问答题

本程序的功能是实时监听文本域中字符的数目。窗口中有一个文本域和一个文字标签，在文本域中输入字符时，文字标签将实时显示当前文本框中字符的个数，如图所示。


import java.awt.*;
import java.awt.event.*;
class exam_64 extends Frame {
TextArea ta= new TextArea(null, 10, 40);
Label caret=new Label("字符数=0");
exam_64() {
super("exam_64");
addWindowListener(new WindowAdapter() {
public void windowClosing(WindowEvent e) {
System.exit(0);
}
});
add(ta, BorderLayout.CENTER);
add(caret, BorderLayout.SOUTH);
caret.addKeyListener(new KeyEventHandler()) ;
ta.addMouseListener(new MouseEventHandler());
pack();
show();
}
class KeyEventHandler implements KeyAdapter {
public void keyTyped(KeyEvent evt) {
caret.setText("字符数="+ta.getText().length());
}
}
void MouseEventHandler extends MouseAdapter {
public void mouseClicked(MouseEvent evt) {
caret.setText("字符数="+ta.getText().length());
}
}
static public void main(String[] args) {
new exam_64();
}
}