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How to populate a ButtonGroup[] array using a for loop

So I have two arrays of JRadioButton[] that I am trying to put into an array of ButtonGroup[]. When I add a single JRadioButton from my array one at a time to a single ButtonGroup, I have no problems. When I use a for-loop to add to an array of ButtonGroup[], there are no syntax errors but I do get a compiler error:
Exception in thread "main" java.lang.NullPointerException: Cannot invoke "javax.swing.ButtonGroup.add(javax.swing.AbstractButton)" because "allButtons[x]" is null
I don't mind creating buttonGroups one at a time, but I was just wondering why the first one works and the other doesn't.
The code that works:
//creates radio buttons
JRadioButton[] profButtons = new JRadioButton[profs.length];
JRadioButton[] expButtons = new JRadioButton[profs.length];
for(int i=0; i<profs.length; i++)
{
profButtons[i] = new JRadioButton(profs[i]);
expButtons[i] = new JRadioButton(profs[i]);
}
//makes the radio button groups.
ButtonGroup acrGroup = new ButtonGroup();
acrGroup.add(profButtons[0]);
acrGroup.add(expButtons[0]);
The code that doesn't work:
//creates radio buttons
JRadioButton[] profButtons = new JRadioButton[profs.length];
JRadioButton[] expButtons = new JRadioButton[profs.length];
for(int i=0; i<profs.length; i++)
{
profButtons[i] = new JRadioButton(profs[i]);
expButtons[i] = new JRadioButton(profs[i]);
}
//makes the radio button groups.
ButtonGroup[] allButtons = new ButtonGroup[profs.length];
for(int x=0; x<profs.length; x++)
{
allButtons[x].add(profButtons[x]);
allButtons[x].add(expButtons[x]);
}
profs is a String[] array that I use to label the radio buttons.

In this part of your code, you're creating the array of ButtonGroups, but you never initialize each element, so each of them is null
//makes the radio button groups.
ButtonGroup[] allButtons = new ButtonGroup[profs.length];
for(int x=0; x<profs.length; x++)
{
allButtons[x].add(profButtons[x]);
allButtons[x].add(expButtons[x]);
}
So, inside your for-loop, add this line as the first line
allButtons[x] = new ButtonGroup();
And then, the rest of your code

How to randomly disable buttons in java

I have this program where I need to randomly disable some buttons so that the user is not able to select all of them. Is there a way to disable random buttons in java?
I was thinking of using Math.random, but I don't know how to start or refer to it.....
These are the buttons.
JButton but1 = new JButton();
JButton but2 = new JButton();
JButton but3 = new JButton();
JButton but4 = new JButton();
JButton but5 = new JButton();
JButton but6 = new JButton();
JButton but7 = new JButton();

Array/Vararg Verson:
public JButton getRandomButton(JButton... buttons) {
int index = (int) (Math.random() * buttons.length);
return buttons[index];
}
List Version:
public JButton getRandomButton(List<JButton> buttons) {
int index = (int) (Math.random() * buttons.size());
return buttons.get(index);
}
Both methods will return a random JButton for you to do stuff with.

You can put them all in a list, then shuffle the list and disable the element at index 0 for example...
but note that:
it could happen that you are disabling an already disabled button, so you need to do some checks
shuffling a list is an operation that will require more time, and it depends on how many buttons you have in the list.
List<JButton> items = Arrays.asList(new JButton(), new JButton(), new JButton());
System.out.println(items);
Collections.shuffle(items);
items.get(0).setEnabled(true);

Put your buttons into a List and iterate over them. Use Random.nextBoolean to determine whether to enable or disable the button. This will give you a 50/50 distribution. If you require something else, use nextInt and a modulus.
List<JButton> myButtons = /*whatever*/;
final Random generator = new Random();
for (JButton button : myButtons)
{
button.setEnabled(generator.nextBoolean());
}
This won't guarantee that any number of the buttons are enabled at any one time. You weren't clear whether that was important.

Dynamically generated jbuttons

I'm trying to make a GUI for that simulates elevators in a building (really to test threading/c-scan), but when generating the buttons for the elevator control panel and the buttons for each individual floor, I'm kind of stuck. I thinking about trying to generate a new pair of buttons for each floor, and generating a new control panel per elevator. Also there's the difficulty of having a variable amount of floors. Anyway my question what is this best way to go about doing this? Perhaps it's not necessary to generate new buttons for everything and just use one set and change what the actions do per floor/elevator? I'm asking because I'm not very familiar with GUIs. Thanks for the help

If all the elevators, and the control panel are the same, you can use a singular method and pass in the elevator or the control panel. CustomPanel extends JPanel and has a method foo.
public void createElevatorButtons(final CustomPanel panel) {
ArrayList<JButton> buttons = new ArrayList<>(); //arraylist of buttons we can keep track of
JPanel buttonPanel = new JPanel(); //the visible component
for(int i = 1; i <= numberOfFloors;i++) {
JButton button = new JButton(String.valueOf(i)); //creates buttons for floors 1 to max
buttons.add(button);
buttonPanel.add(button);
}
panel.add(buttonPanel);
//add the action listeners
for(JButton button : buttons) {
button.addActionListener( new ActionListener() {
public void actionPerformed(ActionEvent e) {
JButton floor = (JButton) e.getSource();
int floorNumber = Integer.parseInt(floor.getText());
panel.foo(floorNumber); //we tell the elevator/panel/whatever to do something, you will have to extend JPanel to do foo
}
});
}
}

In this case that the number of floors is variable you can create an array of buttons:
JButton[] buttons = new JButton[MAX_NUMBER_OF_FLOORS];
Then when you determine the exact of number of floors at runtime, you can go to instantiate and add the buttons:
for(int i=0; i<numberOfFloors; i++) {
buttons[i] = new JButton();
controlPanel.add(buttons[i]);
}
Something like this should work.
Assign MAX_NUMBER_OF_FLOORS a big number like 100, there should be a possible limit given by the problem.

Java Button Action Command

I'm creating a simple Minesweeper game in Java. Size 9x9.
I create an array of JPanels and an array of buttons; I add each button to its respective JPanel. then i add the JPanels to the JFrame.
How do i distinguish between each button on the action event?
Here's some of my code:
int gridx = 9;
int gridy = 9;
JButton[] buttons = new JButton[gridx*gridy];
JPanel[] jpanels = new JPanel[gridx*gridy];
public Minesweeper(){
super("Minesweeper");
setLayout(new GridLayout(9,9));
JPanel panel = new JPanel();
int i = 0;
for(i = 0; i<gridx*gridy; i++){
jpanels[i] = new JPanel();
buttons[i] = new JButton();
buttons[i].addActionListener(buttonEvent);
jpanels[i].setLayout(new GridLayout(1,1));
jpanels[i].add(buttons[i]);
add(jpanels[i]);
}
//buttons[67].setEnabled(false);
setSize(300,300);
setVisible(true);
}
The only way i can think about doing this is adding text to the button like so:
buttons[i] = new JButton(i);
Then calling getActionCommand() but i dont want text to show up on the button. Any other ideas?

You can use AbstractButton#setActionCommand.
In your loop:
buttons[i].setActionCommand(i+"");
Then you'll get i back when you use getActionCommand
Note I did mention in a comment on another answer that I would create a new class Mine which extends JButton which I believe to be a better and more complete solution. This however gets the job done rather quickly.

How to get values of dynamically generated components?

I am dynamically generating a list of name boxes, sliders and labels, and am trying to figure out how to access the values of the sliders and change the labels. Other posts suggest using an array, but I have no idea where to begin with that.
My code is as such:
public Tailoring(int n) {
/*initComponents();*/
JPanel containerPanel = new JPanel();
containerPanel.setLayout(new BoxLayout(containerPanel, BoxLayout.PAGE_AXIS));
this.add(containerPanel);
JLabel Title = new JLabel("Tailoring:");
containerPanel.add(Title);
for(int i = 0; i < n; i++){
JPanel rowPanel = new JPanel(new FlowLayout(FlowLayout.LEFT));
JTextField NameBox = new JTextField("Guest " + (i+1));
JSlider TipSlider = new JSlider();
JLabel TipCost = new JLabel();
rowPanel.add(NameBox);
rowPanel.add(TipSlider);
rowPanel.add(TipCost);
containerPanel.add(rowPanel);
}
}

You can create a new class YourPanel which extends JPanel.
Instead of the statement
JPanel rowPanel = new JPanel(new FlowLayout(FlowLayout.LEFT));
you can use
YourPanel rowPanel = new YourPanel(new FlowLayout(FlowLayout.LEFT));
Define textfield, slider and label as the properties of this YourPanel class.
Provide getters/setters for each field. Then use an array or ArrayList of YourPanel objects in your application. You will be able to reach the nth panel's label with a call like:
panels.get(n).getJLabel();

It appears that you'd like to change the value displayed in the JLabel when the associated JSlider is modified, right? The best way to associate pairs of objects in Java is with a Map structure:
Map<Component, JSlider> sliderToLabel = new HashMap<Component, JSlider>();
for (int i = 0; i < n; i++) {
// after your loop code
sliderToLabel.put(TipSlider, TipCost); // map the slider to its label
}
You will be able to get a reference to the JSlider in the code that listens for changes on that component.
JLabel updateLabel = sliderToLabel.get(targetedSlider);
updateLabel.setText("updated text");
Notes
As a matter of convention, variable names should begin with lower case letters
The event listener I alluded to should also be attached in the loop. See Writing Event ListenersOracle