为什么对对象类型转换通用的是隐式的？

Question

While converting an non-generic class to a generic one, I saw this working :

public class TestException<T>
{
    T t;
    Object getObj(){
        return t;   
    }
}

When I tried to change the return-type from Object to any other say String , I had to type cast it :

String getObj(){
    return (String)t;   
}

Why generic to object type-casting is implicit ? Couldn't find any implementation on java docs.

Answer 1

每个类都继承Object ，因此T保证可以隐式转换为Object 。

Answer 2

Since your generic type T is unbound, it gets the implicit bound T extends Object . This means that it's fine for it to be returned as an Object .

There's no guarantee that T is a String , so you're forced to cast it (and you run the risk of runtime exceptions if you do and T is not a String ).

Answer 3

even if T wasn't generics type it would have worked. try it yourself!

you can always return something as it super type, same goes for generics

Answer 4

Every instance of every class ISA Object, so you can always cast any reference type value to Object. Only a String object, however, ISA String.