What is mean by Collection<? extends EmpApp>?

In my code i am using two different class objects empobj & employee. Eclipse asks to change the code

empobj.addAll(employee);

to

empobj.addAll((Collection<? extends EmpApp>) employee);

What does it mean? I cannot understand the concept开发者_运维知识库 here. Can I get any clarifications about this?

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Collection<? extends EmpApp>

Is a definition of a collection. The "employee into a collection which contains objects of a class which extends EmpApp. The "?" in the generic means that Eclipse isn't sure what classname should be there.

For more on generics in Java, here's the wikipedia article

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addAll takes as its parameter a Collection and then adds all the elements of that collection to itself. In your case, you're trying to give it something that is not a Collection, so the compiler is trying to make it work by casting the argument to the correct type. In this particular case, the correct type is a collection of some object which extends EmpApp, i.e., Collection<? extends EmpApp>.

Judging from the name of your variable, employee probably isn't a collection, so you need to revisit the API for whatever collection empobj is and find out how you can add a single element to it (likely add).

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A collection is group of object to be treated as a single unit.Arbitrary objects can be stored,retrieved,& manipulated as elements of collection.