java 输出全部小写_输入小写 输出大写 为什么报错？

输入小写，输出大写，为什么报错？

要实现在文本框里实现输入小写，输出大写

import java.awt.*;

import java.awt.Event;

import java.awt.TextArea;

import java.awt.event.KeyEvent;

import java.awt.event.KeyListener;

import javax.swing.*;

public class Key implements KeyListener {

public static void main(String[] args)

{

JFrame f=new JFrame("example");

TextArea t=new TextArea();

t.addKeyListener(new Key());

f.add(t);

f.setVisible(true);

}

public void keyPressed(KeyEvent arg0) {

// TODO Auto-generated method stub

TextArea ta=(TextArea)arg0.getSource();

String s=ta.getText();

String b=s.toUpperCase();

ta.setText("" );

ta.append(b);

}

public void keyReleased(KeyEvent arg0) {

// TODO Auto-generated method stub

}

public void keyTyped(KeyEvent arg0) {

// TODO Auto-generated method stub

}

}

报错信息：

java.lang.Error: Do not use javax.swing.JFrame.add() use javax.swing.JFrame.getContentPane().add() instead

at javax.swing.JFrame.createRootPaneException(Unknown Source)

at javax.swing.JFrame.addImpl(Unknown Source)

at java.awt.Container.add(Unknown Source)

at Key.main(Key.java:18)

Exception in thread "main"