将不同类的选项卡添加到main中

Question

我正在创建一个包含四个不同标签的应用程序。我没有将所有四个选项卡的代码放在一个类中，而是选择将每个选项卡放在不同的类中。现在，我在尝试将选项卡集成到应用程序主类的一个窗口时遇到了麻烦。这样做的最佳方法是什么？下面是其中一个类的示例，其中包含一个选项卡的代码，简化，其他类相对相同。这些课程本身很好。任何建议将不胜感激......

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

public class calculate extends JFrame{
    private static final long serialVersionUID = 1L;

    public calculate () {

        //GridBag Constraints
        GridBagConstraints grid = new GridBagConstraints();
        grid.fill = GridBagConstraints.BOTH;
        grid.anchor = GridBagConstraints.NORTH;


        //Create panel add components
        JPanel p2 = new JPanel(new GridBagLayout());

        JLabel floorOp = new JLabel("SELECT FLOORING OPTION", SwingConstants.CENTER);
        grid.gridx = 0;
        grid.gridy = 0;
        grid.gridwidth = 2;
        grid.ipady = 10;
        grid.ipadx = 40;
        floorOp.setOpaque(true);
        floorOp.setBackground(Color.decode("#CCFFFF"));
        floorOp.setBorder(BorderFactory.createMatteBorder(3, 0, 1, 0, Color.lightGray));
        p2.add(floorOp, grid);

        ButtonGroup groupBtn = new ButtonGroup();
        JRadioButton smallBtn = new JRadioButton("Hard Wood");
        groupBtn.add(smallBtn);
        grid.gridx = 0;
        grid.gridy = 1;
        grid.gridwidth = 1;
        grid.ipady = 10;
        smallBtn.setOpaque(true);
        smallBtn.setBackground(Color.WHITE);
        smallBtn.setHorizontalAlignment(AbstractButton.CENTER);
        p2.add(smallBtn, grid);

        JRadioButton mediumBtn = new JRadioButton("Carpet");
        groupBtn.add(mediumBtn);
        grid.gridx = 1;
        grid.gridy = 1;
        mediumBtn.setOpaque(true);
        mediumBtn.setBackground(Color.WHITE);
        mediumBtn.setHorizontalAlignment(AbstractButton.CENTER);
        p2.add(mediumBtn, grid);

        JLabel space = new JLabel("");
        grid.gridx = 0;
        grid.gridy = 2;
        grid.ipady = 25;
        p2.add(space, grid);

        JLabel dims = new JLabel("ENTER DIMENSIONS", SwingConstants.CENTER);
        grid.gridx = 0;
        grid.gridy = 3;
        grid.gridwidth = 2;
        grid.ipady = 10;
        grid.ipadx = 40;
        dims.setOpaque(true);
        dims.setBackground(Color.decode("#CCFFFF"));
        dims.setBorder(BorderFactory.createMatteBorder(3, 0, 1, 0, Color.lightGray));
        p2.add(dims, grid);

        JLabel length = new JLabel("Length:", SwingConstants.CENTER);
        grid.gridx = 0;
        grid.gridy = 4;
        grid.gridwidth = 1;
        length.setOpaque(true);
        length.setBackground(Color.WHITE);
        p2.add(length, grid);

        JTextField lengthTxt = new JTextField(20);
        grid.gridx = 1;
        grid.gridy = 4;
        p2.add(lengthTxt, grid);

        JLabel width = new JLabel("Width:", SwingConstants.CENTER);
        grid.gridx = 0;
        grid.gridy = 5;
        width.setOpaque(true);
        width.setBackground(Color.WHITE);
        p2.add(width, grid);

        JTextField widthTxt = new JTextField(20);
        grid.gridx = 1;
        grid.gridy = 5;
        p2.add(widthTxt, grid);

    }
}

这是我认为我应该在我的主要做的事情，但它会抛出异常

import javax.swing.JFrame;
import javax.swing.JTabbedPane;
import javax.swing.SwingUtilities;

public class appMain {
JFrame frame = new JFrame();
JTabbedPane tabbedPane = new JTabbedPane();
selectTab fp = new selectTab();
calculate sp = new calculate();

public appMain() {
    tabbedPane.add("First", fp);
    tabbedPane.add("Second", sp);
    frame.getContentPane().add(tabbedPane);
    frame.setDefaultCloseOperation(JFrame.DISPOSE_ON_CLOSE);
    frame.setSize(640, 480);
    // frame.pack();
    frame.setLocationByPlatform(true);
    frame.setVisible(true);
}

public static void main(String[] args) {
    SwingUtilities.invokeLater(new Runnable() {
        public void run() {
            new appMain();
        }
    });
}

}