I'm going to try to incorporate the ideas from the other answers to come up with a single answer.
First off, let's take a look at what's going on in the code.
A look at the code
The One
class has a package-private foo
method:
class One {
// The lack of an access modifier means the method is package-private.
void foo() { }
}
The Two
class which subclasses the One
class, and the foo
method is overriden, but has the access modifier private
.
class Two extends One {
// The "private" modifier is added in this class.
private void foo() { /* more code here */ }
}
The issue
The Java language does not allow subclasses to reduce the visibility of a method, field or class in a subclass, therefore, the Two
class reducing the visibility of the foo
method is not legal.
Why is reducing visibility a problem?
Consider the case where we want to use the One
class:
class AnotherClass {
public void someMethod() {
One obj = new One();
obj.foo(); // This is perfectly valid.
}
}
Here, calling the foo
method on the One
instance is valid. (Assuming that the AnotherClass
class is in the same package as the One
class.)
Now, what if we were to instantiate the Two
object and place it in the obj
variable of the type One
?
class AnotherClass {
public void someMethod() {
One obj = new Two();
obj.foo(); // Wait a second, here...
}
}
The Two.foo
method is private, yet, the One.foo
method would allow the access to the method. We've got a problem here.
Therefore, it doesn't make much sense to allow reduction of visibility when taking inheritance into account.
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