If it better to ask for a parameter directly or have it passed in from another class?

[Leo Chen]

Dear all,

I'm tracing through Android framework's source code hoping to learn some design rationales, and I have a design question.

What's the design benefit of having a class fed with the information it requires instead of asking for it directly?

For example, I have a PackageManager class which asks WindowManager for display information, which then passes it on to ResourceManager.
My question is, what's the downside of having ResourceManager asking WindowManager for this information directly since PackageManager doesn't use this display information at all?

code follows:

public class PackageManager
{
   DisplayMetrics mMetrics;
  // some logic..
   Display d = wm.getDefaultDisplay();  // wm is Window Manager
   d.getMetrics(mMetrics);
  // some logic..
   ResourceManager rc = new ResourceManager(mMetrics);
   // uses rc to retrieve resources
}

public class ResourceManager
{
  public ResourceManager(DisplayMetrics _mMetrics)
  {
    // saves mDisplayMetrics for future calculations
  }
}

My guess is that if ResourceManager asks WindowManager directly, it is tightly coupled to WindowManager, therefore in the future if the display target changes to Printer, it won't be able to accommodate such change.
By having display information passed in, in the future if there is another class called PrinterManager, PrinterManager's display info can be passed in to ResourceManager, making ResourceManager independent of which display target it is working with.

Please help me understand this design decision.

thanks

[Lorand Komaromi]

Leo Chen wrote:
My guess is that if ResourceManager asks WindowManager directly, it is tightly coupled to WindowManager, therefore in the future if the display target changes to Printer, it won't be able to accommodate such change.
By having display information passed in, in the future if there is another class called PrinterManager, PrinterManager's display info can be passed in to ResourceManager, making ResourceManager independent of which display target it is working with.

Yes, you've nailed it! This approach is very useful when unit testing, too, you can pass in a fake class to ResourceManager to make unit tests independent of WindowManager. I suggest you read about Dependency Injection and Mock Object!