Dagger2 how to perform constructor injection of a default constructor

Question

Here is a scenario. Let's say there is a class A

Class A{

@Inject
public A(){}

}

And in my Activity

 public class MainActivity extends Activity{

    @Inject
    A a;

     @Override
        protected void onCreate(Bundle savedInstanceState) {
            super.onCreate(savedInstanceState);
    }
}

How to inject a in this scenario.

Answer 1

Ok first of all you need a module:

@Module
class SomeModule{
  @SomeScope
  @Provides
  A proivdeTheAInstance(){
   return new A();
  }
}

Then your component:

@SomeScope
@Component(modules = {A.class}, dependencies={HigherLowerDependencyComponent.class})
interface SomeComponent{
 void inject(MainActivity activity);
}

After that in your activity, after you have performed build, in the onCreate

DaggerSomeComponent.builder().higherLowerDependencnyComponent(implementationHere).build().inject(this)

Than you can @Inject A

But there is one problem. Constructor injection doesn't work like that. In order to perform Constructor Injection, your A() constructor should have at least one dependency. The @Inject annotation on that constructor doesn't call the A() but its dependencies, which in your case would be 0, thus making the @Inject in your A() constructor unnecessary. Your question would stand if your A constructor would be like this:

    @Inject
    public A(SomeDependency dependency){
     this.someDependency = dependency;
    }

SomeDependency is going to be provided in the module as well:

@Module
class SomeModule{
  @SomeScope
  @Provides
  A proivdeTheAInstance(SomeDependency someDependency){ //now dagger will look to find this one
   return new A();
  }

@SomeScope
  @Provides
  SomeDependency proivdeSomeDependencyInstance(){ //which is provided here 
   return new SomeDependency();
  }
}

And you are good to go as you were:

class A{
private SomeDependency someDependency;
 @Inject
 public A(SomeDependency someDependency){ //the inject will require the SomeDependency
  this.someDependency = someDependency;
  }
}

EDIT: If dagger already knows how to provide the instance there is no need for a Module, you can just perform @Inject above the constructor of the class A():

class A{
    @Inject
    public A(){
    }
}