Conditional Logic and GUICE

For Contact Personal Assistant I am using GUICE.   After being introduced to Dependency Injection, or Inversion of Control, I appreciate its awesomeness, if I don’t fully understand it.  One of my main issues with fully adopting this paradigm shift, has been the usual barrage of questions a developer asks and answers in their own head in a daily basis, and the main one for me so far was.

How do I implement a conditional logic for an implementation instance based on runtime parameters?

Sounds a little abstract, I know.  So lets try an example.  Imagine you have a control which renders some information from a IDataServer interface but at runtime you have several Implementations of the IDataServer interface to chose from.

I have thought of many ways this could be achieved, but have known, them all to be hacky, my implementation based on how I currently understand GUICE, and trying to fit it into what I require.  As they say with great power comes great responsibility, and I want to use this framework correctly, not in hacked way that I can get it to work given my current level of knowledge.

It turns out the solution is as simple as it is beautiful:

USE MAP BINDER!

Luckily the answer is discussed on this thread which I replicate here:  

   1:  public class CalculatorModule extends AbstractModule { 

   2:     protected void configure() { 

   3:       MapBinder<Types, Class<ICalculator>> mapbinder 

   4:           = MapBinder.newMapBinder(binder(), Types.class, 

   5:  ICalculator.class); 

   6:       mapbinder.addBinding(Types.A).toInstance(ACalculator.class); 

   7:       mapbinder.addBinding(Types.B).toInstance(BCalculator.class); 

   8:       mapbinder.addBinding(Types.C).toInstance(CCalculator.class); 

   9:     } 

  10:   } 

  11:  @Singleton 

  12:  public class CalculatorFactory { 

  13:      private final Injector injector; 

  14:      private final Map<Types, Class<Calculator>> calculators; 

  15:     @Inject 

  16:     public CalculatorFactory(Injector injector, Map<Types, 

  17:  Class<Calculator>> calculators) { 

  18:          this.injector = injector; 

  19:          this.calculators = calculators; 

  20:      } 

  21:      public ICalculator create(UserGroup group) { 

  22:           injector.getInstance(calculators.get(group.getType)); 

  23:      } 

  24:  } 

This example uses a calculator factory to allow the user to select whether they want implementation A, B or C.  Simply inject the calculator factory into whichever class requires it.

Beautiful!