java - 将新的 JPanel 添加到另一个 JPanel 中会导致 NullPointerException

Question

我使用 IntelliJ 的 GUI 设计器创建了一个 JPanel，其中包含一个子面板。现在我希望子面板成为一个新的 JPanel 并重新添加到我的顶部面板。但这会导致 NullPointerException。为什么以及如何解决它？注意:我的组件都没有标记为“自定义创建”

Exception in thread "main" java.lang.NullPointerException
    at com.intellij.uiDesigner.core.GridLayoutManager.addLayoutComponent(GridLayoutManager.java:134)
    at java.awt.Container.addImpl(Container.java:1127)
    at java.awt.Container.add(Container.java:417)
    at View.<init>(View.java:16)
    at Main.main(Main.java:4)

查看类

import javax.swing.*;
import javax.swing.table.DefaultTableModel;
import java.awt.*;
public class View{
    private JPanel panel;
    private JPanel subPanel;

    public JPanel getPanel() {
        return panel;
    }

    View()
    {
        panel.remove(subPanel);
        subPanel = newPanel();
        panel.add(subPanel);
    }
    JPanel newPanel(){
        JPanel panel = new JPanel();
        panel.setLayout(new GridBagLayout());
        JLabel label = new JLabel("test");
        panel.add(label);
        return panel;
    }

    private void createUIComponents(){
    }
}

查看表单

<?xml version="1.0" encoding="UTF-8"?>
<form xmlns="http://www.intellij.com/uidesigner/form/" version="1" bind-to-class="View">
  <grid id="27dc6" binding="panel" layout-manager="GridLayoutManager" row-count="2" column-count="1" same-size-horizontally="false" same-size-vertically="false" hgap="-1" vgap="-1">
    <margin top="0" left="0" bottom="0" right="0"/>
    <constraints>
      <xy x="20" y="20" width="500" height="400"/>
    </constraints>
    <properties/>
    <border type="none"/>
    <children>
      <grid id="860ea" binding="subPanel" layout-manager="GridLayoutManager" row-count="1" column-count="1" same-size-horizontally="false" same-size-vertically="false" hgap="-1" vgap="-1">
        <margin top="0" left="0" bottom="0" right="0"/>
        <constraints>
          <grid row="0" column="0" row-span="1" col-span="1" vsize-policy="3" hsize-policy="3" anchor="0" fill="3" indent="0" use-parent-layout="false"/>
        </constraints>
        <properties/>
        <border type="none"/>
        <children/>
      </grid>
      <vspacer id="7838f">
        <constraints>
          <grid row="1" column="0" row-span="1" col-span="1" vsize-policy="6" hsize-policy="1" anchor="0" fill="2" indent="0" use-parent-layout="false"/>
        </constraints>
      </vspacer>
    </children>
  </grid>
</form>

主要

import javax.swing.*;
public class Main {
    public static void main(String[] args) {
        View view = new View();
        JFrame frame = new JFrame("Window");
        frame.setContentPane(view.getPanel());
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.pack();
        frame.setVisible(true);
    }
}

Answer 1

GridLayoutManager (IntelliJ) 似乎不允许在不先手动实例化 JPanel 的情况下添加新组件。更改为 GridBagLayout 解决了问题。