向JButton添加ActionListener

Question

When attempting to implement ActionListener I receive the following error

 EmployeesApplet.java:5: error: EmployeesApplet is not abstract and does not override abstract method actionPerformed(ActionEvent) in ActionListener public class EmployeesApplet extends JApplet implements ActionListener ^ 1 error 

I do not want to make EmployeesApplet abstract since it doesn't need to be. My code is below, note I commented out the implements ActionListener and adding the JButtons as ActionListener

import javax.swing.*;
import java.awt.*;
import java.awt.event.*;

public class EmployeesApplet extends JApplet //implements ActionListener 
{
  public JButton              sd   = new JButton ("Salaried");
  public JButton              hr   = new JButton ("Hourly");
  public JButton              cm   = new JButton ("Commissioned");
  public JButton              cl   = new JButton ("Clear"); 

  private final int    FIELDS      =  8,   
                       FIELD_WIDTH = 20;   

  private String[]     strings     = new String[FIELDS];
  private TextFieldWithLabel[] tf  = new TextFieldWithLabel[FIELDS];
  private JTextArea    ta          = new JTextArea(5,25); 

     public void init()
     {
      String[]  s = {"First Name", "Last Name", "Employee ID", "(a) Salaried: Weekly Salary", "(b1) Hourly 1: Rate Per Hour", 
                   "(b2) Hourly 2: Hours Worked" , "(c1) Commissioned: Rate", "(c2) Commissioned: Gross Sales" };

       //----------------------
       //  Set up the Structure
       //----------------------

       Container c = getContentPane();
       JPanel f   = new JPanel(new FlowLayout());
       JPanel b   = new JPanel(new BorderLayout(2,0));

       JPanel glb = new JPanel(new GridLayout(8,1,0,2));
       JPanel gtf = new JPanel(new GridLayout(8,1,0,2));
       JPanel flb = new JPanel(new FlowLayout());


       // Add FlowLayout to the container
       c.add(f);
       // Add BorderLayout to the FlowLayout
       f.add(b);

       //---------------------------------------
       //Add JPanels to the BorderLayout regions
       //---------------------------------------

       // Add JLables to GridLayout in West
       b.add(glb, BorderLayout.WEST);
       for (int i = 0; i < tf.length; i++)
       {
        tf[i] = new TextFieldWithLabel(s[i], FIELD_WIDTH);
        glb.add(tf[i].getLabel());
       }

       // Add JTextFeilds to GridLayout in East
       b.add(gtf, BorderLayout.EAST);
       for (int i = 0; i < tf.length; i++)
       {
        tf[i] = new TextFieldWithLabel(s[i], FIELD_WIDTH);
        tf[i].getTextField();
        gtf.add(tf[i].getTextField());
       }

       // Add JButtons to FlowLayout in South
       b.add(flb, BorderLayout.SOUTH);

       flb.add(sd);
       flb.add(hr);
       flb.add(cm);
       flb.add(cl);

       //sd.addActionListener(this);
       //hr.addActionListener(this);
       //cm.addActionListener(this);
       //cl.addActionListener(this);

       // Add JTextArea and make it not editable   
       f.add(ta);
       ta.setEditable(false);

     }

     public void readFields()
     {
     }

     public void fieldsExist()
     {
     }

     public void fieldsEmpty()
     {
     }

     public void actionPerformed()
     {
     }


}

Answer 1

Your actionPerformed method needs an ActionEvent as it's parameter:

public void actionPerformed(ActionEvent e) {

}

Otherwise you won't be overriding the method defined in ActionListener - you'll just be creating a new method. Since ActionListener is an interface, you are required to implement all the methods defined in the interface, hence the error.

---

The actionPerformed method is declared with the ActionEvent parameter to pass the method details about the event (which component triggered the event, the action command , etc..). Without the ActionEvent parameter, there is no easy way to gather such information. When an action is performed, an ActionEvent object is created, filled with the event information, then your actionPerformed method is invoked, which the ActionEvent object is passed in as an argument.

Answer 2

Your class is implementing the interface ActionListener

hence you have 2 options... make the class abstract or implement the method actionPerformed

the first one looks like what you need....

so try doing something like:

public class EmployeesApplet extends JApplet implements ActionListener {
     ....
     ....
    @Override
    public void actionPerformed(ActionEvent e) {
        // TODO Auto-generated method stub
    }