Exception in thread "main" java.lang.NullPointerException
at java.awt.Container.addImpl(Unknown Source)
at java.awt.Container.add(Unknown Source)
at Volume.<init>(Volume.java:25)
at VolumeDriver.main(VolumeDriver.java:6)
I get the above errors when trying to run my program. My program is not complete but I just want to check what my window looks like so I can make sure it looks correct.
This is my worker class for right now
import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
public class Volume extends JFrame
{
private static final long serialVersionUID = 1L;
private JPanel topPanel;
private JPanel bottomPanel;
private JPanel rightPanel;
private JPanel mainPanel;
private JLabel message;
private final int width = 500;
private final int height = 400;
public Volume()
{
setTitle("Sphere and Box Volumes");
setSize(width,height);
setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
getContentPane().setLayout(new BorderLayout());
getContentPane().add(topPanel, BorderLayout.NORTH);
BuildTopPanel();
getContentPane().add(bottomPanel, BorderLayout.SOUTH);
BuildBotPanel();
getContentPane().add(rightPanel, BorderLayout.EAST);
BuildRightPanel();
getContentPane().add(mainPanel, BorderLayout.WEST);
BuildMainPanel();
setVisible(true);
}
private void BuildTopPanel()
{
topPanel = new JPanel(new FlowLayout());
topPanel.setBackground(Color.WHITE);
JTextField reqVolume = new JTextField(8);
message = new JLabel("Enter the required amount of volume:");
topPanel.add(message);
topPanel.add(reqVolume);
}
private void BuildBotPanel()
{
bottomPanel = new JPanel(new FlowLayout());
bottomPanel.setBackground(Color.WHITE);
JButton intialQuant = new JButton("Set Initial Quantities");
intialQuant.setActionCommand("I");
intialQuant.addActionListener(new ButtonListener());
JButton calcVolume = new JButton("Calculate Volumes");
calcVolume.setActionCommand("V");
calcVolume.addActionListener(new ButtonListener());
JButton close = new JButton("Close");
close.setActionCommand("C");
close.addActionListener(new ButtonListener());
bottomPanel.add(intialQuant);
bottomPanel.add(calcVolume);
bottomPanel.add(close);
}
private void BuildRightPanel()
{
rightPanel = new JPanel(new FlowLayout());
rightPanel.setBackground(Color.WHITE);
}
private void BuildMainPanel()
{
mainPanel = new JPanel(new FlowLayout());
mainPanel.setBackground(Color.WHITE);
}
private class ButtonListener implements ActionListener
{
public void actionPerformed(ActionEvent e)
{
}
}
}
This is my driver class
public class VolumeDriver
{
public static void main(String[] args)
{
Volume frame = new Volume();
frame.setVisible(true);
}
}
Any help would be appreciated.
getContentPane().add(topPanel, BorderLayout.NORTH); // Uh oh! topPanel is not initialized!
BuildTopPanel();
You try to add your topPanel before its initialization in the BuildTopPanel() method, so it will obviously lead to a NPE!
This is also the case for your other build methods. Be sure to actually initialize and build your GUI elements before using them or adding them to your window!
For instance:
// Initialize GUI elements first:
BuildTopPanel();
BuildBotPanel();
BuildRightPanel();
BuildMainPanel();
// Then add the GUI elements to the window:
getContentPane().add(topPanel, BorderLayout.NORTH);
getContentPane().add(bottomPanel, BorderLayout.SOUTH);
getContentPane().add(rightPanel, BorderLayout.EAST);
getContentPane().add(mainPanel, BorderLayout.WEST);
initialize the component before you add it to panel. see the documents as below:
http://docs.oracle.com/javase/7/docs/api/java/awt/Container.html#add%28java.awt.Component%29
That means you need call BuildMainPanel() before getContentPane().add(mainPanel, BorderLayout.WEST);
The same with the other components.
Related
Please help me to understand how this works. I'm having difficulties to understand how, for example, JButton in one class can alter text in JTextArea that is in another class of a same package. I've made a simple app just to ask a question here, I need this for a bigger school project where I need to implement this to work with multiple classes.
When I put everything in the same class it works but I need it in separate classes.
Here is the simple code.
import javax.swing.*;
import java.awt.event.*;
import java.awt.*;
class Button extends JPanel {
private JButton button;
private Panel panel;
public Button() {
button = new JButton("BUTTON");
panel = new Panel();
add(button);
button.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent e) {
JButton clicked = (JButton) e.getSource();
String input = clicked.getText();
panel.setTextArea(input);
//System.out.println(input);
}
});
}
}
class Panel extends JPanel {
private JTextArea textArea;
public Panel() {
setLayout(new BorderLayout());
textArea = new JTextArea();
add(textArea, BorderLayout.CENTER);
}
public JTextArea getTextArea() {
return textArea;
}
void setTextArea(String text) {
this.textArea.setText(text);
}
}
public class Java extends JFrame {
private Button dugme;
private JFrame frame;
private Panel panel;
public Java() {
frame = new JFrame();
dugme = new Button();
panel = new Panel();
//super("test");
frame.setLayout(new BorderLayout());
frame.setTitle("test");
frame.setSize(300, 400);
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.setVisible(true);
frame.add(dugme, BorderLayout.NORTH);
frame.add(panel, BorderLayout.CENTER);
}
public static void main(String[] args) {
Java app = new Java();
}
}
I want action listener to alter the text in the panel, sys-out works so the listener listens the button but I can't make it to alter the text in text area.
As already mentioned by #XtremeBaumer you have two different instances of Panel class. You need to remove the secode one.
public class Button extends JPanel {
private JButton button;
private Panel panel;
public Button(Panel panel) { // we need already created instance of panel here.
this.panel = panel;
button = new JButton("BUTTON");
// panel = new Panel(); <-- this line must be deleted.
// ...
}
}
public class Java extends JFrame {
private Button dugme;
private JFrame frame;
private Panel panel;
public Java(){
frame = new JFrame();
panel = new Panel();
dugme = new Button(panel);
// ...
}
}
Please also replace the line
add(textArea, BorderLayout.CENTER);
by
add(new JScrollPane(textArea), BorderLayout.CENTER);
This allows you to get the scrool bars when text goes larger than the text ara size.
Here is your reworked example
import javax.swing.*;
import java.awt.event.*;
import java.awt.*;
class Button extends JPanel {
private JButton button;
private Panel panel;
public Button(Panel panel) {
this.panel = panel;
button = new JButton("BUTTON");
add(button);
button.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent e) {
JButton clicked = (JButton) e.getSource();
String input = clicked.getText();
panel.setTextArea(input);
//System.out.println(input);
}
});
}
}
class Panel extends JPanel {
private JTextArea textArea;
public Panel() {
setLayout(new BorderLayout());
textArea = new JTextArea();
add(new JScrollPane(textArea), BorderLayout.CENTER);
}
public JTextArea getTextArea() {
return textArea;
}
void setTextArea(String text) {
this.textArea.setText(text);
}
}
public class Java extends JFrame {
private Button dugme;
private JFrame frame;
private Panel panel;
public Java() {
frame = new JFrame();
panel = new Panel();
dugme = new Button(panel);
//super("test");
frame.setLayout(new BorderLayout());
frame.setTitle("test");
frame.setSize(300, 400);
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.setVisible(true);
frame.add(dugme, BorderLayout.NORTH);
frame.add(panel, BorderLayout.CENTER);
}
public static void main(String[] args) {
Java app = new Java();
}
}
I have been trying to pass the info of my JTextField that is in a JDialog into my JFrame. Both the JDialog and JFrame are in separate classes. I have tried to store the JTextField into a JLable using the .setText and .getText and then passing the JLable into the JFrame but with no luck.
I know there are many similar questions but I have tried many different approaches but still no luck. I am relatively new to Java and do not know all the in's and out's. Any help is very appreciated!
My code for the JFrame:
import javax.swing.*;
import java.awt.*;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.beans.PropertyChangeEvent;
import java.beans.PropertyChangeListener;
import javax.swing.JPanel;
public class StockApp extends JFrame implements PropertyChangeListener {
private JPanel main = new JPanel();
private JPanel north = new JPanel();
private JPanel center = new JPanel();
private JPanel south = new JPanel();
private JButton buyStock = new JButton("Buy Stock");
private JButton sellStock = new JButton("Sell Stock");
public TestTest variables = new TestTest();
private JLabel stockNameNorth = new JLabel("Stock Name");
private JLabel stockPriceNorth = new JLabel("Stock Price");
String stockName = variables.getStockName();
String stockPrice = variables.getStockPrice();
public StockApp() {
setTitle("StockApp");
getContentPane().setBackground(Color.white);
setSize(400,400);
setLocation(500,200);
setVisible(true);
main.setLayout(new BorderLayout());
north.setLayout(new FlowLayout());
center.setLayout(new FlowLayout());
south.setLayout(new FlowLayout());
stockNameNorth.setText(stockName);
stockPriceNorth.setText(stockPrice);
add(main);
north.add(stockNameNorth);
north.add(stockPriceNorth);
south.add(buyStock);
south.add(sellStock);
main.add(north, BorderLayout.NORTH);
main.add(center, BorderLayout.CENTER);
main.add(south, BorderLayout.SOUTH);
}
}
And Dialog:
import javax.swing.*;
import java.awt.*;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
public class TestTest extends JDialog implements ActionListener {
private JPanel main = new JPanel();
private JPanel north = new JPanel();
private JPanel center = new JPanel();
private JPanel south = new JPanel();
private JLabel stockNameLabel = new JLabel("Stock name: ");
private JLabel stockPriceLabel = new JLabel("Stock price(£): ");
private JTextField stockNameIn = new JTextField(5);
private JTextField stockPriceIn = new JTextField(5);
private JButton buttonOK = new JButton("OK");
public JLabel stockPrice = new JLabel();
public JLabel stockName = new JLabel();
public TestTest() {
getContentPane().setBackground(Color.white);
setSize(400,400);
setLocation(500,200);
setModal(false);
setVisible(true);
getRootPane().setDefaultButton(buttonOK);
main.setLayout(new BorderLayout());
north.setLayout(new FlowLayout());
center.setLayout(new FlowLayout());
south.setLayout(new FlowLayout());
add(main);
north.add(stockNameLabel);
north.add(stockNameIn);
center.add(stockPriceLabel);
center.add(stockPriceIn);
south.add(buttonOK);
main.add(north, BorderLayout.NORTH);
main.add(center, BorderLayout.CENTER);
main.add(south, BorderLayout.SOUTH);
buttonOK.addActionListener(this);
}
public void actionPerformed(ActionEvent e) {
if (e.getSource() == buttonOK){
stockName.setText(stockNameIn.getText());
stockPrice.setText(stockPriceIn.getText());
dispose();
new StockApp();
}
}
public String getStockName() {
return stockNameIn.getText();
}
public String getStockPrice() {
return stockPriceIn.getText();
}
}
I am trying to pass the stockName and stockPrice variables from the JDialog into the JFrame. I then want the name and price to display at the top of the JFrame.
For demonstration, what the problem is, we need less Fields and Buttons.
So far, no component of StockApp needs to be accessed from different methods, so there is no need to make them visible outside of the ctor.
More explanations in the code.
import javax.swing.*;
import java.awt.*;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import javax.swing.JPanel;
public class StockApp extends JFrame {
public StockApp() {
// move those unreferenced panels here, so we don't have to reason about them:
JPanel main = new JPanel();
JPanel north = new JPanel();
JPanel center = new JPanel();
JPanel south = new JPanel();
// add price later, when name works
JButton buyStock = new JButton("Buy Stock");
JLabel stockNameNorth = new JLabel("Stock Name");
// critical change: Make the label, which you like to update,
// accessible by whom it should be updated:
TestTest variables = new TestTest (stockNameNorth);
setTitle ("StockApp");
getContentPane().setBackground(Color.white);
setSize (600,400);
setLocation (500,200);
setVisible (true);
// make the close-frame action terminate the program:
setDefaultCloseOperation (JFrame.EXIT_ON_CLOSE);
main.setLayout (new BorderLayout());
north.setLayout (new FlowLayout());
center.setLayout (new FlowLayout());
south.setLayout (new FlowLayout());
add (main);
north.add (stockNameNorth);
south.add (buyStock);
main.add (north, BorderLayout.NORTH);
main.add (center, BorderLayout.CENTER);
main.add (south, BorderLayout.SOUTH);
}
// Main method to start the damn thing
public static void main(String[] args) {
SwingUtilities.invokeLater(new Runnable() {
#Override
public void run() {
new StockApp ();
}
});
}
}
// no need to make this class public in a short test:
class TestTest extends JDialog implements ActionListener {
// this are elements, visible outside the construction phase,
// we need to have access to from more than one method.
// Make this important distinction visible to the reader:
JLabel name;
JTextField stockNameIn = new JTextField (5);
JButton buttonOK = new JButton ("OK");
// add the JLabel to update to the ctor, so that it can't be forgotten
// to be set
public TestTest (JLabel pname) {
// we copy the reference to the label, to have access to it in
// the actionPerformed method.
name = pname;
JPanel main = new JPanel();
JPanel north = new JPanel();
JPanel center = new JPanel();
JPanel south = new JPanel();
JLabel stockNameLabel = new JLabel ("Stock name: ");
getContentPane().setBackground(Color.white);
// different size/location than frame, so that they don't hide
// each other completly
setSize (400,600);
setLocation (700,300);
setModal (false);
setVisible (true);
getRootPane().setDefaultButton(buttonOK);
main.setLayout (new BorderLayout());
north.setLayout (new FlowLayout());
center.setLayout (new FlowLayout());
south.setLayout (new FlowLayout());
add (main);
north.add (stockNameLabel);
north.add (stockNameIn);
south.add (buttonOK);
main.add (north, BorderLayout.NORTH);
main.add (center, BorderLayout.CENTER);
main.add (south, BorderLayout.SOUTH);
buttonOK.addActionListener(this);
}
// here we need access to the button - was it the OK-Button, clicked?
// and the textfield stockNameIn, to read the text
// and the name field from the frame, to set the text
public void actionPerformed(ActionEvent e) {
if (e.getSource () == buttonOK) {
name.setText (stockNameIn.getText());
dispose();
}
}
}
Whenever I press "clear" in my Java GUI, it never works, please help me finish this. If i replace "textPanel" with another button, it works, otherwise with "textpanel" it doesn't.
Here is a lightweight version of my code demonstrating the problem:
import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
public class MainFrame extends JFrame {
private TextPanel textPanel;
private FormPanel formpanel;
public MainFrame(){
super("My Frame");
createLayout();
createFrame();
}
public void createFrame(){
setSize(600, 600);
setDefaultCloseOperation(EXIT_ON_CLOSE);
setVisible(true);
}
public void createLayout(){
BorderLayout myLayout = new BorderLayout();
setLayout(myLayout);
textPanel = new TextPanel();
formpanel = new FormPanel();
// adding components
add(textPanel, BorderLayout.CENTER);
add(formpanel, BorderLayout.WEST);
}
public static void main(String[] args){
new MainFrame();
}
public static class FormPanel extends JPanel {
private JButton clear;
private TextPanel textPanel;
public FormPanel(){
clear = new JButton("Clear Cart!");
textPanel=new TextPanel();
add(clear);
clear.addActionListener(new ActionListener(){
public void actionPerformed(ActionEvent aev){
System.out.println("Test");
textPanel.setText("");
}
});
createGrid();
}
/* this methods simply creates the layout */
void createGrid(){
//creating layout
setLayout(new GridBagLayout());
GridBagConstraints gc = new GridBagConstraints();
gc.gridy++;
gc.weightx = .1;
gc.weighty = .1;
gc.gridx = 2;
gc.gridy=5;
gc.anchor = GridBagConstraints.LINE_START;
gc.insets = new Insets(0,0,0,0);
add(clear, gc);
}
}
public static class TextPanel extends JPanel {
private JTextArea textArea;
TextPanel (){
textArea = new JTextArea();
setLayout(new BorderLayout());
JScrollPane p = new JScrollPane(textArea);
add(p, BorderLayout.CENTER);
}
public void appendSomeText(String t){
textArea.append(t);
}
public void setText(String s){
textArea.setText(s);
}
}
}
You have two instances of TextPanel, one in MainFrame and the other one in FormPanel. TextPanel that is defined in FormPanel is actually not added to the panel, so textPanel.setText(""); has no effect on it as it is not visible.
When the text appended with Add To Cart! button it actually goes through a method in MainFrame - formEventOccurred() that executes textPanel.appendSomeText(). This is the other instance of TextPanel that is part of MainFrame and that is actually visible.
Looks like you need to move the duplicated logic from the main frame to the panels. Usually you should not extend JFrame as you are not adding any new functionality.
I made a small program with two buttons. I label
the buttons one exiting the program and the second importing files.
I actually let them both to exit the program when ever someone pressed on it
the problem is the buttons taking all the window, why?
I tried GridBagConstraints to resize the buttons some how but no luck anyway here's the full class without imports..
public class Window2 extends JFrame{
private static final long serialVersionUID = 1L;
public Window2(){
super ("ALANAZ imagtor");
setSize(600,400);
setDefaultCloseOperation(EXIT_ON_CLOSE);
JPanel pnl1 = new JPanel(new GridLayout());
JPanel pnl2 = new JPanel();
//button
JButton butn1 = new JButton("EXIT");
JButton butn2 =new JButton("IMPORT");
butn1.addActionListener(new ActionListener() {
#Override
public void actionPerformed(ActionEvent e) {
// TODO Auto-generated method stub
JOptionPane.showMessageDialog(null, "exiting ... bye...");
System.exit(0);
}
});
butn2.addActionListener(new ActionListener(){
public void actionPerformed(ActionEvent ae){
JOptionPane.showMessageDialog(null, "can't import now exiting");
System.exit(0);
}
});
GridBagConstraints gb1 = new GridBagConstraints();
gb1.insets = new Insets(15,15,15,15);
//Jlabel
JLabel lbl1 = new JLabel("exit or import an image");
pnl1.add(butn1);
pnl1.add(butn2);
pnl2.add(lbl1);
add(pnl2, BorderLayout.SOUTH);
add(pnl1, BorderLayout.CENTER);
}}
You are misusing your layout managers. Your pnl1 JPanel uses GridLayout (without any row or column constants?!), and if you only add one component to it, it will take up the entire JPanel. You seem to have GridBagConstraints in your code, but no GridBagLayout, which is confusing to me.
The solution is to read up on and understand how to use layout managers. Please have a look at the tutorial link: Laying Out Components Within a Container.
Key is to keep remembering that you can nest JPanels within JPanels. For example:
import java.awt.BorderLayout;
import java.awt.Dimension;
import java.awt.GridLayout;
import java.awt.event.*;
import javax.swing.*;
public class Window2 extends JFrame {
private static final long serialVersionUID = 1L;
private static final int PREF_W = 600;
private static final int PREF_H = 400;
public Window2() {
super("ALANAZ imagtor");
setDefaultCloseOperation(EXIT_ON_CLOSE);
int gap = 3;
JPanel buttonPanel = new JPanel(new GridLayout(1, 0, gap, 0));
buttonPanel.setBorder(BorderFactory.createEmptyBorder(gap, gap, gap, gap));
JPanel pnl2 = new JPanel();
JButton butn1 = new JButton("EXIT");
JButton butn2 = new JButton("IMPORT");
butn1.addActionListener(new ActionListener() {
#Override
public void actionPerformed(ActionEvent e) {
JOptionPane.showMessageDialog(null, "exiting ... bye...");
System.exit(0);
}
});
butn2.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent ae) {
JOptionPane.showMessageDialog(null, "can't import now exiting");
System.exit(0);
}
});
JLabel lbl1 = new JLabel("exit or import an image");
buttonPanel.add(butn1);
buttonPanel.add(butn2);
JPanel centerPanel = new JPanel(new BorderLayout());
centerPanel.add(buttonPanel, BorderLayout.SOUTH);
pnl2.add(lbl1);
add(pnl2, BorderLayout.SOUTH);
add(centerPanel, BorderLayout.CENTER);
}
#Override
public Dimension getPreferredSize() {
return new Dimension(PREF_W, PREF_H);
}
public static void main(String[] args) {
Window2 win2 = new Window2();
win2.pack();
win2.setLocationRelativeTo(null);
win2.setVisible(true);
}
}
If you initialize your panel with a BorderLayout instead, and add your buttons using EAST, NORTH, WEST, SOUTH, you will have a quick fix - however I do also recommend reading up on layout managers
JPanel pnl1 = new JPanel(new BorderLayout());
pnl1.add(new JButton(), BorderLayout.SOUTH);
import java.awt.*;
import javax.swing.*;
public class userInput extends JFrame {
private JButton newEntry;
private JButton deleteEntry;
private JButton editEntry;
private JButton saveEntry;
private JButton cancelEntry;
private FlowLayout layout;
public userInput() {
super("My Address Book"); //sets the title!
JTextField field = new JTextField(20);
Container content = getContentPane();
JPanel buttonPanel = new JPanel();
buttonPanel.setLayout(new FlowLayout());
buttonPanel.add(newEntry);
buttonPanel.add(deleteEntry);
buttonPanel.add(editEntry);
buttonPanel.add(saveEntry);
buttonPanel.add(cancelEntry);
add(buttonPanel, BorderLayout.SOUTH);
content.setLayout(new BorderLayout());
content.add(buttonPanel, "South");
setVisible(true);
}
}
Here is my driver program:
import javax.swing.*;
public class AddressBookGui {
public static void main (String[] args)
{
userInput addressBook = new userInput();
addressBook.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); //whenever you hit x you will exit the program
addressBook.setSize(750, 600);
addressBook.setVisible(true);
}
}
you have to initialize newEntry before doing
newEntry = new JButton("foo");
buttonPanel.add(newEntry);
along with the other buttons
You forgot to allocate your Buttons:
newEntry = new JButton();
deleteEntry = new JButton();
...