java监听器不能生效问题

[Huangy远]

求大家帮帮忙..在线等

package J_Fram;

import java.awt.Container;
import java.awt.FlowLayout;
import java.awt.event.KeyAdapter;
import java.awt.event.KeyEvent;
import java.awt.event.KeyListener;

import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JTextArea;
import javax.swing.JTextField;

import com.sun.org.apache.xerces.internal.util.SynchronizedSymbolTable;

public class KeyDemo{
JFrame f;
Container container;
JLabel jl;
JTextField ja;

public KeyDemo() {
// TODO Auto-generated constructor stub
f = new JFrame("键盘输入案例");
f.setLayout(new FlowLayout());
f.setBounds(400, 200, 300, 180);

jl = new JLabel("只能输入数字");
ja =new JTextField(10);
f.add(jl);
f.add(ja);


ja.addKeyListener(new KeyAdapter() {
@Override
public void keyPressed(KeyEvent e) {
// TODO Auto-generated method stub
char c;
c = e.getKeyChar();

if(!(c>='0' && c<='9'))
{
System.out.println(c);
e.consume();
}

}

});



f.setVisible(true);
}






public static void main(String[] args) {
// TODO Auto-generated method stub
KeyDemo keyDemo = new KeyDemo();
}
}