Eclipse shows an error when trying to use protected constructor in a subclass located in other package [duplicate]

Answer 1

This is fun so let me try to summarize it. see JLS#6.6.1

protected can qualify a constructor or a member of a class.

"member" includes field/method (static/instance), nested class/interface (static/inner)

class A {
    protected int f
    protected void m(){}
    protected class X{}
    protected interface Y{}

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First, to access a protected constructor/member, the enclosing class (e.g. A) must be accessible. Assume that's the case, then --

--Inside the package --

A protected constructor or member is accessible anywhere within the same package.

--Outside the package --

A protected constructor is only accessible within subclass constructors, either as a super() call, or as an anonymous class instantiation.

A protected static field/method, nested class/interface is accessible anywhere within subclass bodies.

---

A protected instance field/method is more complex --

• protected "m" is defined in a class A
• obj.m is accessed in class B (outside A's package)
• obj 's type is C

The access obj.m is granted only if B is subclass of A, and C is subclass of B or C is B.

super.m is always allowed; however, it's unclear how JLS frames the issue. It seems that the access should be treated the same as this.m, therefore access is allowed.

Answer 2

See Java Language Specification:

A protected member or constructor of an object may be accessed from outside the package in which it is declared only by code that is responsible for the implementation of that object.

Your class A2 is not responsible for the implementation of A in the new A() call.

Meaning, it is not responsible for the implementation of an instance of A, but it is responsible for the implementation of an instance of A2.