Question

What’s the difference between

bind(FooImpl.class).in(Scopes.SINGLETON);
bind(Foo.class).to(FooImpl.class);

and

bind(Foo.class).to(FooImpl.class).in(Scopes.SINGLETON);

With Google Guice?

edit:

The second declaration create two instances on Singleton in a project I am working on.
Reference here

Answer 1

In reference to Google Guice documentation:

In linked bindings, scopes apply to the binding source, not the
binding target. Suppose we have a class Applebees that implements both
Bar and Grill interfaces. These bindings allow for two instances of
that type, one for Bars and another for Grills:

bind(Bar.class).to(Applebees.class).in(Singleton.class);
bind(Grill.class).to(Applebees.class).in(Singleton.class);

This is because the scopes apply to the bound type (Bar, Grill), not
the type that satisfies that binding (Applebees). To allow only a
single instance to be created, use a @Singleton annotation on the
declaration for that class. Or add another binding:

bind(Applebees.class).in(Singleton.class);

So, It’s possible to have two instances of FooImpl in the second way but not with the first way of writing the binding.