Why can't you reduce the visibility of a method in a Java subclass?

Question

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

Solution 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.

Solution 2

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.

Solution 3

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

Comments

[Pacerier]

But why are we not allowed to override a protected method and change it to private? Since the public interface is still the same, it doesn't break LSP this way.

[Elazar]

The public interface does not change, but the protected does. Code in the parent class cannot access the methods of its own flesh and blood :(