[英]wildcard type matching - Java
Why won't the following code compile? 为什么以下代码无法编译?
public static <T> void foo_test(List<? extends T> src, List<T> dest) {
for (T o : src) {
dest.add(o);
}
}
public static void main(String [] args) {
List<Number> numbers = new ArrayList<Number>();
List<Integer> integers = new ArrayList<Integer>();
foo_test(numbers, integers);
}
You shouldn't and can't add the list of numbers to a list of integers; 您不应该也不能将数字列表添加到整数列表中。 the numbers may not be integers.
这些数字可能不是整数。 However, you can add a list of integers to a list of numbers, the backwards of what you've typed.
但是,您可以将整数列表添加到数字列表中,即您键入的内容的倒数。
This will work: 这将起作用:
foo_test(integers, numbers);
To elaborate, T
is inferred as Integer
in your code, but Number
doesn't extend Integer
, so the call is a compiler error. 详细地说,
T
在您的代码中被推断为Integer
,但是Number
并没有扩展Integer
,因此该调用是编译器错误。
Switching to foo_test(integers, numbers)
makes T
inferred as Number
, and Integer
does extend Number
, so that compiles. 切换到
foo_test(integers, numbers)
可以将T
推断为Number
,并且Integer
确实扩展Number
,以便进行编译。
You need to switch integers
and numbers
. 您需要切换
integers
和numbers
。 The src
list has to be the more specific type than dest
. src
列表必须是比dest
更具体的类型。
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