Why can a separate inner class with a private constructor be subclassed?
class ClassA
{
private ClassA()
{
}
}
class ClassB extends ClassA
{
//here we will get a compiler error that cannot extend a class with private constructor
}
public class GenericTestClass
{
private class TestingInnerPrivateClass
{
private TestingInnerPrivateClass()
{
}
public void display()
{
System.out.print("Test");
};
}
public class InnerPublicClass extends TestingInnerPrivateClass
{
//here i am able to extend a private class
}
public static void main(String[] args)
{
GenericTestClass genericTestClass = new GenericTestClass();
GenericTestClass.InnerPublicClass innerPublicClassInstance = genericTestClass.new InnerPublicClass();
innerPublicClassInstance.display();
}
}
If you look at the code above, you can see that I cannot expand classB
from classA
, but I can expand InnerPublicClass
from InnerPrivateClass
.
I cannot figure out how a class that is private and has a private constructor can also be subclassed when it is an inner class.
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InnerPublicClass
is defined internally GenericTestClass
and therefore has access to all private members of that class, which includes the inner class TestingInnerPrivateClass
. Therefore, it can expand TestingInnerPrivateClass
.
And here is the relevant quote from JLS 6.6.1 :
if a member or constructor is declared private, then access is allowed if and only if it occurs within the body of a top-level class (§7.6) that includes a member or constructor declaration.
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