So in my Java Swing application, I need a button ActionListener to be able to access variables outside of its scope like so:
int x = 13;
JButton btn = new JButton("New Button");
btn.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent e) {
System.out.println(x);
}
});
but I get a variable out of scope error. How can I access it?
The action listener is an anonymous inner class. This means that it can only use final variables from an outer scope. So, either declare x as final or pass it into the class some other way.
This should work:
final int x = 13;
JButton btn = new JButton("New Button");
btn.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent e) {
System.out.println(x);
}
});
Alternatively, see Pass variables to ActionListener in Java for some other options.
Related
public static void main(String[] args) {
ControlledBall ball2 = new ControlledBall(12,2);
JFrame window = new JFrame("Controlled Ball");
window.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
window.setLayout(new GridBagLayout());
GridBagConstraints c = new GridBagConstraints();
JButton stop = new JButton("Stop");
stop.setSize(4,400);
stop.setVisible(true);
stop.setText("Stop");
stop.addActionListener(new Action());
i get an error on the last line that says 'controlledball.this cannot be referenced from a static context'
when i try the following technique instead of calling the stop() method i just change the values i need to change:
stop.addActionListener(new ActionListener() {
#Override
public void actionPerformed(ActionEvent e) {
x= 0;
y = 0;
}
});
i get the error non-static field 'x' cannot be referenced from a static context...
the question is, from the main method how can i change the values of x and y which are declared in another method?
There are may ways you can solve this problem. A good suggestion is to probably create a custom ActionListener that holds a reference to the Object you want to change. For example, you could have:
class StopListener implements ActionListener {
private ControlledBall ball;
public StopListener(ControlledBall ball) {
this.ball = ball;
}
#Override
public void actionPerformed(ActionEvent e) {
ball.stop(); // sets x and y to zero
}
}
Then you can just instantiate and use that class as an ActionListener:
stop.addActionListener(new MyListener(ball2));
This should help you organize your code and keep it clean and maintainable.
I have looked on the internet and can not find any help with understanding action listeners. I am just starting to learn Java and I have yet to find a good tutorial that helps me understand how to use action listeners. Could someone look over my code or point me in the way of a useful tutorial explaining how to use action listeners?
public static void go implements ActionListener(){
JFrame j = new JFrame();
j.setDefaultCloseOperation(EXIT_ON_CLOSE);
j.setSize(640,480);
final Screen screen = new Screen();
j.add(BorderLayout.CENTER, screen);
JButton button = new JButton("Click Me!");
button.addActionListener(new ActionListener(){
public void ActionPerformed(Event e){
screen.repaint();
}
});
j.add(BorderLayout.NORTH, button);
j.setVisible(true);
}
Other way and much better way is to use Anonymous class. You don't need to implement ActionListener
public static void go(){ // no need to implement actionListener
JFrame j = new JFrame();
j.setDefaultCloseOperation(EXIT_ON_CLOSE);
j.setSize(640,480);
final Screen screen = new Screen();
j.add(BorderLayout.CENTER, screen);
JButton button = new JButton("Click Me!");
button.addActionListener(new ActionListener(){ // change are made here
#Override
public void actionPerformed(ActionEvent e) { //& here
screen.repaint();
}
});
j.add(BorderLayout.NORTH, button);
j.setVisible(true);
}
actionPerformed is a method of interface, its not a class.
So use actionPerformed insted of ActionPerformed and use annotation #Ovveride for ovveriding the actionPerformed to provide your own defination.
#Override
public void actionPerformed(ActionEvent e) {
screen.repaint();
}
This seems like a very simple problem, but I'm having a lot of trouble figuring out how to deal with it.
Sample Scenario:
final int number = 0;
JFrame frame = new JFrame();
frame.setVisible(true);
frame.setDefaultCloseOperation(EXIT_ON_CLOSE);
frame.setSize(400, 400);
final JTextArea text = new JTextArea();
frame.add(text, BorderLayout.NORTH);
JButton button = new JButton(number + "");
button.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent arg0) {
number++; // Error is on this line
text.setText(number + "");
}});
frame.add(button, BorderLayout.SOUTH);
I really have no idea where to go.
If you declared number as final, you cannot modified its value. You must remove the final modificator.
Then, you can access to that variable via:
public class Scenario {
private int number;
public Scenario() {
JButton button = new JButton(number + "");
button.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent arg0) {
Scenario.this.number++;
text.setText(Scenario.this.number + "");
}
});
}
}
The notation "ClassName.this" allow you to access to the object of a class where you are in.
Keep atention in when you use "number" first time, -new JButton(number)-, you can access to number directly, because you are in Scenario scope. But when you use it inside your ActionListener, you are in your ActionListener scope instead of Scenario scope. That is why you cannot see the variable "number" directly inside of your action listener and you have to access to the instance of Scenario where you are in. This can be done by Scenario.this
The fastest solution would be to declare number as static, and reference it using the name of your class.
Alternatively you can make a class that implements ActionListener, and pass number and text into it's constructor.
I have the following constructor which defines a board and checks if any of the three JButtons have been clicked:
Timer timer = new Timer(500, this);
private boolean[][] board;
private boolean isActive = true;
private int height;
private int width;
private int multiplier = 40;
JButton button1;
JButton button2;
JButton button3;
public Board(boolean[][] board) {
this.board = board;
height = board.length;
width = board[0].length;
setBackground(Color.black);
button1 = new JButton("Stop");
add(button1);
button1.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent e) {
isActive = !isActive;
button1.setText(isActive ? "Stop" : "Start");
}
});
button2 = new JButton("Random");
add(button2);
button2.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent e) {
this.board = randomBoard();
}
});
button3 = new JButton("Clear");
add(button3);
button3.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent e) {
this.board = clearBoard();
}
});
}
But it returns this error:
Exception in thread "main" java.lang.Error: Unresolved compilation problems:
board cannot be resolved or is not a field
board cannot be resolved or is not a field
Why is this? How do I access this.board in the constructor?
The problem is caused by you trying to access this.board inside the anonymous inner classes. Since there is no board field defined, that causes an error.
For example this:
button2.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent e) {
this.board = randomBoard();
}
});
In order to be able to use the board variable inside your anonymous inner classes you either need to remove this or use something like Board.this.board (if you want to be more explicit).
This should work:
Board.this.board = randomBoard();
The problem is this related to the class ActionListener which did not have a board variable. However with Board.this you specify that you mean the board member of the Board class. This is the syntax you need to use for nested classes to access the variables of the outer class.
The problem is here
button2.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent e) {
this.board = randomBoard();
}
});
your button2 is a JButton, and it doesn't have a board field. Don't access it with this.board. Find another way to access board.
The fast and dirty way to do it would be to make board static, but I'm sure that JButton has a way for you to specify a parent(your board class), and access the board field that way.
After a quick look at The documentation for JButton, I've found a getParent method. I'd start with that.
As others and the compiler said, you don't have a field (which is an instance member), but only a local variable board, which goes out of scope as soon as constructor exits.
public class MainFrame extends JFrame {
MainFrame()
{
JButton zeroButton = new JButton("0");
add(zeroButton);
Handler handler = new Handler();
zeroButton.addActionListener(handler);
}
private class Handler implements ActionListener
{
public void actionPerformed(ActionEvent e) {
if (e.getSource() == **zeroButton**)
JOptionPane.showMessageDialog(null, "hello there!");
}
}
}
The code has an error which underlines what I marked as bold in the code.
here is the error message: "cannot find symbol"
I thought I can access outer class modifiers from the inner class, but it doesn't work!
zeroButton is not a member of outer class (MainFrame), it's a local variable in MainFrame constructor.
Try something like this
public class MainFrame extends JFrame {
private final JButton zeroButton;
MainFrame() {
zeroButton = new JButton("0");
PS I'm also not sure if you're supposed to compare controls with ==. (never used swing)
You are unable to access it because the button is a local variable in another method.
You have two options:
1. Make the button in a instance variable (a class level variable). See Nikita's answer.
Have the handler as an anonymous implementation, in the constructor:
MainFrame() {
final JButton zeroButton = new JButton("0");
add(zeroButton);
Handler handler = new Handler();
zeroButton.addActionListener(new ActionListener()
{
public void actionPerformed(ActionEvent e) {
if (e.getSource() == **zeroButton**)
JOptionPane.showMessageDialog(null, "hello there!");
}
});
}
In this case, the variable should be final because only final local variables are accessible to inner classes.