Understanding java's protected modifier

Question

I have a class called A in package1 and another class called C in package2. Class C extends class A.

A has an instance variable which is declared like this:

protected int protectedInt = 1;

Here is the code for class A

package package1;

public class A {

    public int publicInt = 1;
    private int privateInt = 1;
    int defaultInt = 1;
    protected int protectedInt = 1;

}

And here is the code for class C:

package package2;
import package1.A;

public class C extends A{

    public void go(){
        //remember the import statement
        A a = new A();
        System.out.println(a.publicInt);
        System.out.println(a.protectedInt);

    }
}

Eclipse underlines the last line in C.go() and says "A.protectedInt" is not visible. It seems that this conflicts with the definition of the "protected" keyword, given in the oracle documentation.

The protected modifier specifies that the member can only be accessed within its own package (as with package-private) and, in addition, by a subclass of its class in another package.

What's going on here?

Solution 1

What's going on here?

You've misunderstood the meaning of protected. You can access the protected members declared in A from within C, but only for instances of C or subclasses of C. See section 6.6.2 of the JLS for details of protected access. In particular:

Let C be the class in which a protected member is declared. Access is permitted only within the body of a subclass S of C.

In addition, if Id denotes an instance field or instance method, then:

• [...]

• If the access is by a field access expression E.Id, where E is a Primary expression, or by a method invocation expression E.Id(. . .), where E is a Primary expression, then the access is permitted if and only if the type of E is S or a subclass of S.

(Emphasis mine.)

So this code would be fine:

C c = new C();
System.out.println(c.publicInt);
System.out.println(c.protectedInt);

Solution 2

Since C is inheriting A, C can directly use the protected variable of A like below

public class C extends A{

    public void go(){

       System.out.println(protectedInt);

    }
}

As per your code, you are creating an instance of A and accessing protected variable through that instance, which violates java's rule - A protected variable is not visible outside the package

Solution 3

 public void go(){
        //remember the import statement
        A a = new A();
        System.out.println(a.publicInt);
        System.out.println(a.protectedInt);

    }

When you are doing A a = new A(); and a.protectedInt you trying to access protected member of A which is illegal according to java standards

Instead you can do this.protectedInt directly.

Solution 4

Protected means :

a)This member will be accessible to all classes in same package through A object’s reference.

b) For different package, this will be accessible only inside Subclasses of A say B and the reference used can be of B instance or of any subclass of B.

Let's take an example:

Let A be parent class in some package say com.ex1 Let B ,C be classes in different package w.r.t to A say com.ex2 . Also, B extends A and C extends B. We will see how we can use protected field of A inside B (a subclass of A)

A's code:

public class A {
    protected int a = 10;
}

B's code:

public class B extends A {


    public void printUsingInheritance() {
        // Using this
        System.out.println(this.a);
    }


    public void printUsingInstantiation() {
        // Using instance of B
        B b = new B();
        System.out.println(b.a);

        // Using instance of C as C is subclass of B
        C c = new C();
        System.out.println(c.a);


        A a = new A();
        System.out.println(a.a); // Compilation error as A is not subclass of B
    }
}

C's code:

public class C extends B {

}

For protected Static :

Same rules apply except that in b) now it is accessible in any subclass of A by A's class reference. Reference

Solution 5

No need to instantiate Protection class inside Protection2 Class. You can directly call the protected variable without instantiating the Protection class. Because Protection2 class extends Protection class. So variable automatically inherited by subclass.

Try with below code:

public class Protection2 extends Protection{
Protection2()
{System.out.println("n_pro = " +n_pro);
}}

View more solutions

Comments

[blgt]

The original code will work if both classes are in the same package.

[mahela007]

In that case, why would java have a protected modifier at all? Wouldn't simply making C extend A be enough to make protectedInt visible to C?

[Jon Skeet]

@mahela007: Not without it being protected, no - if it were either default (package) visibility or private, it wouldn't be visible.

[mahela007]

I read this from the JLC doc that Jon skeet posted. "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." How can the code that "implements an object" be outside the package of the same object?

[mahela007]

Hmm... The emphasized part in your answer says "access is permitted if E is a subclass if S".. But in my example, C is a subclass of A..and I still can't access the protected variable.

[Jon Skeet]

@mahela007: But E is A here, and S is C. Although C is a subclass of A, A isn't a subclass of C.

[mahela007]

Ooh.. I think I get it now. Thank you very much for your help. This is a very non-intuitive concept (at least for me). Can you take a look at my comment on sanbhats answer? There's another part of the JLS that I don't understand..

[Jon Skeet]

@mahela007: Look at your example - the members are declared in A, which is in a different package to the class C, which is what "implements" an object of type C.

[Alex Semeniuk]

Please also have in mind that protected methods and variables are visible not only to child classes BUT ALSO to classes within the same package (just like the default objects are). So documentation is wrong saying "if and only if". (Try this yourself everyone).

[overexchange]

To simplify, In OP's code a.protectedInt, protectedInt is accessed using composition relation, so NO to visibility/access. In Jon skeet's example code c.protectedInt, protectedInt is accessed using inheritance relation, so YES to visibility/access.