`试图访问子类的属性时找不到符号',但变量的类型为超类

JavaNoB这里,我有一个OuttoStand类型的变量,在函数中,我有一个条件,当我尝试访问属于任何子类的任何属性时,我要么将OutPoSt流分配给FieleOutToStudio或BytErayOutPutsFipe的新实例。我得到了 Error cannot find symbol . 有没有一种方法可以保持同一父类的变量,并尝试告诉运行时,每当我需要访问属性时,它都是子类类型的?

这是一些伪代码

public void work(Map params)
{
OutputStream output = null;


if(params.isAFile)
{
  output = new FileOutputStream();
}
else
{
  output = new ByteArrayOutputStream();
}

...do some work that makes use of the OutputStreams class
if(isFile)
{
  return output
}
else
{
  mybuffer = output.buf; //it fails here with Cannot find symbol since output is of type OutputStream when it should be treated as type ByteArrayOutputStream
  return myBuffer;
}

}