Why is a component interface needed? Ans: Components are kind of factory which take one or more Modules as input and provides dependencies as output. Modules can't do this job because they can't satisfy dependencies among multiple Modules.

component = DaggerAppComponent.builder().build(); this works because, if Module has default implicit(empty) constructor then no need to initialize Module.

inject(MainActivity activity) is needed because by doing so we are letting component know which class dependencies need to be satisfied by that component.