与BorderLayout对象的使用有关的一些疑问

Question

I am studying Java Swing and I have a doubt related to the use of the BorderLayout object.

I have this simple example program that create a ToolBar:

package com.andrea.menu;

import java.awt.BorderLayout;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;

import javax.swing.ImageIcon;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JMenu;
import javax.swing.JMenuBar;
import javax.swing.JToolBar;
import javax.swing.SwingUtilities;

public class ToolBar extends JFrame {

    public ToolBar() {
        initUI();
    }

    public final void initUI() {

        JMenuBar menubar = new JMenuBar();      // The menu bar containing the main menu voices
        JMenu file = new JMenu("File");         // Creo un menu a tendina con etichetta "File" e lo aggiungo
        menubar.add(file);

        setJMenuBar(menubar);                   // Sets the menubar for this frame.

        JToolBar toolbar = new JToolBar();

        ImageIcon icon = new ImageIcon(getClass().getResource("exit.png"));

        JButton exitButton = new JButton(icon);
        toolbar.add(exitButton);
        exitButton.addActionListener(new ActionListener() {
            public void actionPerformed(ActionEvent event) {
                System.exit(0);
            }

        });

        add(toolbar, BorderLayout.NORTH);

        setTitle("Simple toolbar");
        setSize(300, 200);
        setLocationRelativeTo(null);
        setDefaultCloseOperation(EXIT_ON_CLOSE);
    }

    public static void main(String[] args) {
        SwingUtilities.invokeLater(new Runnable() {
            public void run() {
                ToolBar ex = new ToolBar();
                ex.setVisible(true);
            }
        });
    }
}

So it create a JToolBar object by the line:

JToolBar toolbar = new JToolBar();

and then it put it in the NORTH position of the BorderLayout object by the line:

add(toolbar, BorderLayout.NORTH);

Reading the documentation I know that:

A border layout lays out a container, arranging and resizing its components to fit in five regions: north, south, east, west, and center

My doubt is: the BorderLayout object whom it refers? At the external JFrame container?

It means that it puts the toolbar object in the NORTH position of my JFrame ? Or what?

Answer 1

You put the toolbar in the NORTH position of your ToolBar instance named ex .

Your ToolBar class extends JFrame . The add method is inherited by ToolBar from JFrame . In your main you call ToolBar constructor, which creates a new instance of ToolBar and saves the reference to ex . It also calls the initUI method on ex , which calls add on ex .