How to add action listener that listens to multiple buttons

Question

I m trying to figure out what i am doing wrong with action listeners  I m following multiple tutorials and yet netbeans and eclipse are giving me errors when im trying to use an action listener    Below is a simple program that im trying to get a button working in    What am i doing wrong   import java awt event ActionEvent  import java awt event ActionListener  import javax swing JButton  import javax swing JFrame    public class calc extends JFrame implements ActionListener          public static void main String   args             JFrame calcFrame   new JFrame             calcFrame setSize 100  100           calcFrame setVisible true            JButton button1   new JButton  1            button1 addActionListener this            calcFrame add button1              public void actionPerformed ActionEvent e            if e getSource      button1               the action listener is never registered because with the if e getSource      button1  it cant see button1  errors saying cannot find symbol

User · Answer

You are declaring button1 in main method so you can not access it in actionPerform  You should make it global in class    JButton button1   public static void main String   args         JFrame calcFrame   new JFrame         calcFrame setSize 100  100       calcFrame setVisible true        button1   new JButton  1        button1 addActionListener this        calcFrame add button1      public void actionPerformed ActionEvent e        if e getSource      button1

User · Answer

I m amazed that nobody has mentioned using an action command  This is a pretty standard way of associating sources and listeners  Its really useful if    you have multiple event sources that need to do the same thing  eg if you want the use to be able to press the enter key on a text field as an alternative to clicking a button next to it  you don t have a ref to the component generating the event     see   import java awt FlowLayout  import java awt event ActionEvent      import java awt event ActionListener  import javax swing JButton  import javax swing JFrame  import javax swing JOptionPane   public class DontExtendJFrame implements ActionListener      private enum Actions       HELLO      GOODBYE        public static void main String   args         DontExtendJFrame instance   new DontExtendJFrame         JFrame frame   new JFrame  Test        frame setLayout new FlowLayout         frame setSize 200  100        JButton hello   new JButton  Hello        hello setActionCommand Actions HELLO name         hello addActionListener instance       frame add hello        JButton goodbye   new JButton  Goodbye        goodbye setActionCommand Actions GOODBYE name         goodbye addActionListener instance       frame add goodbye        frame setVisible true           Override   public void actionPerformed ActionEvent evt        if  evt getActionCommand      Actions HELLO name            JOptionPane showMessageDialog null   Hello          else if  evt getActionCommand      Actions GOODBYE name            JOptionPane showMessageDialog null   Goodbye

User · Answer

Using my approach  you can write the button click event handler in the  classical way   just like how you did it in VB or MFC    Suppose we have a class for a frame window which contains 2 buttons  class MainWindow       Jbutton searchButton      Jbutton filterButton     You can use my  router  class to route the event back to your MainWindow class  class MainWindow       JButton searchButton      Jbutton filterButton      ButtonClickRouter buttonRouter   new ButtonClickRouter this            void initWindowContent                create your components here                        setup button listeners         searchButton addActionListener buttonRouter           filterButton addActionListener buttonRouter              void on searchButton                TODO your handler goes here                   void on filterButton                TODO your handler goes here             Do you like it     If you like this way and hate the Java s anonymous subclass way  then you are as old as I am  The problem of  addActionListener new ActionListener         is that it squeezes all button handlers into one outer method which makes the programme look wired   in case you have a number of buttons in one window  Finally  the router class is at below  You can copy it into your programme without the need for any update  Just one thing to mention  the button fields and the event handler methods must be accessible to this router class  To simply put  if you copy this router class in the same package of your programme  your button fields and methods must be package-accessible  Otherwise  they must be public  import java awt event ActionEvent  import java awt event ActionListener  import java lang reflect Field  import java lang reflect InvocationTargetException  import java lang reflect Method   public class ButtonClickRouter implements ActionListener       private Object target       ButtonClickRouter Object target            this target   target              Override     public void actionPerformed ActionEvent actionEvent               get source button         Object sourceButton   actionEvent getSource                find the corresponding field of the button in the host class         Field fieldOfSourceButton   null          for  Field field   target getClass   getDeclaredFields                  try                   if  field get target  equals sourceButton                         fieldOfSourceButton   field                      break                                  catch  IllegalAccessException e                                     if  fieldOfSourceButton    null              return              make the expected method name for the source button            rule  suppose the button field is  searchButton   then the method            is expected to be  void on searchButton            String methodName    quot on  quot    fieldOfSourceButton getName                find such a method         Method expectedHanderMethod   null          for  Method method   target getClass   getDeclaredMethods                  if  method getName   equals methodName                     expectedHanderMethod   method                  break                                   if  expectedHanderMethod    null              return              fire         try               expectedHanderMethod invoke target             catch  IllegalAccessException   InvocationTargetException e               I m a beginner in Java  not in programming   so maybe there are anything inappropriate in the above code  Review it before using it  please

User · Answer

There is no this pointer in a static method    I don t believe this code will even compile    You shouldn t be doing these things in a static method like main    set things up in a constructor   I didn t compile or run this to see if it actually works  but give it a try   public class Calc extends JFrame implements ActionListener        private Button button1       public Calc                 super            this setSize 100  100           this setVisible true            this button1   new JButton  1            this button1 addActionListener this           this add button1               public static void main String   args             Calc calc   new Calc            calc setVisible true              public void actionPerformed ActionEvent e            if e getSource      button1

User · Answer

First  exend JFrame properly with a super   and a constructor then add actionlisteners to the frame and add the buttons   import java awt event ActionEvent  import java awt event ActionListener  import javax swing JButton  import javax swing JFrame    public class Calc extends JFrame implements ActionListener       JButton button1   new JButton  1        JButton button2   new JButton  2         public Calc                  setDefaultCloseOperation JFrame EXIT ON CLOSE            setSize 100  100            button1 addActionListener this            button2 addActionListener this            calcFrame add button1            calcFrame add button2             public void actionPerformed ActionEvent e                Object source   e getSource            if source    button1                          button1 code here           else if source    button2                          button2 code here                      public static void main String   args                 JFrame calcFrame   new JFrame            calcFrame setVisible true

User · Answer

I use  e getActionCommand   contains CharSecuence s    since I  m coming from an  MVC context  and the Button is declared in the View class  but the actionPerformed call occurs in the controller    public View                  buttonPlus   new Button           buttonMinus   new Button  -                public void addController ActionListener controller        buttonPlus addActionListener controller       buttonMinus addActionListener controller       My controller class implements ActionListener  and so  when overriding actionPerformed   public void actionPerformed ActionEvent e        if e getActionCommand   contains                   do some action on the model       else if  e getActionCommand   contains  -               do some other action on the model           I hope this other answer is also useful

User · Answer

The problem is that button1 is a local variable  You could do it by just change the way you add the actionListener   button addActionListener new ActionListener                   public void actionPerformed ActionEvent e                                  button is pressed                 System out println  You clicked the button                       Or you make button1 a global variable

User · Answer

You ve been told how to sort your immediate problem  but I think there are more important problems here    stick with conventions  Even for throw-away code  That means initial cases for class names   Don t extend classes you don t need to  JFrame should rarely be extended  In fact  you don t create an instance of your derived class    Don t bundle a bunch of things into one class  In particular  you should generally only subtype at most one main class or interface at a time  things like Comparable not included   Always interact  including construct  Swing AWT GUIs on the AWT Event Dispatch Thread  EDT   It s ugly and verbose  but that s Java for you  Checking a source of an event is a bit of hack  Listeners are small  so you can t even claim the lame performance excuse     So   import java awt event ActionEvent  import java awt event ActionListener  import javax swing JButton  import javax swing JFrame    public class Calc       public static void main String   args            java awt EventQueue invokeLater new Runnable     public void run                 runEDT                           private static void runEDT             assert java awt EventQueue isDispatchThread             JFrame frame   new JFrame             frame setSize 100  100            JButton button1   new JButton  1            button1 addActionListener new ActionListener                 public void actionPerformed ActionEvent event                                                           frame add button1            frame setVisible true             If you need to access any of the variables from the enclosing method within the listener  make them final

User · Answer

There are good answers here but let me address the more global point of adding action listener that listens to multiple buttons   There are two popular approaches   Using a Common Action Listener  You can get the source of the action in your actionPerformed ActionEvent e  implementation   JButton button1  button2    your button   Override public void actionPerformed ActionEvent e         JButton actionSource    JButton  e getSource         if actionSource equals button1               YOU BUTTON 1 CODE HERE       else if  actionSource equals button2                YOU BUTTON 2 CODE HERE           Using ActionCommand  With this approach you setting the actionCommand field of your button which later will allow you to use switch   button1 setActionCommand  actionName1    button2 setActionCommand  actionName2      And later    Override public void actionPerformed ActionEvent e        String actionCommand     JButton  e getSource    getActionCommand         switch  actionCommand            case  actionName1                   YOU BUTTON 1 CODE HERE         break          case  actionName2                   YOU BUTTON 2 CODE HERE         break            Check out to learn more about JFrame Buttons  Listeners and Fields

User · Answer

The first problem is that button1 is a local variable of the main method  so the actionPerformed method doesn t have access to it   The second problem is that the ActionListener interface is implemented by the class calc  but no instance of this class is created in the main method   The usual way to do what you want is to create an instance of calc and make button1 a field of the calc class

User · Answer

Here is a modified form of the source based on my comment   Note that GUIs should be constructed  amp  updated on the EDT  though I did not go that far   import java awt event ActionEvent  import java awt event ActionListener  import javax swing JButton  import javax swing JOptionPane  import javax swing JFrame   public class Calc        public static void main String   args             JFrame calcFrame   new JFrame                usually a good idea          calcFrame setDefaultCloseOperation JFrame DISPOSE ON CLOSE            final JButton button1   new JButton  1            button1 addActionListener new ActionListener                 public void actionPerformed ActionEvent ae                    JOptionPane showMessageDialog                      button1     is the loneliest number                                       calcFrame add button1               don t do this              calcFrame setSize 100  100               important          calcFrame pack             calcFrame setVisible true

[java] How to add action listener that listens to multiple buttons

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