parserapplet.java

解释器是用于分析简单的算术运算, 适用于初学者开发自己的智能系统.

//import javax.swing.JApplet;  // import class JApplet
//import javax.swing.JApplet.*;  // import class JApplet
//import java.awt.Graphics;    // import class Graphics
//public class ParserApplet extends JApplet {  
//   public void paint( Graphics g )
//   {
//      g.drawString( "Welcome to Java Programming!", 25, 25 );
//   }
//}

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



public class ParserApplet extends Applet 
	implements ActionListener{
//{	
	TextField expText, resText;
	Parser p;
	
	public void init()
	{
		Label heading = new Label("Expression Calculator", Label.CENTER);
		
		Label explab = new Label("Expression ", Label.CENTER);
		Label reslab = new Label("Result ", Label.CENTER);
		
		expText = new TextField(24);
		resText = new TextField(24);
		
		resText.setEditable(false); 
		
		add(heading);
		add(explab);
		add(expText);
		add(reslab);
		add(resText);
		
		expText.addActionListener(this);
		
		// create parser
		p = new Parser();
	}
	
	// User pressed Enter.
	public void actionPerformed(ActionEvent ae)
	{
		repaint();
	}
	
	public void paint(Graphics g)
	{
		double result = 0.0;
		String expstr = expText.getText();
		
		try
		{
			if(expstr.length() != 0)
				result = p.evaluate(expstr);
			
			resText.setText(Double.toString(result));
			showStatus(""); // erase any previous error message
		}catch(ParserException exc)
		{
			showStatus(exc.toString());
			resText.setText("");
		}
		
	}
	
}