收获着，记录着......

少了浏览器的支持，Applet在互联网上永远不会有自己的舞台，而我却很酷爱这种把应用程序放到界面里的东东，今天无聊，把以前写过的一个表达式求值的程序，顺手拿Applet改写一下，再让大家体验一下MVC设计模式的应用吧：


=======================ExpressionModel===================

/*
 *这个就是表达式的模型组件了，它的三个属性，分别代表了它的中则表达式和右则表达式
 *而那两个属性分别是输入的式子和得到的结果
 */
package expression;

import java.text.*;
import java.util.*;

public class ExpressionModel extends Observable
{
 
 private ArrayList<String> middle;
 private ArrayList<String> right;
 private String input, result;

 public ExpressionModel()
 {
  middle = new ArrayList<String>();
  right = new ArrayList<String>();
 }

 //判断一个字符是不是操作符
 private boolean isOperator( String op )
 {
  if( op.equals("+") || op.equals("-") || op.equals("%")
     ||op.equals("*")|| op.equals("/") || op.equals("(")
     ||op.equals(")") )
    return true;
   
  else return false;
 }

 //得到一个操作符的优先级
 private int priority( String op )
 {
  if( op.equals("+") || op.equals("-") || op.equals("(") )
   return 1;
  else if( op.equals("*") || op.equals("/") || op.equals("%") )
   return 2;
  else
   return 0;
 }
 

//计算两个数和一个运算符得到的结果
 private String calTwoNumber( String op, String a, String b ) throws Exception
 {
  double x = Double.parseDouble( b );
  double y = Double.parseDouble( a );
  double z = 0;
  
  if( op.equals("+") )
   z = x + y;
  else if( op.equals("-") )
   z = x - y;
  else if( op.equals("*") )
   z = x * y;
  else if( op.equals("/") )
   z = x / y;
  else if( op.equals("%") )
   z = x % y;
  else
   z=0;
  
  return Double.toString( z );
 }
 
//将我们输入的中则表达式转换成逆波兰式
 private void turnToRight()
 {
  MyStack stack = new MyStack();
  String op;
  int pos = 0;
  
  StringTokenizer st = new StringTokenizer(input,"+-*/%()",true);
  
  while( st.hasMoreElements() )
   middle.add( st.nextToken() );
  
  while(true)
  {
   if( isOperator( middle.get(pos) ) )
   {
    if( stack.getTop() == -1 || (middle.get(pos)).equals("(") )
    {
     stack.push( middle.get(pos) );
    }
    else
    {
     if( (middle.get(pos)).equals(")") )
     {
      if( !(stack.top()).equals("(") )
      {
       op = stack.pop();
       right.add(op);
      }
     }
     else
     {
      if( priority(middle.get(pos)) <= priority(stack.top()) && stack.getTop() != -1 )
      {
       op = stack.pop();
       if( !op.equals("(") )
        right.add(op);
      }
      stack.push( middle.get(pos) );
     }
    }
   }
   else
    right.add( middle.get(pos) );
   pos++;
   if( pos >= middle.size() )
    break;
  }
  
  while( stack.getTop() != -1 )
  {
   op = stack.pop();
   right.add(op);
  }
 }

 private void calculate( String expression ) throws Exception
 {
  input = expression;
  
  turnToRight();
  
  MyStack stack = new MyStack();
  String op1, op2, is = null;
  Iterator<String> it = right.iterator();
  
  while(it.hasNext())
  {
   is = it.next();
   if( isOperator(is) )
   {
    op1 = stack.pop();
    op2 = stack.pop();
    stack.push( calTwoNumber(is,op1,op2) );
   }
   else stack.push(is);
  }
  
  NumberFormat form = NumberFormat.getInstance();
  form.setMaximumFractionDigits(5);
  
  result = input + "=" + form.format( Double.parseDouble( stack.pop() ) );
  
  setChanged();
  notifyObservers();
 }
 
 public void input(String expression) throws Exception
 {
  calculate(expression);
  middle.clear();
  right.clear();
 }
 
 public String getResult(){
  return result;
 }
 
}



===============MyStack==============


//我们用到的自己的堆栈
package expression;

import java.util.*;

public class MyStack
{
 
 private LinkedList<String> list;
 private int top;
 
 public MyStack()
 {
  list = new LinkedList<String>();
  top = -1;
 }
  
 public void push(String value)
 {
  top++;
  list.addFirst(value);
 }
 
 public String pop()
 {
  String temp = list.getFirst();
  top--;
  list.removeFirst();
  return temp;
 }
 
 public String top()
 {
  return list.getFirst();
 }
 
 public int getTop()
 {
  return top;
 }
 
}

===============ExpressionController========------

//控制器组件，包括一个输入域和两个按纽

package expression;

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

public class ExpressionController extends JPanel
{
 
 private ExpressionModel expressionModel;  
 private JTextField calTextField;
 
 public ExpressionController( ExpressionModel controllerModel ) {
  
  super();
  
  expressionModel = controllerModel;
  
  calTextField = new JTextField( 15 );
  JButton calButton = new JButton( "计算" );
  JButton delButton = new JButton( "取消" );
  
  calButton.addActionListener(
   new ActionListener()
   {
    public void actionPerformed( ActionEvent event )
    {
     try
     {
      expressionModel.input( calTextField.getText() );
      calTextField.setText("");
     }
     catch ( Exception exception )
     {
      JOptionPane.showMessageDialog (
       ExpressionController.this,
       "请输入有效表达式！", "错误",
       JOptionPane.ERROR_MESSAGE );
      calTextField.setText("");
     }
    }
   }
  );
  
  delButton.addActionListener(
   new ActionListener()
   {
    public void actionPerformed( ActionEvent event )
    {
     try
     {
      calTextField.setText("");
     }
     catch ( Exception exception )
     {
      calTextField.setText("");
     }
    }
   }
  );
  
  setLayout( new FlowLayout() );
  
  add( calTextField );
  add( calButton );
  add( delButton );
 
 }
 
 public Dimension getPreferredSize()
 {
  return new Dimension( 320, 40 );
 }
 
 public Dimension getMinimumSize()
 {
  return getPreferredSize();
 }
 
 public Dimension getMaximumSize()
 {
  return getPreferredSize();
 }
 
}



================ExpressionView ==================
//用来显示结果的视图
package expression;

import java.awt.*;
import java.text.*;
import java.util.*;

import javax.swing.*;
import javax.swing.border.*;

public class ExpressionView extends JPanel implements Observer
{
 
 private ExpressionModel expressionModel;
 private JLabel display = new JLabel();
 
 public ExpressionView( ExpressionModel model ) {
  
  if( model == null )
   throw new NullPointerException();
  
  expressionModel = model;
  expressionModel.addObserver( this );
  
  add( display );
  
  setBackground( Color.white );
  setBorder( new MatteBorder( 1, 1, 1, 1, Color.black ) );
  
 }
 
 public void update( Observable observable, Object object )
 {
  display.setText( expressionModel.getResult() );
 }
 
 public Dimension getPreferredSize()
 {
  return new Dimension( 320, 60 );
 }
 
 public Dimension getMinimumSize()
 {
  return getPreferredSize();
 }
 
 public Dimension getMaximumSize()
 {
  return getPreferredSize();
 }
 
 
}



===================Expression=====================

//最后把这些组装到这里吧

package expression;

import java.awt.*;
import java.awt.event.*;

import javax.swing.*;
import javax.swing.border.*;

public class Expression extends JApplet
{
  
 public void init()
 {
  ExpressionModel model = new ExpressionModel();
  ExpressionView view = new ExpressionView( model );
  ExpressionController controller = new ExpressionController( model );
  
  Container contentPane = getContentPane();
  contentPane.add( view, BorderLayout.CENTER );
  contentPane.add( controller, BorderLayout.SOUTH );
  
  setSize( 320, 100 );
  
 }
  
}

把这几个类打包到一个jar文件后，再创建个htm文件来访问它

<object
    classid = "clsid:8AD9C840-044E-11D1-B3E9-00805F499D93"
    codebase = "http://java.sun.com/update/1.5.0/jinstall-1_5-windows-i586.cab#Version=5,0,0,5"
    WIDTH = 320 HEIGHT = 100 >
    <PARAM NAME = CODE VALUE = "expression.Expression" >
    <PARAM NAME = ARCHIVE VALUE = "expression.jar" >
    <param name = "type" value = "application/x-java-applet;version=1.5">
    <param name = "scriptable" value = "false">
</object>

这样就行了，不过你的机器需要安装jre 5