Java Swing How to - Position components gap-less and one by one

Question

We would like to know how to position components gap-less and one by one.

Answer

import java.awt.GridBagConstraints;
import java.awt.GridBagLayout;
//from  w  ww . java  2 s.c o m
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JPanel;

public class Main extends JFrame {
  public Main() {
    JPanel bigPanel = new JPanel(new GridBagLayout());
    JPanel panel_a = new JPanel();
    JButton btnA = new JButton("button a");
    panel_a.add(btnA);

    JPanel panel_b = new JPanel();
    JButton btnB = new JButton("button b");
    panel_b.add(btnB);

    GridBagConstraints c = new GridBagConstraints();
    c.gridwidth = GridBagConstraints.REMAINDER;
    c.fill = GridBagConstraints.HORIZONTAL;
    c.weightx = 1.0;
    bigPanel.add(panel_a, c);
    bigPanel.add(panel_b, c);
    c.weighty = 1.0;

    bigPanel.add(new JPanel(), c);
    this.add(bigPanel);
  }

  public static void main(String[] args) {
    Main gui = new Main();
    gui.pack();
    gui.setVisible(true);
  }
}