This is a design question. I’m trying to decide between 2 implementations.
In order to properly explain this I need an example. So, for the sake of example:
I have a class that generates a report about, let’s say, certain Stock Market values on a specific day. I create a StockMarketReportGenerator object, passing it today’s date, and it generates a report based on today’s market values.
The StockMarketReportGenerator “has a” StockMarketData object. The purpose of the StockMarketData object is to contain all the stock market values that are stored in a table (probably called StockMarket 🙂 ) in the database, and some further values calculated from the table data. It has private methods that connect with the database, retrieve the data, do the necessary calculations, and store the final values in the object’s member variables. (It then has getter methods to expose these values, but no setter methods.) The StockMarketData class is basically a “holder” for stock market data values. I have one central function called something like “calculateStockMarketData()” that calls all these private helper methods and sets up the object. (I am aware that all this processing could really be more easily handled by a framework such as Hibernate; but the decision was made to do it manually as it’s a very small, somewhat temporary project and not worth the setup.)
My question is this – from my ReportGenerator class, I only need the StockMarketData object in order to access it’s properties /
member variables – post-processing and post-calculations. Which means that really, I want to get the object pre-filled
with data. And so I keep the calculateStockMarketData method private and call it automatically from the StockMarketData
constructor. But I’m feeling somewhat uneasy about doing all my processing in a constructor and then not having any public methods. Is this a design flaw? Or is it really the most logical way to do this? Basically, which of the following 2 implementations is better?
1) (My current implementation) Make the central calculateStockMarketData() method private and call it from the StockMarketData method constructor (passing today’s date), so that whenever you have a StockMarketData object, it’s already filled. So all I would need from the ReportGenerator class before I start using the object properties is the line:
StockMarketData myData = new StockMarketData(someDate);
2) Make the central calculateStockMarketData() method public, so that in order to set up a StockMarketData object you need to explicitly call the method. So from the ReportGenerator class I would code:
StockMarketData myData = new StockMarketData(someDate);
myData.calculateStockMarketData();
The first strikes me as the better design, especially since there is then no possibility of using the object properties before they are initialized. But I am also unsure about the standards regarding executing a great deal of code from a constructor… Which should I choose?
I would go with number 2, especially if there is a possibility of adding methods to the class that don’t rely on that data being there.
On the other hand, if you would consider the class to be in an invalid state without the data populated, you should do it on construction.