将JPanel从一个类添加到另一个类中的JPanel

Question

我有框架JFrame，我向其中添加了gui JPanel，它由多个不同的JPanels组成，其中一个位于不同的类中。

问题是我无法将其他类JPanel添加到我的gui JPanel中，我没有收到任何错误，但没有显示任何内容

PurchaseStock.java

    guiC.gridx=0;
    guiC.gridy=0;
    gui.add(StockDropdown.getP(), guiC);

    guiC.gridx=1;
    guiC.gridy=0;
    gui.add(quantityP, guiC);

    guiC.insets = new Insets(30,0,0,0);
    guiC.gridwidth=2;
    guiC.gridx=0;
    guiC.gridy=1;
    gui.add(checkout, guiC);

    frame.add(gui);
    frame.pack(); //Size to components

    frame.setResizable(false);
    frame.setVisible(true);

StackDropdown.java

public class StockDropdown {

    private static JPanel ui = new JPanel(new BorderLayout());
    private JButton bt = new JButton("tsfdsfds");
    private JPanel top = new JPanel();
    private JPanel middle = new JPanel();


    public StockDropdown(){
       ui.add("North", top);
       top.add(bt);
    }
    public static JPanel getP(){
        return ui;
    }

}

更新：

PurchaseStock.java

public class PurchaseStock extends JFrame implements ActionListener {
    JFrame frame = new JFrame();

    JPanel gui = new JPanel(new GridBagLayout());
    GridBagConstraints guiC = new GridBagConstraints();

    public PurchaseStock(){
        frame.setTitle("Update Stock");
        frame.setDefaultCloseOperation(JFrame.DISPOSE_ON_CLOSE);


        guiC.gridx=0;
        guiC.gridy=0;
        gui.add(new StockDropdown(), guiC);


        frame.add(gui);
        frame.pack(); //Size to components

        frame.setResizable(false);
        frame.setVisible(true);
   }


}

StackDropdown.java

package stock;

import java.awt.BorderLayout;
import javax.swing.JButton;
import javax.swing.JPanel;

public class StockDropdown extends JPanel{

    private JPanel ui = new JPanel(new BorderLayout());
    private JButton bt = new JButton("tsfdsfds");
    private JPanel top = new JPanel();
    private JPanel middle = new JPanel();


    public StockDropdown(){

       ui.add("North", top);
       top.add(bt);

    }


}

更新2：

StockDropdown.java

package stock;

import java.awt.BorderLayout;
import java.awt.Dimension;
import java.awt.Image;
import javax.swing.ImageIcon;
import javax.swing.JButton;
import javax.swing.JComboBox;
import javax.swing.JLabel;
import javax.swing.JPanel;

public class StockDropdown extends JPanel{
    private JComboBox itemsDropdown = new JComboBox();

    private JPanel top = new JPanel();
    private JPanel middle = new JPanel();
    JButton t = new JButton("tt");

    public StockDropdown(){

       add("North", top);
       add("Center", middle);
       top.add(itemsDropdown);

       for(String key : StockData.getStock().keySet())  {
         itemsDropdown.addItem(StockData.getName(key));
       }
       middle.add(t);

    }

     private ImageIcon setImageSize(String path) {
        ImageIcon image1 = new ImageIcon(path);
        Image image2 = image1.getImage().getScaledInstance(100,100,0);
        return new ImageIcon(image2);
    };

}

Answer 1

您确实获得了另一个JPanel，但由于在其构造函数中添加了组件并且从未调用过StockDropdown构造函数，因此未对其添加任何内容。 除非您打算创建一个真正的有效静态工厂方法，该方法将调用适当的构造函数，否则我将摆脱这些静态字段和方法。

例如，

import javax.swing.*;

public class StockDropdown extends JPanel {
    private JButton bt = new JButton("tsfdsfds");
    private JPanel top = new JPanel();
    private JPanel middle = new JPanel();

    // make this private if using factory method
    private StockDropdown(){
       add("North", top);
       top.add(bt);
    }

    // public static factory method
    public static StockDropdown getInstance() {
        StockDropdown ui = new StockDropdown();
        return ui;
    }
}

请注意，如果StockDropdown扩展了JPanel，请摆脱ui变量，因为使用ui变量，您正在将组件添加到未显示的容器中：

public class StockDropdown extends JPanel {
    private JButton bt = new JButton("tsfdsfds");
    private JPanel top = new JPanel();
    private JPanel middle = new JPanel();

    public StockDropdown() {
        setLayout(new BorderLayout());
        top.add(bt);
        // add("North", top);
        add(top, BorderLayout.PAGE_START);
    }
}