java - 无法重新添加相同的面板

Question

这让我发疯。当您单击 3 个不同的按钮时，我尝试更改面板并且它有效，但对于一个面板 - 仅一次。

如果您单击 addPerson - Person 面板显示，如果您随后单击 addCD - CD 面板显示； (同样适用于 view store)如果您随后单击 addPerson - 它不起作用。它抛出空指针异常。即使您单击 addCD/viewstore 然后添加 Person 它也会显示，但它不会显示第二次。

在一个测试文件中，我创建了一个带有添加和删除功能的 GUI:如果我点击添加，它会抛出空指针异常，但如果我已经添加并编译了，就没问题了......

/* PERSON PANEL */
public JPanel create_PersonPnl()
{
    personPnl = new JPanel(new FlowLayout(FlowLayout.CENTER));
    personPnl.setBackground(Color.WHITE);
    personPnl.setPreferredSize(minPnl);

    /* VERTICAL BOX for Person boxes */
    Box personBox = Box.createVerticalBox();
    personBox.setBorder(new TitledBorder(new LineBorder(Color.DARK_GRAY), "Person"));

        /* Horizontal Box for Name Lbl & TF */
        Box nameBox = Box.createHorizontalBox();
            nameBox.add(Box.createHorizontalStrut(10));
            nameLbl = new JLabel("Name: ");
            nameBox.add(nameLbl);
            nameBox.add(Box.createHorizontalStrut(5));
            nameTF = new JTextField();
            nameBox.add(nameTF);
            nameBox.add(Box.createHorizontalStrut(10));

        personBox.add(nameBox);
        personBox.add(Box.createVerticalStrut(10));
        Box limitBox = Box.createHorizontalBox();
            limitBox.add(Box.createHorizontalStrut(10));
            limitLbl = new JLabel("CD Limit: ");
            limitBox.add(limitLbl);
            limitTF = new JFormattedTextField();
            limitBox.add(limitTF);
            limitBox.add(Box.createHorizontalStrut(10));
        personBox.add(limitBox);
        personBox.add(Box.createVerticalStrut(10));
        personBox.add(addPersonBtn = new JButton("Add Person"));
        personBox.add(Box.createVerticalStrut(10));
            personList = new JList(temp);
            personList.setSelectionMode(ListSelectionModel.SINGLE_SELECTION);
            scrollp = new JScrollPane(personList);
                scrollp.setHorizontalScrollBarPolicy(JScrollPane.HORIZONTAL_SCROLLBAR_AS_NEEDED);
                scrollp.setVerticalScrollBarPolicy(JScrollPane.VERTICAL_SCROLLBAR_AS_NEEDED);
            scrollp.setSize(100, 45);
            personBox.add(scrollp);
            personBox.add(Box.createVerticalStrut(10));
        Box optionsBox = Box.createHorizontalBox();
            editPrsnBtn = new JButton("Edit");
            editPrsnBtn.addActionListener(this);
            optionsBox.add(editPrsnBtn);
            removePrsnBtn = new JButton("Remove");
            removePrsnBtn.addActionListener(this);
            optionsBox.add(removePrsnBtn);
        personBox.add(optionsBox);

    personPnl.add(personBox);
    return personPnl;
}

这就是我的 ActionPerformed 方法中的内容。

if(e.getSource() == addPersonBtn)
    {
        changePnl.removeAll();
        changePnl.add(create_PersonPnl());
        changePnl.revalidate();
        System.out.println("PersonPnl added");
    }

    if(e.getSource() == addCDBtn)
    {
        changePnl.removeAll();
        changePnl.add(create_CDPnl());
        changePnl.revalidate();
    }

    if(e.getSource() == viewStoreBtn)
    {
        changePnl.removeAll();
        changePnl.add(create_StorePnl());
        changePnl.revalidate();
    }

只有 ChangePnl 的代码。

  changePnl = new JPanel();
  changePnl.setBackground(Color.WHITE);
defaultPnl = new JPanel();
defaultPnl.setBackground(Color.WHITE);
defaultPnl.add(new JLabel("Welcome to the CD Store"));
defaultPnl.add(new JLabel("Click an option from the left"));
changePnl.add(defaultPnl);

S.O.P 是调试以查看正在运行的内容.. 并且只打印一次，仅此而已，除非我取出 .add(create_PersonPnl());所以我缩小了范围，但我不知道，因为它是第一次工作。

提前致谢!

单独的测试文件来证明它是 create_PersonPnl()

import java.awt.Color;
import java.awt.Dimension;
import java.awt.FlowLayout;
import java.awt.Toolkit;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;

import javax.swing.*;
import javax.swing.border.LineBorder;
import javax.swing.border.TitledBorder;

public class Test implements ActionListener
{
JButton add, remove;
JButton addPersonBtn, editPrsnBtn, removePrsnBtn;
JFrame frame;
JFormattedTextField limitTF;
JLabel nameLbl, limitLbl;
JList personList;
JPanel  TotalGUI, welcomePnl, mainPnl, imagePnl, changePnl, defaultPnl, cdPnl,       storePnl;
JPanel personPnl;
JScrollPane scrollp;
JTextField nameTF;

Dimension dim = Toolkit.getDefaultToolkit().getScreenSize();
Dimension minPnl = new Dimension(300, 400);

/* Test for JList */
String[] temp = {"1", "2", "3", "4", "1", "2", "3", "4","1", "2", "3", "4","1", "2", "3", "4" };

public Test()
{
    frame = new JFrame("CD Store");
    frame.setExtendedState(JFrame.NORMAL);

    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    //frame.pack(); //sets size based on components size in TotalGUI
    //frame.setJMenuBar(create_MenuBar());
    //frame.setMinimumSize(minDim);

    frame.setSize(725, 550);

    //set frame location (central to screen)
    int fw = frame.getSize().width;
    int fh = frame.getSize().height;
    int fx = (dim.width-fw)/2;
    int fy = (dim.height-fh)/2;


    frame.getContentPane().add(create_Content_Pane());
    frame.setVisible(true);

    //moves the frame to the centre
    frame.setLocation(fx, fy);
}

public JPanel create_Content_Pane()
{
    JPanel TotalGUI = new JPanel();

    TotalGUI.add(remove = new JButton("Remove"));
    remove.addActionListener(this);

    TotalGUI.add(add = new JButton("Add")); <- this crashes when selected
    add.addActionListener(this);

    //TotalGUI.add(create_PersonPnl()); <- works just fine

    return TotalGUI;
}

public JPanel create_PersonPnl()
{
    personPnl = new JPanel(new FlowLayout(FlowLayout.CENTER));
    personPnl.setBackground(Color.WHITE);
    personPnl.setPreferredSize(minPnl);

    /* VERTICAL BOX for Person boxes */
    Box personBox = Box.createVerticalBox();
    personBox.setBorder(new TitledBorder(new LineBorder(Color.DARK_GRAY), "Person"));

        /* Horizontal Box for Name Lbl & TF */
        Box nameBox = Box.createHorizontalBox();
            nameBox.add(Box.createHorizontalStrut(10));
            nameLbl = new JLabel("Name: ");
            nameBox.add(nameLbl);
            nameBox.add(Box.createHorizontalStrut(5));
            nameTF = new JTextField();
            nameBox.add(nameTF);
            nameBox.add(Box.createHorizontalStrut(10));

        personBox.add(nameBox);
        personBox.add(Box.createVerticalStrut(10));
        Box limitBox = Box.createHorizontalBox();
            limitBox.add(Box.createHorizontalStrut(10));
            limitLbl = new JLabel("CD Limit: ");
            limitBox.add(limitLbl);
            limitTF = new JFormattedTextField();
            limitBox.add(limitTF);
            limitBox.add(Box.createHorizontalStrut(10));
        personBox.add(limitBox);
        personBox.add(Box.createVerticalStrut(10));
        personBox.add(addPersonBtn = new JButton("Add Person"));
        personBox.add(Box.createVerticalStrut(10));
            personList = new JList(temp);
            personList.setSelectionMode(ListSelectionModel.SINGLE_SELECTION);
            scrollp = new JScrollPane(personList);
                scrollp.setHorizontalScrollBarPolicy(JScrollPane.HORIZONTAL_SCROLLBAR_AS_NEEDED);
                scrollp.setVerticalScrollBarPolicy(JScrollPane.VERTICAL_SCROLLBAR_AS_NEEDED);
            scrollp.setSize(100, 45);
            personBox.add(scrollp);
            personBox.add(Box.createVerticalStrut(10));
        Box optionsBox = Box.createHorizontalBox();
            editPrsnBtn = new JButton("Edit");
            editPrsnBtn.addActionListener(this);
            optionsBox.add(editPrsnBtn);
            removePrsnBtn = new JButton("Remove");
            removePrsnBtn.addActionListener(this);
            optionsBox.add(removePrsnBtn);
        personBox.add(optionsBox);

    personPnl.add(personBox);
    return personPnl;
}

public static void main(String[] args)
{
    new Test();
}

public void actionPerformed(ActionEvent e) 
{
    if(e.getSource() == remove)
    {
        TotalGUI.removeAll();
        TotalGUI.revalidate();
    }

    if(e.getSource() == add)
    {
        TotalGUI.add(create_PersonPnl());
        TotalGUI.revalidate();
    }
}
}

Answer 1

在你的方法中

public JPanel create_Content_Pane()
{
    JPanel TotalGUI = new JPanel();

    TotalGUI.add(remove = new JButton("Remove"));
    remove.addActionListener(this);

    TotalGUI.add(add = new JButton("Add")); <- this crashes when selected
    add.addActionListener(this);

    //TotalGUI.add(create_PersonPnl()); <- works just fine

    return TotalGUI;
}

出现NullPointerException是因为你的private field TotalGUI为null...去掉TotalGUI = new JPanel();前面的JPanel声明即可解决空指针问题。这可能只能解决您在测试场景中的问题...要解决您原始场景中的问题，最好拥有该类的完整源代码..