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

Question

我使用IntelliJ的图形用户界面设计器创建了一个带有子面板的JPanel。现在我想要的子面板是一个新的JPanel和readd到我的顶部面板。但这会导致NullPointerExceptiion。为什么以及如何才能解决这个问题？注意:我的组件都没有标记为"Custom Create“

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(){
    }
}

View.form

<?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>

Main

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

如果不先手动实例化JPanel，GridLayoutManager (IntelliJ)似乎不允许添加新组件。改用GridBagLayout解决了这个问题。

Answer 2

您需要添加GridConstraints

https://intellij-support.jetbrains.com/hc/en-us/community/posts/206192269-Hello-noob-question-about-intellij-layout-manager

panel.add(sth, new com.intellij.uiDesigner.core.GridConstraints());

maven对此有依赖关系：

<!-- https://mvnrepository.com/artifact/com.intellij/forms_rt -->
    <dependency>
        <groupId>com.intellij</groupId>
        <artifactId>forms_rt</artifactId>
        <version>7.0.3</version>
    </dependency>