`Cannot find symbol` when trying to access a property of a child class but the variable is of type superclass

Question

Java noob here, I have a variable of type OutputStream and later on in the function I have a condition where I either assign OutputStream to a new instance of FileOutputStream or ByteArrayOutputStream, however whenever I try to access any property that belongs to any of the subclasses. I get a Error cannot find symbol . Is there a way to keep the variable of the same parent class and try to tell the runtime that whenever I need to access the property it would be of the child's class type?

Here is some pseudo code

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;
}

}

Answer 1

You have to cast it like this:

(ByteArrayOutputStream output).buf

Java doesn't know that it can safely call the method in ByteArrayOutputStream since OutputStream doesn't have that method.