OBSERVER PATTERN HOW TO
OBSERVER PATTERN CODE

The observers are dependent on the subject such that when the subject's state changes, the observers get notified. This can be achieved by making your event handler a field of the class and setting the same event handler to handle all ActionEvents coming from the text fields.The subject and observers define the one-to-many relationship. This class then provides a number of utility methods called things like doubleValue(), intValue() and so forth, allowing you to get hold of this number in any number format you wish.įinally, make the calculation happen automatically upon changes to the values in the text fields. However, you can be guaranteed that whatever is typed, the class of the resulting object will be a subclass of the abstract class Numeric. Note: if you setup your JFormattedTextField to contain numeric data, then depending on what the user types its value field may end up containing a number object of any numeric type ( Integer, Double etc.) depending on the format of the number typed by the user. Now correct the products application for a second time, this time using JFormattedTextField components to ensure that users can only enter numeric information into its input fields. This information is then used by the component to make sure that users obey those formatting rules.įind out more about JFormattedTextField components by reading the corresponding topic in the Sun Java tutorial. New ActionListener() encapsulates formatting information which specifies what text it should regard as legal user input and how that input should be converted into a value property for that field. mHelloButton.addActionListener( // This constructs an action Here are the steps to making a live button:ĭeclare and create a button: JButton mHelloButton = new JButton() Īdd some text to it: tText('Say Hello!') Īdd it to a previously defined JPanel : mButtonPanel.add(mHelloButton) Īdd an action listener to the button, and define what happens when the button is clicked. The libraries you’ll need for listeners are these ones: import What happens when they are clicked is something you (as programmer) must decide by implementing an action event.

Live buttons essentially listen until they are clicked. A button is “made live” i.e. reacts to a mouse click by adding an event listener to it. To make your buttons do something you’ll need to use event “listeners”. To use the layout managers you’ll need: import To create simple images using the above components, you’ll need the following libraries: import Notice that the layout can be altered by changing the parameters. Notice how Barbie’s image –stored in a jpeg file– has been added to the mImageLabel. The two JButton ’s are exceptions: they are first added to mButtonPanel, which in turn is added to mMainPanel.

As each (sub)component is created it is added to mMainPanel.

The contents of mMainPanel consist of a JLabel, a JTextField and another JPanel containing two JButton ’s.

There are other layouts eg GridLayout, also used to arrange the buttons in their panel (see below).

The contents of the JFrame are contained in a Container which has been organised according to a BoxLayout.

The outermost container is an instance of a JFrame.

The components of Hello WorldĪs illustrated in the figure below the image, buttons and messages are made up of instances of purpose-built classes.

OBSERVER PATTERN CODE

All you have to do is write the program code so that the GUI responds in the way you want it to. The arrangement is done by adding “components” and then invoking Swing’s layout managers making the buttons sensitive to mouse clicks is done using Swing’s “action listeners”.

When the buttons are clicked (using the mouse) the GUI responds in various ways. By studying this program you will understand these concepts well enough to start writing your own GUI applications.įrom the figure you can see that the GUI is organised inside a rectangle, with two buttons. Although this is an extremely simple application it illustrates many of the Swing libraries you’ll need. Have a look at the Hello World GUI discussed in lectures.

OBSERVER PATTERN HOW TO

Most of the libraries that we will be using will be buttons and panels which you’ll learn how to put together in various ways to create a simple GUI (graphical user interface) for a simple game. But Swing is cleverer than just displaying images - it can be used to receive input, and react to it by changing what it displays. Swing is a collection of libraries which can be used to make your programs display simple “graphical” style images. Consider the java tutorial on anonymous inner classes to be your lecture notes for this section.Īs you learn about Swing, keep in mind that it is an implementation of the Observer Pattern.

Observer pattern