为什么变量的未定义通用参数会丢弃其方法的返回类型的显式通用参数？

Question

public abstract class A<T> {
    public static void test(A i) { for (String s : i.get()) {} }
    public abstract Iterable<String> get();
}

Why for the code above I get:

incompatible types
required: String
found:    Object

But if I will change argument of test method to A<Object> , it will compile OK? Why undefined generic parameter of variable drops explicit generic parameter of it's method's return type?

Answer 1

Why undefined generic parameter of variable drops explicit generic parameter of it's method's return type?

A is a raw type. That's a type with all generics dropped in the API dropped, even ones with fixed type arguments.

See the JLS section 4.8 and the Raw Types section of the Java Generics FAQ for more information.

In this case, if you want any A , you can use a wildcard:

public static void test(A<?> i)