Java迭代器返回的结果的通用类型不兼容

Question

I have an object which is a type of Map, and I am trying to iterate it with iterator. However, I cannot pass the compile, the code are as following.

// Returned from other method, and the run time type is Map<String, B>
Map<String, ? extends A> map = ...

// Option 1: cannot pass.
Iterator<Map.Entry<String, ? extends A>> iterator = map.entrySet().iterator();

// Option 2: cannot pass either.
// Where B is a subclass extends from A
Iterator<Map.Entry<String, B>> iterator = map.entrySet().iterator();

Can someone help me out? Thanks.

Answer 1

I suggest you use the foreach loop instead of using the Iterator directly

Map<String, ? extends A> map = new HashMap<>();
for (Map.Entry<String, ? extends A> entry : map.entrySet()) {
    String key = entry.getKey();
    A value = entry.getValue();
}

Answer 2

If you really want to store the Iterator in a variable, instead of this

// ERROR
Iterator<Map.Entry<String, ? extends A>> iterator = map.entrySet().iterator();

you have to do this:

Iterator<? extends Map.Entry<String, ? extends A>> iterator = map.entrySet().iterator();

The reason for the extra ? extends ? extends is the wildcard nested in in the Map.Entry<String, ? extends A> Map.Entry<String, ? extends A> declaration. If you had a variable of type

Iterator<Map.Entry<String, ? extends A>>

then in principle each entry could have a value of a different subtype of A, so each Map.Entry would be of a different type. By contrast,

Iterator<? extends Map.Entry<String, ? extends A>>

means that each entry is of a single type. (But you don't know what type it is, since it's a wildcard.)

In practice, this doesn't matter for Iterator since you're only getting objects of type Map.Entry<String, ? extends A> Map.Entry<String, ? extends A> out of it. But if you had some other type that allowed you to put things into it (such as a Set ) then the difference would be significant. For example,

Set<? extends Map.Entry<String, ? extends A>>

would only allow getting entries from the set, but

Set<Map.Entry<String, ? extends A>>

would allow adding entries to the set, and those entries might contain different subtypes of A, possibly polluting the set.

For further discussion on this, see

• Angelika Langer - Generics FAQ - What do multilevel wildcards mean?

as well as these Stack Overflow questions:

• Java generics, nested collection of wildcard

• Nested Generics with Wildcards

• Mixing nested type parameters and wildcards in Java

Answer 3

I this this should do the trick

for (Map.Entry<String, ? extends A> entry : map.entrySet())
{
   String key = entry.getKey();
    A value = entry.getValue();
}