Note: If you're not sure what a dialog box is then have a read of What Is a Dialog Box?

A message box is a simple pop-up window that displays a message to the user and is dismissed with the click of a button. To avoid having to build your own dialog box from scratch, the JOptionPane class provides methods to make a variety of dialog boxes by simply feeding them some parameters to determine the appearance of the dialog box.

Note: If you're looking to get a specific input from a user then have a look at Building an Input Dialog Box. It still uses a method from the JOptionPane class but it's worth distinguishing it from a simple message box as the options the user can choose are more varied.

The JOptionPane Class

To work with the JOptionPane class it isn't necessary to create an instance of a JOptionPane because the methods and fields that you need to use are static methods and static fields.

To create a simple message dialog box you have a choice of three JOptionPane methods:

• showMessageDialog - This method is the most basic. It allows you to set the message, a title for the dialog box, a message type (e.g., warning message, error message), and only has an OK button. 
• showConfirmDialog - Again, you can set the message, a title, a message type, but you can also choose between a selection of button combinations (e.g., YES/NO, OK/CANCEL). It returns an int representing which button was clicked.
• showOptionDialog - This is the most flexible method. In addition to setting the message and title, you have the choice of how to customize the dialog box. You can pick a message type and button combination, or you can provide a specific icon and an Object array for the button options. The Object array gives the flexibility to make the button names from an String array or even give them all images using an Icon array. It returns an int representing the user's choice of button.

Using the showMessageDialog Method

To create a simple message box use the showMessageDialog:

JOptionPane.showMessageDialog(this, "This is the dialog title","This is the dialog message", JOptionPane.PLAIN_MESSAGE);
This will create a dialog with no icon - just a message, a title and an OK button.

That sounds like a pretty plain message box and that's because I've passed the static field JOptionPane.PLAIN_MESSAGE as the message type. The message type tries to give the dialog box a certain feel by determining the icon that will be used. A PLAIN_MESSAGE has no icon but the other fields do:

• ERROR_MESSAGE
• INFORMATION_MESSAGE
• WARNING_MESSAGE
• QUESTION_MESSAGE
• PLAIN_MESSAGE

For example, JOptionPane.ERROR_MESSAGE uses a red octagon with an exclamation mark in the middle to try and give the feel that something unexpected has happened.

Have another look at the first parameter in the above example. I'm using this to refer to the JFrame object as the parent of the dialog box. The first parameter of any of the three JOptionPane methods is always the parent component (e.g., JFrame, JPanel) of the dialog box. This allow the dialog box to determine its position on the screen relative to its parent. This parameter can be null if you want to create a dialog box without a container like a JFrame or JPanel.

The final thing to note about the JOptionPane.showMessageDialog method is it does not return the user's response. There is only an OK button for them to click. As their options are so limited it's not worth trying to capture a response.

Read Creating a Message Dialog Box - Part II to see how the other JOptionPane methods are used.

An example Java code program can be viewed in Simple Message Dialog Boxes Program. A more in-depth Java application example can viewed in JOptionPane Option Chooser Program.