I have two classes
package a;
public class A {
protected void doSomething() {
}
protected static class C {
protected C(int c) {
}
}
}
package b;
public class B extends A {
#Override
protected void doSomething() {
C c = new C(0); //compile error
C c2 = new C(0){}; //legal
}
}
I have read chapter 6.6.2.2. Access to a protected Constructor of JLS (https://docs.oracle.com/javase/specs/jls/se11/html/jls-6.html) but i am still confused with the explanation. What is wrong with the call of the super constructor new C(0); even if B is a child of A?
Thank you :-)
Variables, methods, and constructors, which are declared protected in a superclass can be accessed only by the subclasses in other package or any class within the package of the protected members' class.
Now, the constructor of C class is protected therefore is accessible outside the a package only by subclasses of C. But B is not a subclass of C ...
As #Amongalen pointed out, the second statement
C c2 = new C(0){};
is legal because it creates an anonymous class that extends C, therefore the protected constructor is visible here.
Related
I'm new to java and getting the hang of protected visibility.
I have Class A which has protected method method1 as follows -
public abstract class A {
public A(...){}
protected void method1(){
// do something
}
Now I have another Class B which extends a different Class C. Class B has a nested static class, Class D extending Class A
public class B extends C {
public B(...){
super(...);
}
private static class D extends A {
D(...){super(...);}
}
public void method2() {
D record = new D();
record.method1(); // need to access the protected method1 of class A, which is the superclass of nested class D. Getting the error here
}
}
I get this error - method1 has protected access in Class A
Now my question is how can I access method1? I cannot change it to public in class A
You need to create a proxy method in class D as outlined below. Class D can access Class A's protected methods, but an instance of Class D can't call protected methods directly, and must have a proxy method to call the protected method of the super class.
In class D, you need to create a public getter for the method in Class A, so:
public class A {
protected void protectedMethod() {}
}
public class D extends A {
public void callProtectedMethod() {
super.protectedMethod();
}
}
final D record = new D();
record.callProtectedMethod();
Since A.method1() is protected, it is only accessible from same class/same package/child of A.
In your code, your are calling A.method1() in B, that is not accessible if B not in same package as A
The solution is using a wrapper as #Alex Ander or change modifier of A.method1() to public
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 the Oracle documentation says:
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?
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);
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
Protected means:
a) This member will be accessible to all classes in the same package through A object’s reference.
b) For different packages, this will be accessible only inside subclasses of A, say B, and the reference used can be a B instance or of any subclass of B.
Let's take an example:
Let A be a parent class in some package, say com.ex1.
Let B and C be classes in different packages w.r.t to A, say com.ex2. Also, B extends A and C extends B.
We will see how we can use protected fields 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 a 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 a 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
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.
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);
}}
Within the same package where the protected member is declared, access is permitted:
package package1;
public class C extends A{
public void go(){
A a = new A();
System.out.println(a.protectedInt); // got printed
C c = new C();
System.out.println(c.protectedInt); // got printed as well
}
}
Outside the package where the protected member is declared, access is permitted if and only if by code that is responsible for the implementation of that object. In this case, C is responsible for the implementation of that object, so it could access the protected.
package package2;
public class C extends A{
public void go(){
A a = new A();
System.out.println(a.protectedInt); // compiler complains
C c = new C();
System.out.println(c.protectedInt); // got printed
}
}
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 the Oracle documentation says:
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?
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);
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
Protected means:
a) This member will be accessible to all classes in the same package through A object’s reference.
b) For different packages, this will be accessible only inside subclasses of A, say B, and the reference used can be a B instance or of any subclass of B.
Let's take an example:
Let A be a parent class in some package, say com.ex1.
Let B and C be classes in different packages w.r.t to A, say com.ex2. Also, B extends A and C extends B.
We will see how we can use protected fields 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 a 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 a 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
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.
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);
}}
Within the same package where the protected member is declared, access is permitted:
package package1;
public class C extends A{
public void go(){
A a = new A();
System.out.println(a.protectedInt); // got printed
C c = new C();
System.out.println(c.protectedInt); // got printed as well
}
}
Outside the package where the protected member is declared, access is permitted if and only if by code that is responsible for the implementation of that object. In this case, C is responsible for the implementation of that object, so it could access the protected.
package package2;
public class C extends A{
public void go(){
A a = new A();
System.out.println(a.protectedInt); // compiler complains
C c = new C();
System.out.println(c.protectedInt); // got printed
}
}
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 the Oracle documentation says:
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?
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);
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
Protected means:
a) This member will be accessible to all classes in the same package through A object’s reference.
b) For different packages, this will be accessible only inside subclasses of A, say B, and the reference used can be a B instance or of any subclass of B.
Let's take an example:
Let A be a parent class in some package, say com.ex1.
Let B and C be classes in different packages w.r.t to A, say com.ex2. Also, B extends A and C extends B.
We will see how we can use protected fields 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 a 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 a 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
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.
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);
}}
Within the same package where the protected member is declared, access is permitted:
package package1;
public class C extends A{
public void go(){
A a = new A();
System.out.println(a.protectedInt); // got printed
C c = new C();
System.out.println(c.protectedInt); // got printed as well
}
}
Outside the package where the protected member is declared, access is permitted if and only if by code that is responsible for the implementation of that object. In this case, C is responsible for the implementation of that object, so it could access the protected.
package package2;
public class C extends A{
public void go(){
A a = new A();
System.out.println(a.protectedInt); // compiler complains
C c = new C();
System.out.println(c.protectedInt); // got printed
}
}
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 the Oracle documentation says:
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?
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);
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
Protected means:
a) This member will be accessible to all classes in the same package through A object’s reference.
b) For different packages, this will be accessible only inside subclasses of A, say B, and the reference used can be a B instance or of any subclass of B.
Let's take an example:
Let A be a parent class in some package, say com.ex1.
Let B and C be classes in different packages w.r.t to A, say com.ex2. Also, B extends A and C extends B.
We will see how we can use protected fields 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 a 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 a 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
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.
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);
}}
Within the same package where the protected member is declared, access is permitted:
package package1;
public class C extends A{
public void go(){
A a = new A();
System.out.println(a.protectedInt); // got printed
C c = new C();
System.out.println(c.protectedInt); // got printed as well
}
}
Outside the package where the protected member is declared, access is permitted if and only if by code that is responsible for the implementation of that object. In this case, C is responsible for the implementation of that object, so it could access the protected.
package package2;
public class C extends A{
public void go(){
A a = new A();
System.out.println(a.protectedInt); // compiler complains
C c = new C();
System.out.println(c.protectedInt); // got printed
}
}