One of the benefits of having a web application is that it can be very easily accessed by everyone around the world. One of the downsides of this is that when so many people use your application, they are going to have errors in their input. Some people are not attentive, others are tired and, finally, everyone in this world has his or her individual style of thinking, so something that seems obvious to the developers of the application might puzzle someone else.
A well-designed web application should immediately be able to define that the input is wrong and stop—otherwise all kinds of errors can happen inside of the application. If this application is user-friendly, it should:
Clearly and unambiguously inform the user that some part of the input is erroneous, and should be corrected.
Identify the field that is erroneous and mark it in some way.
If possible, display the erroneous value, and maybe even explain why exactly it is wrong.
Let's see how Tapestry 5...