Question

I’m writing an API that uses Guice for all its DI and want to hide all Guice “stuff” from the API developers. I have the following:

public class MyAppModule extends AbstractModule {
    @Override
    public void configure(Binder binder) {
        // Omitted for brevity...
        // For instance, binds interface type Animal to impl type Dog, etc.
    }
}

public class MyInjector {
    public abstract<?> inject(Class<?> clazz) {
        MyAppModule module = new MyAppModule();
        Injector injector = Guice.createInjector(module);

        return injector.getInstance(clazz);
    }
}

// So that API developers can just use Animal references and Guice will
// automatically inject instances of Dog
MyInjector injector = new MyInjector();
Animal animal = injector.inject(Animal.class); // <-- Guice returns a Dog instance

The problem is with my MyInjector#inject(Class<?>) method. Written the way it is, I’m getting a compiler error:

Multiple markers at this line
- Return type for the method is missing
- Syntax error on token "?", invalid TypeParameter

According to the Guice docs, Injector#getInstance returns an abstract<T>. If possible I’d like to avoid generics as well as explicit typecasting, to make things simpler for my API developers. Do I have any options here? If so, what are they? If not, why? Thanks in advance.

Answer 1

Don’t use the wildcard ? use something like T

Also, an abstract method can’t have an implementation so you need to remove it (you are overriding the method)

public <T> T inject(Class<T> clazz) {
    MyAppModule module = new MyAppModule();
    Injector injector = Guice.createInjector(module);

    return injector.getInstance(clazz);
}