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

Joachim Sauer
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ria
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3 Answers3

65

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.

Joey
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Joachim Sauer
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    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. – Pacerier Aug 23 '14 at 07:32
  • 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 :( – Elazar Sep 07 '15 at 22:08
18

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.

sepp2k
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1

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

z -
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