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

Question

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

Answer 1

Since each instance of the subclass must still be a valid instance of the base class (see the Liskov substitution principle ).

If a subclass suddenly lost one property of the base class (for example, a public method, for example), then it will no longer be a valid replacement for the base class.

Answer 2

Because, if allowed, the following situation is possible:

The Sub class inherits from the Parent class. The parent has a public method foo , Sub makes this method private. Now, the following code will compile fine, because the declared type bar is Parent:

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

However, it is not clear how this should behave. One possibility is to allow it to cause a runtime error. Another would be to simply allow it, which would allow you to call the private method from the outside, just by clicking on the parent class. None of these alternatives is acceptable, so it is not allowed.

Answer 3

Because subtypes should be used as instances of their supertype.