Java泛型：无界通配符不能与Object类型参数一起使用

Question

Explaining the issue with an example:

public class DataWrapper<T> {
    T data;
};

DataWrapper<Object> obj1 = new DataWrapper<Object>();

List<DataWrapper<?>> anyDataList = Arrays.asList(obj1); //this doesn't work

DataWrapper<Integer> objInt = new DataWrapper<Integer>();
anyDataList = Arrays.asList(obj1, objInt); //this work

I couldn't understand why "Arrays.asList(obj1)" doesn't work?

Answer 1

Java 7 was stupid (not it wasn't) when inferring type arguments for generic methods. For example, it would use the declared type of an argument to infer the type regardless of the context of the method invocation. So in

Arrays.asList(obj1);

the type argument would be inferred as

DataWrapper<Object>

the method's return type would then be List<DataWrapper<Object>> which is no assignable to a List<DataWrapper<?>> .

In

Arrays.asList(obj1, objInt);

the type is inferred from both arguments. A common type is found between the two. In this case, that's ? extends Object ? extends Object . The method return type becomes List<DataWrapper<? extends Object>> List<DataWrapper<? extends Object>> which is assignable to List<DataWrapper<?>> .

In Java 8, what you posted works out of the box. In Java 7, you can provide an explicit type argument to make it work

List<DataWrapper<?>> anyDataList = Arrays.<DataWrapper<?>>asList(obj1);