import javax.swing.*;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
public class Form1 {
private JPanel panel1;
private JButton button1;
public Form1() {
button1.addActionListener(new ActionListener() {
#Override
public void actionPerformed(ActionEvent e) {
System.out.println("Button clicked");
}
});
}
public static void main(String args[]){
JFrame frame = new JFrame("Form 1");
frame.setContentPane(new Form1().panel1);
frame.pack();
frame.setVisible(true);
}
}
The error relates to the setContentPane in main method and then also the actionListener. I've posted the exact error below. Why is this? I created this using the IntelliJ IDEA GUI form creator.
Exception in thread "main" java.lang.NullPointerException
at Form1.<init>(Form1.java:12)
at Form1.main(Form1.java:22)
In Java you can't use variables that are not initialized, else you will receive a NPE. See the following code and please read a Java book :)
public class Form1 {
private JPanel panel1;
private JButton button1;
public Form1() {
panel1 = new JPanel();
button1 = new JButton1("Press Me");
panel1.add(button1);
button1.addActionListener(new ActionListener() {
#Override
public void actionPerformed(ActionEvent e) {
System.out.println("Button clicked");
}
});
}
public static void main(String args[]){
JFrame frame = new JFrame("Form 1");
frame.setContentPane(new Form1().panel1);
frame.pack();
frame.setVisible(true);
}
}
So there are 2 issues that you're going to run into that are causing that error. After you create an object (JButton, JPanel in this case), you have to instantiate them. There are a lot of different ways to do this, and it depends on what you're trying to do, but the simplest fix for your issue here is to add the following lines in the constructor:
panel1 = new JPanel();
button1 = new JButton();
Your code should run fine after that.
Related
I am trying to get the buttons to at least execute something when they are pressed and BlueJ doesn't show any errors, but when I execute the Program and I try to press the buttons, nothing happens. I am really unsure why that is the case. I would appreciate any help!
import javax.swing.*;
import java.awt.*;
import java.awt.event.ActionListener;
import java.awt.event.ActionEvent;
class MainMenu
{
JFrame frame= new JFrame();
JButton button = new JButton("Singleplayer");
JButton button2 = new JButton("Multiplayer");
MainMenu(){
prepareGUI();
}
public void prepareGUI(){
frame.setTitle("Game");
frame.getContentPane().setLayout(null);
frame.add(button);
frame.add(button2);
button.setBounds(100,200,100,40);
button2.setBounds(200,200,100,40);
frame.setVisible(true);
frame.setBounds(200,200,400,400);
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
}
public void setUpButtonListeners(){
ActionListener buttonlistener = new ActionListener() {
#Override
public void actionPerformed(ActionEvent e) {
frame.getContentPane().setBackground(Color.green);
System.out.println("Singleplayer Selected");
}
};
ActionListener buttonlistener2 = new ActionListener() {
#Override
public void actionPerformed(ActionEvent ae) {
frame.getContentPane().setBackground(Color.red);
System.out.println("Multiplayer Selected");
}
};
button.addActionListener(buttonlistener);
button2.addActionListener(buttonlistener2);
}
public class MainClass {
public void main(String args[] )
{
new MainMenu();
}
}
}
setUpButtonListeners() are not executed in the program. So action listeners are not available. you can include setUpButtonListeners() in prepareGUI method.
To be short, I create a class Something witch have a function with a JFrame where I have a label and a button on it. On the button I have an addActionListener(new changeLabel()).
I did class changeLabel in the src package for the listener but when I start the application and I click the button throw an NullPointerException on the changeLabel at
nameLabel.setText("Name changed");
line. I want to mention that if I create this listener class in Something class, work perfectly.
I don't know why throw null exception because the label is initialized firstly and after that, the button just want to change the text.
I tryed to make a getFunction, to call that label, I tryed with object Something, with object changeLabel etc... but doesn't work.
Here is some code
package trying;
import javax.swing.*;
import java.awt.BorderLayout;
import java.awt.event.*;
public class Something {
JFrame frame;
JLabel changeName;
JButton button;
public void gui(){
frame = new JFrame();
frame.setSize(200, 200);
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
//is just an example
changeName = new JLabel("Stefan");
//is just an example
button = new JButton("Change");
button.addActionListener(new changeLabel());
frame.getContentPane().add(changeName, BorderLayout.NORTH);
frame.getContentPane().add(button, BorderLayout.SOUTH);
frame.setVisible(true);
}
public static void main(String args[]){
new Something().gui();
}
}
The listener class
package trying;
import java.awt.event.*;
public class changeLabel extends Something implements ActionListener{
#Override
public void actionPerformed(ActionEvent e) {
changeName.setText("Andrei");
}
}
How can I solve this problem?
The problem is that because the changeLabel class extends Something, it will contain it's own changeName variable which is not initialized == null.
You can:
make the changeLabel implementation private class of Something (good practice) or
pass the JLabel to its constructor.
In both ways changeLabel should not extend Something.
Code Sample #1:
public class Something {
JFrame frame;
JLabel changeName;
JButton button;
public void gui(){
frame = new JFrame();
frame.setSize(200, 200);
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
//is just an example
changeName = new JLabel("Stefan");
//is just an example
frame.getContentPane().add(changeName, BorderLayout.NORTH);
button = new JButton("Change");
button.addActionListener(new changeLabel());
frame.getContentPane().add(button, BorderLayout.SOUTH);
frame.setVisible(true);
}
public static void main(String args[]){
new Something().gui();
}
class changeLabel implements ActionListener{
#Override
public void actionPerformed(ActionEvent e) {
changeName.setText("Andrei");
}
}
}
Code Sample #2:
public class Something {
...
public void gui() {
...
button.addActionListener(new changeLabel(changeName));
}
}
public class changeLabel implements ActionListener {
private final JLabel label;
public changeLabel(JLabel label) {
this.label = label;
}
#Override
public void actionPerformed(ActionEvent e) {
label.setText("Andrei");
}
}
By now, I already know my code is flawed. I just want to know why it's flawed. I want to activate the "webButton" so that when it gets clicked, it prints a message on the console that reads "This opens Mozilla Firefox."
package smartphone;
import java.awt.*;
import java.awt.event.ActionEvent;
import javax.swing.*;
import java.util.Scanner;
public class Smartphone implements ActionListener {
public static void main(String[] args) {
{
{
JFrame container = new JFrame();
container.setLayout(new BorderLayout());
Scanner daniel = new Scanner(System.in);
JButton webButton = new JButton(new ImageIcon("Firefox.png"));
JButton phoButton = new JButton(new ImageIcon("Facebook.png"));
JButton texButton = new JButton(new ImageIcon("Phone.png"));
JButton setButton = new JButton(new ImageIcon("Settings.png"));
JButton smsButton = new JButton(new ImageIcon("sms.png"));
container.setTitle("Smartphone Interface!");
container.setSize(240,340);
container.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
container.add(setButton, BorderLayout.CENTER);
container.add(webButton, BorderLayout.SOUTH);
container.add(texButton, BorderLayout.NORTH);
container.add(phoButton, BorderLayout.EAST);
container.add(smsButton, BorderLayout.WEST);
container.setVisible(true);
webButton.addActionListener(instanceofSmartphone);
}
}
}
}
Do this.
webButton.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent e)
{
//Execute when button is pressed
}
});
If you want to use the ActionListener interface then, implement it in your Frame class and then replace that instanceOfSmartphone with
webButton.addActionListener(this);
And put this outside the method
public void actionPerformed(ActionEvent e) {
if(e.getSource() == webButton) {
}
}
You need to implement the actionPerformed method. An example follows.
public void actionPerformed(ActionEvent e) {
System.out.println("This method opens Mozilla Firefox.");
}
Additionally, you need to change how you add the action listener to the following.
webButton.addActionListener(this);
There are also a number of other issues. Here is a modified version of your code to get it working but far from perfect or what you will want in the end. I strongly recommend you walk through all of the tutorials in order at the below website. It doesn't get any easier than what they have. Also, if your not already using it, you should try Netbeans or some other IDE. It give you feedback that may help while you are starting out.
https://docs.oracle.com/javase/tutorial/
package smartphone;
import java.awt.*;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import javax.swing.*;
import java.util.Scanner;
public class Smartphone extends Frame implements ActionListener {
static JButton webButton = new JButton(new ImageIcon("Firefox.png"));
static JButton phoButton = new JButton(new ImageIcon("Facebook.png"));
static JButton texButton = new JButton(new ImageIcon("Phone.png"));
static JButton setButton = new JButton(new ImageIcon("Settings.png"));
static JButton smsButton = new JButton(new ImageIcon("sms.png"));
Smartphone(){
webButton.addActionListener(this);
}
public static void main(String[] args) {
Smartphone container = new Smartphone();
container.setLayout(new BorderLayout());
Scanner daniel = new Scanner(System.in);
container.setTitle("Smartphone Interface!");
container.setSize(240, 340);
container.add(setButton, BorderLayout.CENTER);
container.add(webButton, BorderLayout.SOUTH);
container.add(texButton, BorderLayout.NORTH);
container.add(phoButton, BorderLayout.EAST);
container.add(smsButton, BorderLayout.WEST);
container.setVisible(true);
}
#Override
public void actionPerformed(ActionEvent e) {
System.out.println("This opens a Firefox Webbrowser.");
}
}
You need to add an ActionListener to the buttons INSIDE of the constructor: buttonName.addActionListener(this);.
Then you need to create the following method:
public void actionPerformed(ActionEvent event) {
Object control = event.getSource();
if (control == buttonName) {
//Run code...
}
}
I am trying to call the actionPerformed() in normal method of class. I know that it get automatically executed on whenever button get pressed. But I want to call that method when ENTER button get pressed on Specific textfield. Is it possible to call actionPerformed() in keyPressed() or in normal function/method.
The following code will give you rough idea what I want to do.
void myFunction()
{
actionPerformed(ActionEvent ae);
}
public void actionPerformed(ActionEvent ae)
{
//my code
}
Thanks in advance
If you want, some actionPerformed() method of a JButton to be executed on pressing ENTER inside a JTextField, then I guess you can use the doClick(), method from AbstractButton class to achieve this. Though this approach, might can override the original behaviour of the JTextField on press of the ENTER key :(
Please have a look at this code pasted below, to see if this is what, stands fit for your needs :-) !!!
import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
public class ButtonClickExample
{
private JTextField tfield;
private JButton button;
private JLabel label;
private ActionListener actions = new ActionListener()
{
#Override
public void actionPerformed(ActionEvent ae)
{
if (ae.getSource() == button)
{
label.setText(tfield.getText());
}
else if (ae.getSource() == tfield)
{
button.doClick();
}
}
};
private void displayGUI()
{
JFrame frame = new JFrame("Button Click Example");
frame.setDefaultCloseOperation(JFrame.DISPOSE_ON_CLOSE);
JPanel contentPane = new JPanel();
contentPane.setLayout(new BorderLayout(5, 5));
JPanel centerPanel = new JPanel();
tfield = new JTextField("", 10);
button = new JButton("Click Me or not, YOUR WISH");
tfield.addActionListener(actions);
button.addActionListener(actions);
centerPanel.add(tfield);
centerPanel.add(button);
contentPane.add(centerPanel, BorderLayout.CENTER);
label = new JLabel("Nothing to show yet", JLabel.CENTER);
contentPane.add(label, BorderLayout.PAGE_END);
frame.setContentPane(contentPane);
frame.pack();
frame.setLocationByPlatform(true);
frame.setVisible(true);
}
public static void main(String[] args)
{
EventQueue.invokeLater(new Runnable()
{
#Override
public void run()
{
new ButtonClickExample().displayGUI();
}
});
}
}
I know this is an old thread, but for other people seeing this my recomendation is something like this:
// This calls the method that you call in the listener method
void performActionPerformedMethod(){
actionPerformed(ActionEvent e);
}
// This is what you want the listener method to do
void actionPerformedMethod(){
// Code...
}
// This is the interface method
public void actionPerformed(ActionEvent e){
actionPerformedMethod()
}
I'm creating a very simple program.
I have created this classes :
MainJframeClass, JDesktopPaneClass, JinternalFrameClass1 and JinternalFrameClass2.
what ive done is that i instantiated my jdesktoppaneclass and named it desktoppane1 and i added it to the MainJframeclass. i have also instantiated the 2 jinternalframes and named it internal1 and internal2. Now, i have button in mainjframeclass that when i press, i add the internal1 to desktoppane1. what my problem now is how to add the internal2 to desktoppane1 using a button placed somewhere in internal1. i know that why could i just add another button to desktoppane1 and add the internal2. but i have done it already, i just want to solve this problem. if you can help me please. sorry for my english by the way.
It's simply a matter of references. The code that adds something to the JDesktopPane must have a reference to it, and so you will need to pass that reference into the class that needs it say via either a constructor parameter or a method parameter.
Edit 1
For example:
import java.awt.*;
import java.awt.event.*;
import java.util.Random;
import javax.swing.*;
public class ReferenceExample extends JPanel {
private JDesktopPane desktop = new JDesktopPane();
private Random random = new Random();
public ReferenceExample() {
JButton addInternalFrameBtn = new JButton("Add Internal Frame");
addInternalFrameBtn.addActionListener(new ActionListener() {
#Override
public void actionPerformed(ActionEvent e) {
addInternalFrame();
}
});
JPanel btnPanel = new JPanel();
btnPanel.add(addInternalFrameBtn);
setPreferredSize(new Dimension(600, 450));
setLayout(new BorderLayout());
add(new JScrollPane(desktop), BorderLayout.CENTER);
add(btnPanel, BorderLayout.SOUTH);
}
public void addInternalFrame() {
MyInternalFrame intFrame = new MyInternalFrame(ReferenceExample.this);
int x = random.nextInt(getWidth() - intFrame.getPreferredSize().width);
int y = random.nextInt(getHeight() - intFrame.getPreferredSize().height);
intFrame.setLocation(x, y);
desktop.add(intFrame);
intFrame.setVisible(true);
}
private static void createAndShowUI() {
JFrame frame = new JFrame("Reference Eg");
frame.getContentPane().add(new ReferenceExample());
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.pack();
frame.setLocationRelativeTo(null);
frame.setVisible(true);
}
public static void main(String[] args) {
java.awt.EventQueue.invokeLater(new Runnable() {
public void run() {
createAndShowUI();
}
});
}
}
class MyInternalFrame extends JInternalFrame {
// pass in the reference in the constructor
public MyInternalFrame(final ReferenceExample refEg) {
setPreferredSize(new Dimension(200, 200));
setClosable(true);
JButton addInternalFrameBtn = new JButton("Add Internal Frame");
addInternalFrameBtn.addActionListener(new ActionListener() {
#Override
public void actionPerformed(ActionEvent e) {
// use the reference here
refEg.addInternalFrame();
}
});
JPanel panel = new JPanel();
panel.add(addInternalFrameBtn);
getContentPane().add(panel);
pack();
}
}
how to add the internal2 to desktoppane1 using a button placed somewhere in internal1.
In the ActionListener added to your button you can use code like the following to get a reference to the desktop pane:
Container container = SwingUtilities.getAncestorOfClass(JDesktopPane.class, (Component)event.getSource());
if (container != null)
{
JDesktopPane desktop = (JDesktopPane)container;
JInternalFrame frame = new JInternalFrame(...);
desktop.add( frame );
}