if I have
ComboBox box = b;
b.addActionListener(this);
shouldn't I expect this.actionPerformed(event) to be called
when the combobox is operated?
I have a test frame with a few combo boxes, which seem to operate
normally, but no actionPerformed is ever called. Perhaps the frame
itself needs to be armed in some way?
Your question is not so clear and you didn't give it a proper title.
If you want to add ActionListener to a ComboBox, this is how you do it:
ComboBox box = new ComboBox();
box.addActionListener(new ActionListener() {
#Override
public void actionPerformed(ActionEvent evt) {
}
});
From what I understand you just want to create a ComboBox from within a class that with be handling the action events. To do so I would suggest that the class inherits from ActionListener and override (use the #Override tag) the actionPerformed. Just Overriding the action perform is not enough if the class is not inheriting from ActionListener.
public class MyListener extends ActionListener {
#Override
public void actionPerformed (ActionEvent evt){
//code you want to execute when the event happens
}
public void methodCreatingComboBox(){
ComboBox b = new ComboBox();
b.addActionListener(this);
//other stuffs
}
}
that would work like a charm ! And you can use that same instance of MyListener for multiple events.
Here's the correct answer. I was using com.codename1.ui.Dialog as
the top level window. I switched to using com.codename1.ui.Form
and now the actions are firing as expected.
Something in the environment constructed by Dialog (which extends Form)
is interfering with the event mechanism. Perhaps by design.
Related
Which way of implementing an ActionListener is more correct?
Is there any major performance differences?
Implementing an ActionListener to the class:
public class MainFrame implements ActionListener {
JButton exampleButton1 = new JButton();
JButton exampleButton2 = new JButton();
JButton exampleButton3 = new JButton();
public MainFrame(){
exampleButton1.addActionListener(this);
exampleButton2.addActionListener(this);
exampleButton3.addActionListener(this);
}
#Override
public void actionPerformed(ActionEvent e) {
Object src = e.getSource();
if(src.equals(exampleButton1)){
//Do something
}else if(src.equals(exampleButton2)){
//Do something
}else if(src.equals(exampleButton3)){
//Do something
}
}
}
Versus adding ActionListeners to each JButton:
public class MainFrame {
JButton exampleButton1 = new JButton();
JButton exampleButton2 = new JButton();
JButton exampleButton3 = new JButton();
public MainFrame(){
exampleButton1.addActionListener(new ActionListener() {
#Override
public void actionPerformed(ActionEvent e) {
//Do something
}
});
exampleButton2.addActionListener(new ActionListener() {
#Override
public void actionPerformed(ActionEvent e) {
//Do something
}
});
exampleButton3.addActionListener(new ActionListener() {
#Override
public void actionPerformed(ActionEvent e) {
//Do something
}
});
}
}
Or perhaps even using Lambdas..?
I would prefer to use an individual Action as the listener for the button. An Action is a slightly more advanced listener that can be used anywhere an ActionListener can be used.
It provides additional functionality such as:
The same Action can be used by multiple components, such as a JButton, JMenuItem
You can disable the Action, which will disable all components that use the Action
It allows to assign mnemonics and accelerators to your components
See the Swing tutorial on How to Use Actions for more information and examples on this concept.
As you can spot, for single ActionListener-approach there are three more branches for each single if test, which button was pressed. There was nothing done yet, so no real action, just testing which button was pressed.
Now, if you want to achieve high quality work there are metrics like branch coverage. Firstly, If you go for the single ActionListener-approach each of your if is creating two branches. So you have to come up with 6 tests to just test if the base idea of your ActionListener is working correctly, so to find out which button was pressed and the correct if part was used. This is some overhead effort.
Secondly, there is the Single Responsibility Paradigm (SRP). It states that each class should be responsibly for one task only. Now, there are three things this single ActionListener is handling.
Thirdly, the reusage of the single ActionListener is very limited and highly depending on the buttons.
Fourthly, I also call this kind of single ActionListener-approach Manual written Object Orientation, because this would be an approach if there was no object orientation and you have to switch or if/else for calling different methods, like exampleButton1Pressed(), exampleButton2Pressed() etc. But, this can be achieved by three dedicated ActionListeners.
So go for dedicated ActionListeners.
I understand how to create a button and it's application in Java. Would anyone be able to show me the code to be able to make the button in the code below be able to print something as simple as hello world in the terminal. I am using bluej if that is of any matter. I am very sorry I am a beginner coder.
JButton button = new JButton();
button.setActionListener(e -> System.out.println("Clicked"));
This uses a lambda expression. Inside it, you can add as much code as you like, but add it between {} if it's more than a line.
More on buttons here
You need a listener for your button.
JButton button= new JButton("Button");
button.addActionListener(new ActionListener()
{
public void actionPerformed(ActionEvent e)
{
System.out.println("Hello World");
}
});
the button will 'listen' for the action and preform whatever task you define for it.
ActionListener is what you are looking for. There is a very nice guide on Oracle's website. You should look into this tutorial and understand different ways of creating ActionListeners. I will give you a simple example which doesn't involve Anonymous Classes because I am not sure of how much you know about them.
public class Frame extends JFrame implements ActionListener {
public Frame() {
super("Test"); // calling the superclass
setLayout(new FlowLayout()); // creating a layout for the frame
setDefaultCloseOperation(EXIT_ON_CLOSE);
// create the button
JButton jbTest = new JButton("Click me!");
/* 'this' refers to the instance of the class
because your class implements ActionListener
and you defined what to do in case a button gets pressed (see actionPerformed)
you can add it to the button
*/
jbTest.addActionListener(this);
add(jbTest);
pack();
}
// When a component gets clicked, do the following
#Override
public void actionPerformed(ActionEvent ae) {
System.out.println("Hello!");
}
}
private JButton jBtnDrawCircle = new JButton("Circle");
private JButton jBtnDrawSquare = new JButton("Square");
private JButton jBtnDrawTriangle = new JButton("Triangle");
private JButton jBtnSelection = new JButton("Selection");
How do I add action listeners to these buttons, so that from a main method I can call actionperformed on them, so when they are clicked I can call them in my program?
Two ways:
1. Implement ActionListener in your class, then use jBtnSelection.addActionListener(this); Later, you'll have to define a menthod, public void actionPerformed(ActionEvent e). However, doing this for multiple buttons can be confusing, because the actionPerformed method will have to check the source of each event (e.getSource()) to see which button it came from.
2. Use anonymous inner classes:
jBtnSelection.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent e) {
selectionButtonPressed();
}
} );
Later, you'll have to define selectionButtonPressed().
This works better when you have multiple buttons, because your calls to individual methods for handling the actions are right next to the definition of the button.
2, Updated. Since Java 8 introduced lambda expressions, you can say essentially the same thing as #2 but use fewer characters:
jBtnSelection.addActionListener(e -> selectionButtonPressed());
In this case, e is the ActionEvent. This works because the ActionListener interface has only one method, actionPerformed(ActionEvent e).
The second method also allows you to call the selectionButtonPressed method directly. In this case, you could call selectionButtonPressed() if some other action happens, too - like, when a timer goes off or something (but in this case, your method would be named something different, maybe selectionChanged()).
Your best bet is to review the Java Swing tutorials, specifically the tutorial on Buttons.
The short code snippet is:
jBtnDrawCircle.addActionListener( /*class that implements ActionListener*/ );
I don't know if this works but I made the variable names
public abstract class beep implements ActionListener {
public static void main(String[] args) {
JFrame f = new JFrame("beeper");
JButton button = new JButton("Beep me");
f.setVisible(true);
f.setSize(300, 200);
f.add(button);
button.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent e) {
// Insert code here
}
});
}
}
To add an action listener, you just call addActionListener from Abstract Button.
I am currently working on Java Swing lessons after finishing my first set of lessons in Java. In this lesson, we are working on communication between different components (buttons,toolbars,etc.) that we have been studying. The problem is, "this" is being passed as a method argument for the addActionListener() method. This works, however, I do not completely understand what it is doing. I did some research regarding "this," and found that the most popular usage for the "this" keyword would be in constructors with variables of the same names. I could not find an example that would fit my case. By looking at the code below, I will explain that parts of the code that I understand.
import java.awt.FlowLayout; Implements FlowLayout class
import java.awt.event.ActionEvent; //Imports ActionEvent Class
import java.awt.event.ActionListener; //Imports ActionListener Interface
import javax.swing.JButton; //Imports JButton class
import javax.swing.JPanel; //Imports JPanel class
public class Toolbar extends JPanel implements ActionListener {
// ^ Inherits JPanel & Implements ActionListener Interface
private JButton helloButton; //Creating variable of JButton type
private JButton goodbyeButton; //Creating variable of JButton type
public Toolbar() { //Constructor
helloButton = new JButton("Hello"); //Creates new JButton
goodbyeButton = new JButton("Goodbye"); //Creates new JButton
helloButton.addActionListener(this); //Question 1
goodbyeButton.addActionListener(this); //Question 1
setLayout(new FlowLayout(FlowLayout.LEFT)); //Sets Layout Type to Left
add(helloButton); //Adds button to FlowLayout (Layout Manager) Interface
add(goodbyeButton); //Adds button to FlowLayout (Layout Manager) Interface
}
public void setTextPanel(TextPanel textPanel) {
//No Usage Yet!
}
public void actionPerformed(ActionEvent arg0) { //Unimplemented Method from ActionListener
System.out.println("A button was clicked"); //Prints after button is clicked.
}
}
Question 1: As you can see, after creating two JButtons, we are adding an addActionListener() method in order to see detect if the button has been clicked. If it has been clicked, then it will do any code typed in actionPerformed that is implemented from the ActionListener interface. In previous lessons, we had a different way of making a button respond to a click. It did the same thing, but it looked like this:
btn.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent arg0) {
textPanel.appendText("Hello\n");
}
});
In this example, rather than printing to the console to test to see if the actionListener() worked, we just appended the text to a text area from a customized component. In the code directly above this text, we are working with an anonymous class.
So... The question is, what exactly is "this" doing as it is being passed as a method argument in addActionListener()? I know that it is printing "A button was clicked" to the console, but I don't understand the logic behind what "this" is doing to send that the button has been clicked to actionPerformed().
This is what the applications looks like:
This is the application in motion, and the buttons have already printed to the console after being clicked. I thought it might give you a better idea of what I am working on. I hope somebody can shed some light on this, and explain how "this" is working in this context. Thank you!
this represents the current instance of your class.
Toolbar class implements ActionListener interface, that means it provides an implementation of method actionPerformed.
So, helloButton.addActionListener(this); becomes possible (try to remove implements ActionListener from class declaration, the code won't compile).
Saying this, Toolbar instances can be considered as ActionListener objects and can be passed to button.addActionListener.
When using
btn.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent event) {
}
}
you create a new implementation of the ActionListener interface in the fly. This kind of implementation is named anonymous.
The both solutions are valid. Prefer the anonymous solution if the code in actionPerformed cannot be used from another place.
this is just an instance of your Toolbar class. It is used inside instance methods to represent the current instance. Essentially you can imagine your class has another private member variable named this, that points to a Toolbar object.
Since Toolbar implements ActionListener you are assigning the newly instantiated Toolbar object as a listener to the buttons (meaning the Toolbar.actionPerformed() method will be called when the buttons are clicked.)
By implementing ActionListener and then passing this to addActionListener method you are asking to be informed of any actions that are performed through a call to your actionPerformed method.
Or, more simply, you are giving the button your phone number and asking it to call you whenever something happens.
In the earlier lessons you were stating what should happen - print this text or add some text to the textPanel. To do this you were making an ActionListener on-the-fly. Now you implement ActionListener yourself you can request a callback to you anstead of making a listener one on-the-fly.
The addActionListener method for a swing component takes an argument of type ActionListener. A class that implements an ActionListener contains code that specifies what should be done when someone interacts with a swing component.
When you pass this to the addActionListener method, you are passing a reference to the current object that is being instantiated. As it happens, the current object being instantiated is also an ActionListener (as it implements ActionListener) and can therefore be passsed to the addActionListener method.
When you interact with the JButton in the GUI, the actionPerformed method of Toolbar class will be called since that is the ActionListener that you registered the JButton with.
These two examples would do the same thing (assuming all imports are there, and that capitalization would be correct):
public static void main(String args[]){
myFrame frame = new myFrame();
myFrame.addActionListener(new myListener);
}
public myFrame extends JFrame{
public myFrame(){
super("myFrame");
}
}
public myListener implements ActionListener{
public void ActionPerformed(ActionEvent e){
//Do Stuff
}
}
and
public static void main(String args[]){
myFrame frame = new myFrame();
}
public myFrame extends JFrame implements ActionListener{
public myFrame(){
super("myFrame");
this.add(this);
}
public void ActionPerformed(ActionEvent e){
//Do Stuff
}
}
The advantage of having the ActionListener be this is that it would have more direct access to fields and methods, if you want to make the modifier on the methods private.
However, if you want to avoid if/else if monstrosities for handling multiple buttons, I recommend either using a separate ActionListener(and providing a means to change what needs to be changed), or taking a look at anonymous classes and lambda expressions.
Often with object-oriented coding, it's worth role-playing the part of an object of the class you're coding. If you do that, "this" means "me".
And because Java is pass-by-reference, "this" is an arrow pointing to "me".
public class Foo implements ActionListener {
public void actionPerformed(ActionEvent e) {
// do something useful with e
}
}
Here we've written a class Foo. Since we've said it implements ActionListener, it must have an actionPerformed() method. Anything can call that:
ActionListener listener = new Foo(...);
ActionEvent event = ...;
foo.actionPerformed(event);
And we can create a Foo and give it to something that generates events:
ActionListener listener = new Foo(...);
button.addListener(listener);
If we role-play the object, you can think of this last line as "Hey button! Tell listener whenever an action happens.".
Now, what if we want the listener to take control of who tells it what on its own? "Hey button, tell me whenever an action happens".
public class Foo implements ActionListener {
public attachToButton(JButton button) {
button.addActionListener(this);
}
public void actionPerformed(ActionEvent e) {
// do something useful with e
}
}
It may help to imagine what the button's addActionListener code might look like:
public class JButton { // not the real JButton code, but it will be similar
private List<ActionListener> actionListeners = new ArrayList<ActionListener>();
public void addActionListener(ActionListener a) {
actionListeners.add(a);
}
// called internally when an event happens
private void onEvent(ActionEvent e) {
for(ActionListener listener : actionListeners) {
listener.actionPerformed(e);
}
}
}
So, if you're the listener that called addActionListener(this), then that List contains a reference that points right back at you, which the button uses to tap you on the shoulder every time an action occurs.
I have a GUI designed in Swing with all of the Components laid out. For example I have a JComboBox with a JList and a JTextField,
When I select a different item from the JComboBox I am trying to use a ListSelectionListener to call a method in a subclass to update the JTextField based on the choice,
How would I go about doing that properly? How do I call the subclass and then from the subclass update the GUI object's value?
public class Parent {
private void init() {
// ...
combo.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent e) {
Object selected = combo.getSelectedItem();
textField.setText(getTextBasedOnSelection(selected));
}
});
// ...
}
/**
* Returns the text to display when the given object is selected.
* Subclasses may override this method to display what they want
*/
protected String getTextBasedOnSelection(Object selected) {
return selected.toString();
}
// ...
}
I hope I get your problem right. You have a View Component with several subviews and you want to update one because of the changes done inside the other one.
Therefore you write an action listener for your combobox in the main View:
comboBox.addActionListener(new ActionListener(){
#Override
public void actionPerformed(ActionEvent arg0) {
textField.setText(comboBox.getSelectedItem());
}
});
Instead of inter-connecting components directly, I recommend to apply the Mediator pattern:
Create a subclass of JPanel (e.g. XyzPane) where you put all your components in. This class becomes the Mediator. It
listens for events of its components
updates the components as needed
fires its own events, if needed (this allows it to be a part of a parent Mediator: grouping components in Panes and then nesting the Panes)