All geek questions in one place

Unable to re-add the same panel

It drives me crazy. I try to change panels when you click on 3 different buttons, and this works, but for one panel only once.

If you click the AddPerson button - the Person panel, If you then click addCD - the CD panel will appear; (the same for the browse store) If you then click on addPerson, this will not work. It throws a nullpointer exception. Even if you click on addCD / viewstore and THEN add the Person that it shows, but it just won’t appear a second time.

In the test file, I created a graphical interface with adding and removing: if I clicked on the add button, it would throw a null pointer exception, but if I just added it and compiled, everything was fine ...

/* 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; }

This is what is in my ActionPerformed method.

 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(); }

Only code for ChangePnl.

  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);

SOP had to debug in order to see what was running .. and that only prints once and that it if I did not pull out .add (create_PersonPnl ()); so I narrowed it down, but I have no clue as it works the first time.

Thanks in advance!

Separate the test file to prove it 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(); } } }
+4
source share
1 answer

In your method

 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; }

The NullPointerException occurs because your TotalGUI private field is null ... Delete the JPanel declaration before TotalGUI = new JPanel(); . This will solve the null pointer problem. This probably only solves your problem in the test script ... To solve the problem in the source script, it would be nice to have the full class source code.

+1
source

Source: https://habr.com/ru/post/1395692/

More articles:


All Articles