what does List<? extends SomeClass> mean?

Question

I saw that definition in a protobuf generated java file:

java.util.List<? extends xxx.yyy.zzz.proto.BasicMessage.DestInfoOrBuilder> foo();

But what dose <? and extends mean? I can understand List<SomeClass> I can't understand List<? extends SomeClass>..

Solution 1

This is a generic declaration.

It is used to check types at compile time. You could put any object into a list, but that would make it more difficult to maintain and could cause ClassCastExceptions, if not used appropriate.

<? extends xxx.yyy.zzz.proto.BasicMessage.DestInfoOrBuilder> means "allow every class which extends DestInfoOrBuilder".

Solution 2

In java generics programing,there are two kinds of bounds when using wildcards.

1)Upper Bounded Wildcards . like: ArrayList <? extends Number> p,it means you can use anything extends Number to fill the ArrayList.

2)Lower Bounded Wildcards. like:ArrayList<? super Integer> list, it means you have to pass anything who is the super class (such as Number,Object)of Integer to fill the ArrayList.

for more information,refer wildcards.

Solution 3

In java generic code, the question mark (?), called the wildcard, represents an unknown type. The wildcard can be used in a variety of situations: as the type of a parameter, field, or local variable; sometimes as a return type (though it is better programming practice to be more specific). The wildcard is never used as a type argument for a generic method invocation, a generic class instance creation, or a supertype.

For more information read Java Generic's Wildcards and Generics: The wildcard operator