Override “private” method in java

You cannot override a private method. It isn’t visible if you cast A to B. You can override a protected method, but that isn’t what you’re doing here (and yes, here if you move your main to A then you would get the other method. I would recommend the @Override annotation when you intend to override,

class A extends B {
    @Override
    public void don() { // <-- will not compile if don is private in B.
        System.out.println("hoho public");
    }
}

In this case why didn’t compiler provide an error for using t.don() which is private?

The Java Tutorials: Predefined Annotation Types says (in part)

While it is not required to use this annotation when overriding a method, it helps to prevent errors. If a method marked with @Override fails to correctly override a method in one of its superclasses, the compiler generates an error.