焦点监听事件FocusListener

public class Demo extends JFrame {

    public Demo() {

        setBounds(100, 100, 200, 120);

        setDefaultCloseOperation(WindowConstants.EXIT_ON_CLOSE);

        Container c = getContentPane();

        c.setLayout(new FlowLayout());

        JTextField tf1 = new JTextField();

        JTextField tf2 = new JTextField();

        JTextField tf3 = new JTextField();

        tf1.setColumns(15);

        tf2.setColumns(15);

        tf3.setColumns(15);

        c.add(tf1);

        c.add(tf2);

        c.add(tf3);

        tf1.addFocusListener(new MyFocusListener());//调用自定义焦点监听方法类

        tf2.addFocusListener(new MyFocusListener());

        tf3.addFocusListener(new MyFocusListener());

        setVisible(true);

    }

    //在MyFocusListener上，Alt+Ins，Implement Methods，快速创建方法。有光标为绿，无为红

    class MyFocusListener implements FocusListener {//自定义类，实现焦点监听（一种接口）

        public void focusGained(FocusEvent e) {

            JTextField tf = (JTextField) e.getSource();//获取触发焦点事件的控件，强制转换为文本框

            tf.setBorder(BorderFactory.createLineBorder(Color.GREEN));//绿色边框

        }

        public void focusLost(FocusEvent e) {

            JTextField tf = (JTextField) e.getSource();//获取触发焦点事件的控件，强制转换为文本框

            tf.setBorder(BorderFactory.createLineBorder(Color.RED));//红色边框

        }

    }

    public static void main(String[] args) {

        new Demo();

    }

}