extends

问题 I am just trying to understand the extends keyword in Java Generics. List<? extends Animal> means we can stuff any object in the List which IS A Animal then won\'t the following also mean the same thing: List<Animal> Can someone help me know the difference between the above two? To me extends just sound redundant here. Thanks! 回答1: List<Dog> is a subtype of List<? extends Animal> , but not a subtype of List<Animal> . Why is List<Dog> not a subtype of List<Animal> ? Consider the following

问题 If I have several classes with functions that I need but want to store separately for organisation, can I extend a class to have both? i.e. class a extends b extends c edit: I know how to extend classes one at a time, but I\'m looking for a method to instantly extend a class using multiple base classes - AFAIK you can\'t do this in PHP but there should be ways around it without resorting to class c extends b , class b extends a 回答1: Answering your edit : If you really want to fake multiple

问题 I need to create a base class that extends Activity which does some common tasks in my application and extend my activities from it,in the following form: public BaseActivity extends Activity{....} public SubActivity extends BaseActivity{...} in SubActivity I need to give values to some variables and UI components defined in BaseActivity , I may need to define a different layout for SubActivity according to some flag value, also(in SubActivity ) I want to execute asyncTask that is defined in

问题 Please explain in an easy to understand language or a link to some article. 回答1: extends is for extending a class. implements is for implementing an interface The difference between an interface and a regular class is that in an interface you can not implement any of the declared methods. Only the class that "implements" the interface can implement the methods. The C++ equivalent of an interface would be an abstract class (not EXACTLY the same but pretty much). Also java doesn't support