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Most, if not all, business/commercial applications of computer systems are using record-type data handling processes.
Field names at the top
An important element of record structure is that each field holds data of a specified type .
These diagrams are also found in the book Computer Science although it originated on the ib-computing.com website.
IBID Press Complete coverage of the IB Computer Science Program in a textbook by Richard Jones & Andrew Meyenn .
Here I've used a convention of putting vt before the data fields and fd for the fields that just label the fields.
It makes life easier for me and for other people reading the code.
To put the labels close to the data fields I used right-justification ( Label.RIGHT ).Simple set of actions
Messages as usual...
VideoTape is the inner class, defined later in the code. We use an array of these objects to store the data.
This could be handled much better in a real application. There is no scope for the user to change them.
Data members of inner class.
Constructor that sets data fields.
I've scrunched these up to save space in the listing.
Remember the use of the this keyword to make sure we are returning the Class data member and not any local identifiers .
Notice the semi-colon at the end of the inner class definition.
The layout was achieved through the use of Panels and a lot of trial and error.
BorderLayout defines 5 areas of the container surface: CENTER, NORTH, SOUTH, EAST and WEST where Panels or objects can be placed.
add Button was pressed.
Get the details from the fields the user has (hopefully) filled in.
Check there is more space to store details.
Add the tape at the end.
let the user know.
Something went wrong in getting the details (see later).
If else chain - the Next button.
Display record at element current (zero-based, so adjust message to a more natural "human" concept).
Similar checks for moving backwards through the collection.
This method call is used at the end of the init() method.
It adds itesm from the DESC array to the Choice list box.
Notice the return type of VideoTape - our inner class.
If these fields are empty the tape is not valid.
Possible exceptions are bad number format for the expected integer and there being no item selected in the drop down list.
Given the position of a valid record in the display, this method displays it.
Notice that collection[current] is of type VideoTape.
A special method is required to convert from stored code to descriptions in the Choice list.
Use of vt prefix helps us to remember which GUI object the identifier refers to.
Simple search of the array.
If we find the code. return the corresponding description.
The Applet in action, not very good yet, but something to build on.
Refer to the classes page if neccessary to find out how to create a custom class.
Technically, not showing how to do this could be regarded as a cop out!
You can also add special validation checks (see if the tape length is reasonable, for example).
If the inner class adds all these techniques then we end up with very cluttered code.
This is a hard exercise but if you succeed you can practically write a Standard Level Dossier right now.
All you need is files (da, da, dada da).
Well, maybe searching or sorting too...
Sometimes data items of different types belong logically together. For example, all of the data about a student in a school could be held in a single object known as a record . Traditional programming languages and applications such as databases provide ways of grouping such data together. The following terminology is associated with records:
The fields are in italic type and the DATA is in CAPITALS . All three records together would make up a data file.
The same information can often be presented in column layout, when all three records can be seen together with the fields used as labels at the top of each column:
Now the file is represented by all of the information in the table . The data in each field can be of any primitive type or of other classes too. Traditional languages provide data structures called records which allow data to be kept together as a single unit:
newtype StudentRecord Record
However, since Java is already based around the idea of grouping objects with a common structure we can simply define a class and any associated methods needed to access the data associated with each object created.
This class can be an explicit class, like the Fraction Class we saw already, or it can be an "inner class" - that is, combined with an existing Class. We look at both of these methods
The Class definition and data members are as follows:
// labels for field data
// control buttons
// system messages display
// Set up an array to hold the tape collection
Actually, you should be able to do this easily enough. All that is required is to take (cut and paste) the inner Class code out of the Applet and create a new VideoTape.java class file.
This won't need any GUI objects or Applet methods. In BlueJ the Applet will automatically link to the new VideoTape Class.
The advantages of having a completely separate Class are:
You should also be able to add a custom exception class VideoTapeException if you followed the earlier pages on this technique. You would then need to change the Applet code which adds a new tape to the collection to catch these exceptions - you might find it easier to put these into an addTape() helper method.
Custom exceptions you could create could be:
The different types of video tape should be extendable easily by the user. One way to do this is to have the VideoTape Class keep an array of type CODE and DESC arrays internally. You would need to provide a mechanism to add data to these arrays.
One possible way to do this is to have a Panel with code and description TextFields in the NORTH of the Applet layout. You could add an "Add Type" Button which collects the data and adds it to the arrays in Class VideoTape (perhaps using a public addType() method).
Databases such as Access, Paradox and mySQL are general purpose applications which are often used to handle data records.
I went over the whole object/record/database type app with my class back in Nov 2004.
Feel free to follow along on these pages.
They relies quite heavily on BlueJ but could be adapted.
This work is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 2.5 License. © 2001 - 2009 Richard Jones, PO BOX 246, Cambridge, New Zealand;
This page was last modified: October 28, 2013