这是因为您没有在此处指定 Parent 的类型参数:

This is because you didn't specify the type parameter of Parent here:

Foo<Parent, String> foo = new Foo<>(stringParent, "bonjour");
Foo<Child, Integer> childFoo = new Foo<>(intChildBar, 42);

Object b = foo.parent.b;

如果您指定 Parent 的类型参数，则代替 Foo< Parent，String> ，即 Foo< Parent< String>，String> ，那么您可以获得正确的 b 类型:

Instead of Foo<Parent, String>, if you specify the type parameter of Parent, that is Foo<Parent<String>, String>, then you can get the correct type of b:

Foo<Parent<String>, String> foo = new Foo<>(stringParent, "bonjour");
Foo<Child, Integer> childFoo = new Foo<>(intChildBar, 42);

String b = foo.parent.b;