[英]Generics - incompatible types
public class HelloWorld{
public static void main(String[] args) {
List<? extends Number> callback = new ArrayList<Long>();
new Container<>().test(callback);
}
public static class Container<T extends Number> {
public void test(List<T> some) {
}
}
} }
this code produces 该代码产生
HelloWorld.java:7: error: incompatible types: List<CAP#1> cannot be converted to List<Number>
new Container<>().test(callback);
^
where CAP#1 is a fresh type-variable:
CAP#1 extends Number from capture of ? extends Number
Can you explain in details this code incorrect. 您能否详细说明此代码不正确。
I expect that new Container will be generalized with type compatible with callback 我希望新的Container将被泛化为与回调兼容的类型
Firstly, there cannot be a type that extends String
as String
is a final class. 首先,不能存在扩展
String
的类型,因为String
是最终类。
Secondly, the compiler cannot determine if the type ? extends SomeType
其次,编译器无法确定类型是否为
? extends SomeType
? extends SomeType
that you use for your List
is the same type as T extends SomeType
that you use for the Container
class, so it produces an error. ? extends SomeType
你用你的List
是相同类型的T extends SomeType
您使用的Container
类,所以它产生一个错误。
What you would need to do is either declare a generic type in the method signature, and use that type for both the List
and Container
: 您需要做的就是在方法签名中声明一个泛型类型,然后对
List
和Container
使用该类型:
public <T extends SomeClass> void example() {
List<T> callback = new ArrayList<>();
new Container<T>().test(callback);
}
Or declare the list with a non-bounded type: 或使用不受限制的类型声明列表:
List<SomeClass> callback = new ArrayList<>();
new Container<>().test(callback);
Well, you're using generics. 好吧,您正在使用泛型。 as you define Container, it tells to Container default constructor must support a type which is instance of String.
当您定义Container时,它告诉Container默认构造函数必须支持类型为String的实例。 here is a correct example which String is subClass of Object, so i use that.
这是一个正确的示例,其中String是Object的子类,所以我使用它。
public class HelloWorld{
public static void main(String[] args) {
List<String> callback = new ArrayList<String>();
new Container<String>().test(callback);
}
public static class Container<T extends Object> {
public void test(List<T> some) {
}
}
}
I hope this helps you. 我希望这可以帮助你。
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