Why do we use super when there is no superclass?

Question

Hello,
I have a question about the word "super " and the constructor. I have read about super and the constructor but there is somethong that I do not understand.
In the example that I am studying there is a constructor calles " public MultiListener()" and there you can see " super" to use a constructor from a superclass. But there is no superclass. Why does super mean here?
import javax.swing.*;
import java.awt.GridBagLayout;
import java.awt.GridBagConstraints;
import java.awt.Insets;
import java.awt.Dimension;
import java.awt.Color;
import java.awt.event.ActionListener;
import java.awt.event.ActionEvent;
public class MultiListener extends JPanel
                           implements ActionListener {
    JTextArea topTextArea;
    JTextArea bottomTextArea;
    JButton button1, button2;
    final static String newline = "\n";
    public MultiListener() {
        super(new GridBagLayout());
        GridBagLayout gridbag = (GridBagLayout)getLayout();
        GridBagConstraints c = new GridBagConstraints();
        JLabel l = null;
        c.fill = GridBagConstraints.BOTH;
        c.gridwidth = GridBagConstraints.REMAINDER;
        l = new JLabel("What MultiListener hears:");
        gridbag.setConstraints(l, c);
        add(l);
        c.weighty = 1.0;
        topTextArea = new JTextArea();
        topTextArea.setEditable(false);
        JScrollPane topScrollPane = new JScrollPane(topTextArea);
        Dimension preferredSize = new Dimension(200, 75);
        topScrollPane.setPreferredSize(preferredSize);
        gridbag.setConstraints(topScrollPane, c);
        add(topScrollPane);
        c.weightx = 0.0;
        c.weighty = 0.0;
        l = new JLabel("What Eavesdropper hears:");
        gridbag.setConstraints(l, c);
        add(l);
        c.weighty = 1.0;
        bottomTextArea = new JTextArea();
        bottomTextArea.setEditable(false);
        JScrollPane bottomScrollPane = new JScrollPane(bottomTextArea);
        bottomScrollPane.setPreferredSize(preferredSize);
        gridbag.setConstraints(bottomScrollPane, c);
        add(bottomScrollPane);
        c.weightx = 1.0;
        c.weighty = 0.0;
        c.gridwidth = 1;
        c.insets = new Insets(10, 10, 0, 10);
        button1 = new JButton("Blah blah blah");
        gridbag.setConstraints(button1, c);
        add(button1);
        c.gridwidth = GridBagConstraints.REMAINDER;
        button2 = new JButton("You don't say!");
        gridbag.setConstraints(button2, c);
        add(button2);
        button1.addActionListener(this);
        button2.addActionListener(this);
        button2.addActionListener(new Eavesdropper(bottomTextArea));
        setPreferredSize(new Dimension(450, 450));
        setBorder(BorderFactory.createCompoundBorder(
                                BorderFactory.createMatteBorder(
                                                1,1,2,2,Color.black),
                                BorderFactory.createEmptyBorder(5,5,5,5)));
    public void actionPerformed(ActionEvent e) {
        topTextArea.append(e.getActionCommand() + newline);
        topTextArea.setCaretPosition(topTextArea.getDocument().getLength());
     * Create the GUI and show it.  For thread safety,
     * this method should be invoked from the
     * event-dispatching thread.
    private static void createAndShowGUI() {
        //Create and set up the window.
        JFrame frame = new JFrame("MultiListener");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        //Create and set up the content pane.
        JComponent newContentPane = new MultiListener();
        newContentPane.setOpaque(true); //content panes must be opaque
        frame.setContentPane(newContentPane);
        //Display the window.
        frame.pack();
        frame.setVisible(true);
    public static void main(String[] args) {
        //Schedule a job for the event-dispatching thread:
        //creating and showing this application's GUI.
        javax.swing.SwingUtilities.invokeLater(new Runnable() {
            public void run() {
                createAndShowGUI();
}

Answer 1

OP wrote:
>
Isn't that simplier to use the constructor instead of using "super"? I mean using the constructor jpanel () ?
>
You can't call the super class constructor directly if you are trying to construct the sub-class. When you call it directly it will construct an instance of the super class but that instance will NOT be an integral part of the 'MultiListener' sub class you are trying to create.
So since 'MultiListener' extends JPanel if you call the JPanel constructor directly (not using super) your code constructing a new instance of JPanel but that instance will not be an ancestor of your class that extends JPanel. In fact that constructor call will not execute until AFTER the default call to 'super' that will be made without you even knowing it.
A 'super' call is ALWAYS made to the super class even if your code doesn't have an explicit call. If your code did not include the line:
super(new GridBagLayout()); Java would automatically call the public default super class constructor just as if you had written:
super();If the sub class cannot access a constructor in the super class (e.g. the super class constructor is 'private') your code won't compile.