java 中的协变量返回类型是什么？

package org.arpit.java2blog;
 
public class BaseClass {
 
 public A m1() {
 System.out.println("In BaseClass method");
 return new A();
 }
 
 public static void main(String args[])
 {
 BaseClass b=new SubClass();
 b.m1();
 }
}
 
class SubClass extends BaseClass {
 public B m1() {
 System.out.println("In SubClass method");
 return new B();
 
 }
}
 
class A {
}
 
class B extends A {
 
}