Question

In the first code example of this beginners guide to Dependency Injection I encountered some new constructs of which I am not sure that I totally understand:

 // Instantiate CabAgency, and satisfy its dependency on an airlineagency.

 Constructor constructor = cabAgencyClass.getConstructor
  (new Class[]{AirlineAgency.class});
 cabAgency = (CabAgency) constructor.newInstance
  (new Object[]{airlineAgency});

What does new Class[]{AirlineAgency.class} actually mean and do?

I understand that its goal is to create a Constructor instance for AirlineAgency.class but how does the syntax new Class[]{} achieve this?

Why the array notion [] when there is only one object involved?

What is the {} syntax here? Why not ()?

Answer 1

new Class[] { AirlineAgency.Class } creates an one-element array of Class objects and initializes the only element to be AirlineAgency.class. It is similar to new int[] { 42 }.

The code is essentially equivalent to this:

Class[] parameterTypes = new Class[1];
parameterTypes[0] = AirlineAgency.class;

Constructor constructor = cabAgencyClass.getConstructor(parameterTypes);

Object[] arguments = new Object[1];
arguments[0] = airlineAgency;

cabAgency = (CabAgency)constructor.newInstance(arguments);

The Class.getConstructor method wants an array of parameter types for the constructor (to find the right overload to use), and similarly Constructor.newInstance wants an array of arguments. That’s why it’s done that way.