please check this example,
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
<title>Java Installed?</title>
<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1">
<script type="text/javascript">
function isJava(){
if(!navigator.javaEnabled())
{
var message = confirm("You either have java disabled or you currently don't have java installed. Would you like to download java?");
if(message)
{
window.location = "http://java.sun.com";
}
}
}
window.onload = isJava;
</script>
</head>
<body>
</body>
</html>
Greetings.