Which button has clicked. Java,Netbeans

I make an app in java with Netbeans. At a Jframe Form,I use four buttons. I need to know which of them has clicked by user. Everyone who can help me? Thanks

public class Color extends javax.swing.JFrame implements ActionListener {


public Color() {
        initComponents();


        /////////////////////////////////

        //Register a listener for the  buttons.
        up_button.addActionListener(this);
        down_button.addActionListener(this);
        left_button.addActionListener(this);
        right_button.addActionListener(this);
       }


private int k=1;
    public void actionPerformed(ActionEvent e) {

       k=k+1;


       if (k==1)
       {
         image.setIcon(createImageIcon("color1"
                                        + e.getActionCommand()
                                        + ".PNG"));
       }
       else ...  }

       private void up_buttonActionPerformed(java.awt.event.ActionEvent evt)     {                                          
        // TODO add your handling code here:

    }  

    private void down_buttonActionPerformed(java.awt.event.ActionEvent evt) {                                          
        // TODO add your handling code here:

    }

    public static void main(String args[]) {

        /* Create and display the form */

        java.awt.EventQueue.invokeLater(new Runnable() {
            public void run() {
                new Color().setVisible(true);
            }
        });
    }