第18章 图形界面 -> 习题4 编写程序，在文本框中输入英文字母，再根据“大写字母按钮”或“小写字母按钮”将输入的内容进行转换， 并将转换后的结果显示在标签上。

CharacterCast.java

import javax.swing.*;
import java.awt.*;

public class CharacterCast {

    private JFrame frame;
    private JTextField textField;
    private JButton lowerButton;
    private JButton upperButton;
    private JLabel label;

    public CharacterCast() {
        prepareGUI();

        frame.setVisible(true);
    }

    private void prepareGUI() {
        // 窗口
        frame = new JFrame("CharacterCast");
        frame.setBackground(Color.LIGHT_GRAY);
        frame.setSize(400, 200);
        frame.setDefaultCloseOperation(WindowConstants.EXIT_ON_CLOSE);
        frame.setLayout(null);

        // textfield
        textField = new JTextField();
        textField.setBounds(10, 24, 360, 21);

        // button
        lowerButton = new JButton("转为小写");
        upperButton = new JButton("转为大写");
        lowerButton.setBounds(10, 60, 120, 30);
        upperButton.setBounds(250, 60, 120, 30);

        lowerButton.addActionListener(e -> label.setText(textField.getText().toLowerCase()));
        upperButton.addActionListener(e -> label.setText(textField.getText().toUpperCase()));

        // label
        label = new JLabel();
        label.setBounds(10, 100, 360, 21);

        // pack
        frame.add(textField);
        frame.add(lowerButton);
        frame.add(upperButton);
        frame.add(label);
    }

    public static void main(String[] args) {
        CharacterCast characterCast = new CharacterCast();
    }
}