/")) {num1 = Double.parseDouble(input.getText());str = "";flag = 4;}// 输入等号if (s.equals("=")){num2 = Double.parseDouble(input.getText().toString());if (flag == 1) {result = num1 + num2;}if (flag == 2) {result = num1 - num2;}if (flag == 3) {result = num1 * num2;}if (flag == 4) {result = num1 / num2;} input.setText(String.valueOf(result)); str="";}if (s.equals("清空")) {input.setText("0");str="";}}}