qr code reader program in java Forms and Event Handling in Java

Making QR Code in Java Forms and Event Handling

CHAPTER 7 Forms and Event Handling
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Figure 7-2 A warning message is displayed if the @ symbol was not entered in the e-mail address
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We re only interested if the value of the Location variable is 1 Therefore, we use an if statement (see 3) to determine whether the @ symbol wasn t entered by the person If the @ symbol was not entered, an alert dialog box is displayed with a message warning that the e-mail address is invalid (Figure 7-2)
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When you look at a form within a web page, you probably don t necessarily think about how the form relates to everything else that you re seeing However, relationships on a web page are very important when you are a JavaScript programmer, because you need to know them to access them Everything that you see on a web site is considered an object The rst object you see is the window, which is referred to in a JavaScript as window A window contains an HTML document referred to as document You ve referenced the document throughout this book whenever you called the documentwrite() function A document can have one or more forms, and a form can have one or more elements Form objects are stored in an array (see 4) called forms and appear in the order in which the forms appear in the document You can reference each form by referencing the form s index Suppose you wanted to reference the third form You d write this:
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windowdocumentforms[2]
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You re telling the browser to go to the window object and then within the window object go to the document object and then reference the form that is assigned to the 2 index value of the forms array (Remember that the index 2 is referencing the third form, because the rst form is index 0)
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TIP Although this is a good syntax to reference the window object, this is not required You can use this instead:
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documentforms[2] (might be worth mentioning)
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Forms are assigned to elements of the forms index in the order that each form appears in the document You can reference a form using its index instead of using the name of the form Remember that the name of the form is the value that is assigned to the form s name attribute Here s how to reference a form by using the name of the form In this example, we re referencing the order form:
windowdocumentformsorder
TIP Referencing by name is better practice than referencing by index because the display and ordering of elements changes all the time, and it requires less maintenance if you reference by name Also, referencing by name makes your code easier for humans to understand and maintain The following example shows how to access an attribute of a form We de ned the display() function in the JavaScript within the <head> tag This function receives the value of the form s Reset element and displays it in an alert dialog box (Figure 7-3) The function is called in response to an onclick event that occurs when the user clicks the Reset button
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 10 Transitional//EN"> <html xmlns="http://wwww3org/1999/xhtml"> <head> <title>Accessing form attributes</title> <script language="Javascript" type="text/javascript"> <!-function display() { alert ('Value: ' + documentformsorderResetvalue) } --> </script> </head> <body> <FORM action= "http://wwwjimkeoghcom" method="post" name="order"> <P> First Name: <INPUT type="text" name="Fname"/><BR> Last Name: <INPUT type="text" name="Lname"/><BR>
CHAPTER 7 Forms and Event Handling
Email: <INPUT type="text" name="Email"/><BR> <INPUT name="Submit" value="Submit" type="submit"/> <INPUT name="Reset" value="Reset" type="reset" onclick="display()"/> </P> </FORM> </body> </html>
Elements on a form are stored in an array called elements in the order in which the elements appear on the form Here s how you access an element by using the element s index within the elements array:
windowdocumentformsorderelements[2]
This tells the browser to go to the window object and within the window object go to the document object Within the document object go to the forms and access the form named order And within the order form access the element that has index 2, which is the third element
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