Method overriding and visibility in Java

Question

Why does the compiler give an error message when you reduce the visibility of a method while overriding it in the subclass?

Answer 1

Because every instance of the subclass still needs to be a valid instance of the base class (see Liskov substitution principle).

If the subclass suddenly has lost one property of the base class (namely a public method for example) then it would no longer be a valid substitute for the base class.

Answer 2

Because subtypes have to be usable as instances of their supertype.

Answer 3

Because if this was allowed, the following situation would be possible:

Class Sub inherits from class Parent. Parent has a public method foo, Sub makes that method private. Now the following code would compile fine, because the declared type of bar is Parent:

Parent bar = new Sub();
bar.foo();

However it is not clear how this should behave. One possibility would be to let it cause a runtime error. Another would be to simply allow it, which would make it possible to call a private method from outside, by just casting to the parent class. Neither of those alternatives are acceptable, so it is not allowed.