一个奇怪的问题（setText()无法再文本框中显示内容）

import java.awt.BorderLayout;
import java.awt.GridLayout;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.awt.event.MouseEvent;
import java.awt.event.MouseListener;

import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JPanel;
import javax.swing.JTextField;


public class Calculator extends JFrame{
    private JButton[] btn = new JButton[16];
    private JButton CE = new JButton();
    private JPanel p1 = new JPanel();
    private JPanel p2 = new JPanel();
    private JTextField txt = new JTextField("0");
    String A = new String();//表达式里的数字
    int a,result = 0;
   
    public Calculator(){
        this.setLayout(new BorderLayout(0,20));
        for(int i = 0; i <= 9; i++){
            btn[i] = new JButton(String.valueOf(i));
            
            //整型转化为字符串
//            System.out.println(a+"");
//            System.out.println(String.valueOf(a));
//            System.out.println(((Integer)a).toString());
        }
        btn[10] = new JButton(String.valueOf("+"));
        btn[11] = new JButton(String.valueOf("-"));
        btn[12] = new JButton(String.valueOf("*"));
        btn[13] = new JButton(String.valueOf("/"));
        btn[14] = new JButton(String.valueOf("="));
        btn[15] = new JButton(String.valueOf("清空"));
        p1.setLayout(new GridLayout(4,4,10,30));
        this.add(txt,BorderLayout.NORTH);
        
        OperatorHandle oh = new OperatorHandle();
        for(int j = 0; j < 16; j++){
            p1.add(btn[j]);
            btn[j].addActionListener(oh);
        }
        //System.out.println("A="+A);
        
        txt.setSize(60, 40);
        this.add(p1,BorderLayout.CENTER);
        this.setBounds(100, 200, 300, 350);
        
    }
   

    public static void main(String[] args) {
        Calculator f =new Calculator();
        f.setVisible(true);

    }
    class OperatorHandle implements ActionListener{

        String s;//监听器类：每次点击下按钮产生的操作，只是重复执行的 actionperformed函数，其在类中定义的变量如 string s;只会执行一次
        String oprate = "$";
        @Override
        public void actionPerformed(ActionEvent e) {
            // TODO Auto-generated method stub
            String cmd=e.getActionCommand();
            int idx= "0123456789".indexOf(cmd);
            System.out.println("idx="+idx);
            if (idx!=-1){//is number
                s = "0123456789".substring(idx,idx+1);
                A += s;
                a = Integer.parseInt(A);
                System.out.println("a="+a+ "    A="+A+"   result="+result);
                txt.setText(A);
                if(!oprate.equals("$")){
                    switch(oprate){
                    case "+": result += a; break;
                    case "-": result -= a; break;
                    case "*": result *= a; break;
                    case "/": result /= a; break;
                    }
                }
                txt.setText(String.valueOf(result));
                System.out.println("result="+result);
            }
            else{//  is oprate
                int x = "+-*/".indexOf(cmd);
                System.out.println("a="+a+"   x="+x+"   result="+result);
                switch(x){
                case 0: oprate = "+"; break;
                case 1: oprate = "-"; break;
                case 2: oprate = "*"; break;
                case 3: oprate = "/"; break;
                }
                result = Integer.parseInt(A);
                A = "0";
                //txt.setText(String.valueOf(result));
            }
               
               
            
        }
        
    }
   
   

}