Connecting Beans

Now that the interface is ready, we have to add behaviour. This is done via message passing between beans. For that purpose each bean has a list of events to which it can react. Events can be initiated by the user (a click on a button or an <ENTER> in a text field) or they can be system generated (timer events or events generated by other beans). When an event occurs, a bean can react directly and/or send an appropriate message to another bean, which in turn reacts to the incoming message.

In a Java Beans environment one implements this by drawing connections between beans and defining the proper messages. In many cases this can be done completely in a graphical way, but sometimes it is necessary - or even simpler - to put in a few lines of Java code that define the concrete message.

In our example applet we will need four almost identical messages. One of them can be defined as a connection between the plus button and the result text field and can be formulated as:

"When I (the button) am pressed, you (the output field) have to get the numbers from the two input fields, add them and display the result."

This message is a bit too complicated to be created graphically, though this would be possible by inserting an intermediate bean for addition. To see what is possible without any hand code, we first implement the following simpler message:

"When I (the button) am pressed, you (the output field) have to get the number from the first input field and display it."

and will then change it to work as requested.

To implement all this with NetBeans, you need the following steps:

1. Change to connection mode and define sender and receiver of the message:
   • Click on the connection mode icon () to the right of the selection mode icon.
   • In the form or the inspector window click first on the sender bean jButton1, then on the receiver bean jTextField3.
   Now the connection wizard window pops up and asks for all the information that will be needed to define the message.
2. Select the source event that is the reason for the message (here: the +-button has been pressed):
• From the list of event types open the action folder and select actionPerformed.
• Confirm by clicking on the Next > button.
3. Define the kind of operation that will be done by the receiver (namely its text property will be set):
• Make sure that the Set Property radio button is active.
• Scroll down the list of possible properties of the text field and select text.
• Confirm again with Next >.
4. Determine the origin of the text parameter that will be used to set the receiver's text property:
   • From the radio button list of possibilities choose Property.
   • Click on the ellipsis icon to the right. This opens a property chooser window.
   • Select jTextField1 from the upper combo box. Now all properties of this component are displayed that can be used to set the text property (i.e. that have the correct type).
   • Choose text from the properties list and confirm with OK.
   • Finally close the wizard with Finish.
   This creates all code that is needed for the message transfer and switches to the message code in the source editor. Most of the code is "guarded", which means, it is completely under the control of NetBeans and can not be changed manually in the source editor. It is marked by a light blue background. The message itself is not guarded, it is the simple line
   • jTextField3.setText(jTextField1.getText());
   This is easy to understand even with very little Java experience:
   • Get the text from the text field jTextField1 and use it to set the text of jTextField3.
5. To see whether this works just run the applet:
• Choose Build/Execute from the main menu or simply press the execute button (). A pop up window appears asking for access to the internal server. Answer with Yes.
• The applet viewer starts and displays the running applet. Unfortunately it doesn't use the size of the form, so it looks ugly. Simply resize the window so that it looks similar to the design view.
• Enter any text into the first text field and press the +-button. The text appears in the result field as requested. Try other text fields or buttons to see that they have no effect.
• Finish the applet simply by closing the window. Afterwards close the output window of NetBeans.
6. To start the applet NetBeans has automatically created a simple html file for it. You can edit it to set the requested applet size for future runs:
• Open the filesystems window with Window/Filesystems.
• If necessary open the simpleDemo directory.
• Double-click the simpleDemoApplet html file (). Its source code is now displayed in the source editor.
• Locate the APPLET tag and change the width to 400 and the height to 150.
• Save the changes with File/Save or the save icon ().
• Close the filesystems window and switch to the form window tab.
• Run the applet again to see that it now starts with the correct size. Close it afterwards.
7. Now define the correct message by manually editing the message code:
• Switch to the Java source code tab and find the previously defined message line.
• Replace it with the following four lines
  • double val1 = Double.parseDouble(jTextField1.getText());
    double val2 = Double.parseDouble(jTextField2.getText());
    double result = val1 + val2;
    jTextField3.setText(result + "");
• Save the changes and start the applet again. Enter numbers into the input fields and press the +-button to see that it works. Close the applet window afterwards.
8. Finally create the messages for the three remaining buttons:
   • Repeat the steps 1 - 4 exactly, only changing the receiving button.
   • Replace the generated message code with the four lines from above. Change the operator + in the third line to -, *, / respectively.
   • Run the applet and test it.

That we could not create the complete message without inserting manual code was due to two difficulties:

• We had to combine input from two sources to compute the result.
• The text strings used for input and output had to be converted to/from numerical values (doubles) to make arithmetic computations.

These problems have been addressed by the PhysBeans library:

• It contains function beans that have no visual counterpart, but can be used to compute new results from several inputs.
• Many bean properties are defined as double values and the conversion is done in the internal bean code.