相关疑难解决方法(0)

有谁知道为什么下面的代码不能编译？add()和addAll()都不能按预期工作.删除"？extends"部分会使一切正常,但之后我将无法添加Foo的子类.

 List<? extends Foo> list1 = new ArrayList<Foo>();
 List<? extends Foo> list2 = new ArrayList<Foo>();

 /* Won't compile */
 list2.add( new Foo() ); //error 1
 list1.addAll(list2);    //error 2 

错误1:

IntelliJ说:

add(capture<? extends Foo>) in List cannot be applied to add(Foo)

编译器说:

cannot find symbol
symbol  : method addAll(java.util.List<capture#692 of ? extends Foo>)
location: interface java.util.List<capture#128 of ? extends Foo>

错误2:

IntelliJ给了我

addAll(java.util.Collection<? extends capture<? extends Foo>>) in List cannot be applied to addAll(java.util.List<capture<? extends Foo>>)

而编译器只是说

cannot find symbol
symbol …