帮忙看看我的这个问题

想用java实现一个计算器，其中用到了栈，我的代码如下：
Calculate.java

import java.util.*;



class Calculate {

    private Stack<Character> sign = new Stack<Character>();

    private ArrayList<Character> list = new ArrayList<Character>();

    public void addToStack( char c ){

	    System.out.println("addToStack");

        sign.push( c );

		System.out.println("符号栈中元素个数为"+sign.size());

    }

	public void addToList( char c ){

	    System.out.println("addToList");

		list.add( c );

	}

	public void deleteChar(){

	System.out.println("deleteChar");

	}

	public void emptyStack(){

	    System.out.println("emptyStack");

		

	}

	public void emptyList(){

	System.out.println("emptyList");

	}

	public String result(){

	    return "结果";

    }	

}  

UI.java

import javax.swing.*;    

import java.awt.*;   



class UI {

    private JFrame myCalculate;

	private JPanel contentPanel;

	private JPanel buttonPanel;

	private JTextField result;

	private JButton bracketLeft;

	private JButton bracketRight;

	private JButton num0;

	private JButton num1;

	private JButton num2;

	private JButton num3;

	private JButton num4;

	private JButton num5;

	private JButton num6;

	private JButton num7;

	private JButton num8;

	private JButton num9;

	private JButton add;

	private JButton sub;

	private JButton mul;

	private JButton div;

	private JButton dot;

	private JButton equal;

	private JButton delete;

	private JButton empty;

	private Listener myListener;

	public UI(){

	    myCalculate = new JFrame("计算器");

		contentPanel = new JPanel();

		BorderLayout borderLayout = new BorderLayout();

		borderLayout.setVgap(20);

		contentPanel.setLayout(borderLayout);

		myCalculate.setContentPane(contentPanel);

		result = new JTextField(20);

		result.setEditable(false);

		bracketLeft = new JButton("(");

		bracketRight = new JButton(")");

		num0 = new JButton("0");

		num1 = new JButton("1");

		num2 = new JButton("2");

		num3 = new JButton("3");

		num4 = new JButton("4");

		num5 = new JButton("5");

		num6 = new JButton("6");

		num7 = new JButton("7");

		num8 = new JButton("8");

		num9 = new JButton("9");

		add = new JButton("+");

		sub = new JButton("-");

		mul = new JButton("×");

		div = new JButton("÷");

		dot = new JButton(".");

		equal = new JButton("=");

		delete = new JButton("删除");

		empty = new JButton("清空");

		myListener = new Listener();

		bracketLeft.addActionListener(myListener);

		bracketRight.addActionListener(myListener);

		num0.addActionListener(myListener);

		num1.addActionListener(myListener);

		num2.addActionListener(myListener);

		num3.addActionListener(myListener);

		num4.addActionListener(myListener);

		num5.addActionListener(myListener);

		num6.addActionListener(myListener);

		num7.addActionListener(myListener);

		num8.addActionListener(myListener);

		num9.addActionListener(myListener);

		add.addActionListener(myListener);

		sub.addActionListener(myListener);

		mul.addActionListener(myListener);

		div.addActionListener(myListener);

		dot.addActionListener(myListener);

		equal.addActionListener(myListener);

		delete.addActionListener(myListener);

		empty.addActionListener(myListener);

		buttonPanel = new JPanel();

		GridLayout gridLayout = new GridLayout(5,4);

		gridLayout.setHgap(10);

		gridLayout.setVgap(30);

		buttonPanel.setLayout(gridLayout);

		buttonPanel.add(bracketLeft);

		buttonPanel.add(bracketRight);

		buttonPanel.add(delete);

		buttonPanel.add(empty);

		buttonPanel.add(num7);

		buttonPanel.add(num8);

		buttonPanel.add(num9);

		buttonPanel.add(div);

		buttonPanel.add(num4);

		buttonPanel.add(num5);

		buttonPanel.add(num6);

		buttonPanel.add(mul);

		buttonPanel.add(num1);

		buttonPanel.add(num2);

		buttonPanel.add(num3);

		buttonPanel.add(sub);

		buttonPanel.add(equal);

		buttonPanel.add(dot);

		buttonPanel.add(num0);

		buttonPanel.add(add);

		contentPanel.add("North",result);

		contentPanel.add("Center",buttonPanel);

		myCalculate.setSize(300,350);

		myCalculate.setVisible(true);

		myCalculate.setDefaultCloseOperation( JFrame.DISPOSE_ON_CLOSE );

	}

	public void setString( String s ){

	    result.setText( s );

	}

	public void addString( String s ){

	    result.setText( result.getText() + s );

	}

	public void deleteChar(){

	    System.out.println("deleteChar");

	}

}

Listener.java

import java.awt.event.*;



class Listener implements ActionListener {

    private boolean isSign( char c ){

	    switch( c ){

		    case '(':

			case ')':

			case '+':

			case '-':

			case '×':

			case '÷':return true;

			default:return false;

		}

	}

	private boolean isNum( char c){

	    if( c >= '.'&&c <= '9')

		    return true;

		else

		    return false;

	}

    public void actionPerformed( ActionEvent e ){

	    Calculate myCalculate = new Calculate();

//	    System.out.println( e.getActionCommand() );

//		Main.calculate.setString( e.getActionCommand() );

        String button = e.getActionCommand();

		char c = (button.toCharArray())[0];

		if( isSign( c ) )

		    myCalculate.addToStack( c );//把符号添加到栈里

        else if( isNum( c ) ){

            Main.calculate.addString( button );//把操作数添加到文本框里

			myCalculate.addToList( c );//把操作数添加到输出序列中

        }

        else if( button.equals( "删除" ) ){

		    Main.calculate.deleteChar();

            myCalculate.deleteChar();

        }

        else if( button.equals( "清空" ) ){

            Main.calculate.setString("");

			myCalculate.emptyStack();

			myCalculate.emptyList();

        }	

        else if( c == '=' )

            Main.calculate.setString( myCalculate.result() );		

	}

}

Main.java

public class Main{

    static UI calculate;

    public static void main( String[] args ){

        calculate = new UI();

    }

}

当我在界面中按一个运算符按键（+,-,*,/和括号）时，我想把运算符存到Calculate中定义的栈中，当我按多个运算符后，栈中应该有多个元素，可是为什么我的Calculate类中的addToStack（）方法每次都输出“个数为1”呢？我看了半天没找出原因在哪里，旁观者清，请各位帮忙看看。