Layout

You should read

Basic

What is a component?

JLabel, JButton are components. I can't give the exact definition of component, but they are the "atoms" of Java swing application. We use them to build up complicated applications.
With the exception of top-level containers, all Swing components whose names begin with "J" descend from the JComponent class. For example, JPanel, JScrollPane, JButton, and JTable all inherit from JComponent. However, JFrame doesn't because it implements a top-level container.

JLabel and JButton

We will concentrate 2 simple components JLabel and JButton.

some useful constructors

JLabel(String text)
JLabel(String text, int horizontalAlignment)
JButton(String text)

LabelFrame.java

You can get your program here .

   1:import javax.swing.*;
   2:public class LabelFrame extends JFrame {
   3:    private String text;
   4:    private JLabel label;
   5:    private int alignment;
   6:    public LabelFrame(String text, int alignment) {
   7:        super(text);
   8:        label = new JLabel(text, alignment);
   9:        getContentPane().add(label);
  10:        setDefaultCloseOperation(javax.swing.JFrame.EXIT_ON_CLOSE);
  11:    }
  12:    
  13:    public static void main(String[] args) {
  14:        LabelFrame f1 = new LabelFrame("Left", JLabel.LEFT);
  15:        // setLocation(x,y) is the set location of the frame, the upper left 
  16:        // corner will be at (x,y)
  17:        f1.setLocation(100,100);
  18:        f1.setSize(100,100);
  19:        f1.setVisible(true);
  20:        LabelFrame f2 = new LabelFrame("Center", JLabel.CENTER);
  21:        f2.setLocation(200,100);
  22:        f2.setSize(100,100);
  23:        f2.setVisible(true);
  24:        LabelFrame f3 = new LabelFrme("Right", JLabel.RIGHT);
  25:        f3.setLocation(300,100);
  26:        f3.setSize(100,100);
  27:        f3.setVisible(true);
  28:    }
  29:}

super

At line 7, we call the JFrame's constructor JFrame(String title) to set up the title of JFrame.
You don't have to call JFrame's constructor everytime. If you don't insert that line. The compiler will call the construtor with no argument JFrame() for you.
Question:
what happens if your superclass doesn't have a constructor with no argument but you don't call superclass' constructor in the constructors?

JLabel.LEFT ....

JLabel.LEFT is a static final int. It has an integer value 2. But we never use new JLabel("hi", 2) because the code is not readable. Who knows what the 2 stands for? Besides what happens if the Java designer decides to change the value of JLabel.LEFT? So a better way to write this is new JLabel("hi", JLabel.LEFT).

setLocation

setLocation(x,y) set the location of the JFrame such that its upper left corner is at (x,y).

alignment

The meaning of aligment is straight forward from the pictures.

What happen if you want to put more than one components on the JFrame? We need to use layout manager to set the layout. In below we will discuss different layout manager. We will concentrate on 3 layout manager BorderLayout, FlowLayout, GridLayout.

BorderLayout

   1:import java.awt.*;
   2:import javax.swing.*;
   3:
   4:public class BorderTest extends JFrame {
   5:    private String title = "Border Test";
   6:    private JButton northButton = new JButton("North");
   7:    private JButton eastButton = new JButton("East");
   8:    private JButton southButton = new JButton("South");
   9:    private JButton westButton = new JButton("West");
  10:    private JButton centerButton = new JButton("Center");
  11:    private Container contentPane = getContentPane();
  12:
  13:    public BorderTest() {
  14:        // the following line is useless because the default layout of JFrame is border layout
  15:        contentPane.setLayout(new BorderLayout()); 
  16:        contentPane.add(northButton, BorderLayout.NORTH);
  17:        contentPane.add(eastButton, BorderLayout.EAST);
  18:        contentPane.add(southButton, BorderLayout.SOUTH);
  19:        contentPane.add(westButton, BorderLayout.WEST);
  20:        contentPane.add(centerButton, BorderLayout.CENTER);
  21:        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
  22:    }
  23:    
  24:    public static void main(String[] args) {
  25:        BorderTest test = new BorderTest();
  26:        test.setSize(250,200);
  27:        int x = 100, y = 200;
  28:        test.setLocation(x,y);
  29:        test.setVisible(true);
  30:    }
  31:}

After you resize it:

Set the layout

To set the layout, use setLayout(new BorderLayout()) You are going to learn different layouts later.

Constructor

There are 2 constructors of BorderLayout:
BorderLayout(): Constructs a new border layout with no gaps between components.
BorderLayout(int hgap, int vgap): Constructs a border layout with the specified gaps between components. The horizontal gap is specified by hgap and the vertical gap is specified by vgap.

Initiate member variables outside of the constructor

Notice how I initiate the variables outside of the constructor from line 5 to line 11.

Add components

To add components, we use add(Component component, int position) method. The position can be

 BorderLayout.CENTER, BorderLayout.NORTH, BorderLayout.EAST,
BorderLayout.SOUTH, BorderLayout.WEST

. If you remove approprate lines form line 16-line 21 and recompile it, we can get

BorderTest2.java

   1:import java.awt.*;
   2:import javax.swing.*;
   3:
   4:public class BorderTest2 extends JFrame {
   5:    private String title = "Border Test";
   6:    private JButton northButton = new JButton("North");
   7:    private JButton eastButton = new JButton("East");
   8:    private JButton southButton = new JButton("South");
   9:    private JButton westButton = new JButton("West");
  10:    private JButton centerButton = new JButton("Center");
  11:    private Container contentPane = getContentPane();
  12:   
  13:    public BorderTest2() {
  14:        // horizontal gap and vertical gap
  15:        int hgap = 10, vgap = 5;
  16:        contentPane.setLayout(new BorderLayout(hgap, vgap)); 
  17:        contentPane.add(northButton, BorderLayout.NORTH);
  18:        contentPane.add(eastButton, BorderLayout.EAST);
  19:        contentPane.add(southButton, BorderLayout.SOUTH);
  20:        contentPane.add(westButton, BorderLayout.WEST);
  21:        contentPane.add(centerButton); // same as contentPane.add(centerButton, BorderLayout.CENTER);
  22:        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
  23:    }
  24:    
  25:    public static void main(String[] args) {
  26:        BorderTest2 test = new BorderTest2();
  27:        test.setSize(250,200);
  28:        // get the dimension of the screen
  29:        Dimension  screenSize = Toolkit.getDefaultToolkit().getScreenSize();
  30:        // get the dimension of the frame
  31:        Dimension  frameSize = test.getSize();
  32:        // draw the frame at the center of the screen
  33:        int x = screenSize.width/2 - frameSize.width/2;
  34:        int y = screenSize.height/2 - frameSize.height/2;
  35:        test.setLocation(x,y);
  36:        test.setVisible(true);
  37:    }
  38:}

Pay attentions to the gaps between the components.
bordertest2

Constructor

Notice I use other constructor for BorderLayout to set the vgap and hgap.

getSize()

getSize() is to get the size of the frame.

Find the size of the screen

You won't be tested on this part for the midterm and final.
I want to show the frame at the center. In order to find the size of the screen I use

Dimension  screenSize = Toolkit.getDefaultToolkit().getScreenSize();