java - Why can't I add SuperType to List<T> where <T extends SuperType>? -

with

public class supertype { } 

and

public class testclass<t extends supertype > {      public void dosomething() {         list<t> list = new arraylist<t>();         list.add(new supertype ()); // error     } } 

it won't compile, giving me

the method add(t) in type list not applicable arguments (supertype)

but why?

assume have

public class supertype { }  public class subtype extends supertype { } 

and use testclass<subtype> code becomes

public class testclass<subtype> {      public void dosomething() {         list<subtype> list = new arraylist<subtype>();         list.add(new supertype ());     } } 

that has error because new supertype() not specific enough. has @ least subtype.