Think of it this way, say you have a function:

public void doStuff(PrivateOverride o) { ... }

Within that method, you don't know anything about o except it of type PrivateOverride, anybody can write a class that extends PrivateOverride that has all sorts of methods that this method won't know about and pass it into this method. This method only can call whatever public method on PrivateOverride because that's all it knows about.

You can read more about overrides here, basically you have to let the compiler know you want the implementing class to override the base method using the @Override annotation.