Here in this article, we are going to study why we are not allowed to use these keywords while declaring outer classes. But before understating the reason behind this we need to understand Java access specifiers and their use cases. There is total 4 access specifier in Java mentioned below in the order of their accessibility.
- private: anything (field, class, method, interface etc.) defined using private keyword is only accessible inside the entity (class or package or interface) in which it is defined.
- default: only accessible inside the same package and it is also known as package-private (No modifiers needed).
- protected: only accessible inside the same package plus outside the package within child classes through inheritance only.
- public: can be accessed from anywhere.
Why an outer class can not be private
As we already know a field defined in a class using private keyword can only be accessible within the same class and is not visible to outside world.So what will happen if we will define a class private, that class will only be accessible within the entity in which it is defined which in our case is its package?
Let’s consider below example of class A
package com.example;
class A {
private int a = 10;
// We can access a private field by creating object of same class inside the same class
// But realy no body creates object of a class inside the same class
public void usePrivateField(){
A objA = new A();
System.out.println(objA.a);
}
}
Field ‘a’ is declared as private inside ‘A’ class and because of it ‘a’ field becomes private to class ‘A' and can only be accessed within ‘A’. Now let’s assume we are allowed to declare class ‘A’ as private, so in this case class ‘A’ will become private to package ‘com.example’ and will not be accessible from outside of the package.
So defining private access to the class will make it accessible inside the same package which default keyword already do for us, Therefore there is no benefit of defining a class private it will only make things ambiguous.
Why an outer class can not be protected
Access specifier protected is sometimes got confused with the default keyword, for some programmers it becomes hard to identify the exact difference between default and protected accesses. But it is very clear as mentioned belowdefault → only accessible within the same package.
protected → accessible within the same package as well as outside of the package in child classes through inheritance only.
Let’s consider below example of class A
package com.example;
public class A {
protected int a = 10;
}
And suppose there is one more class in another package
package com.experiment;
public class B extends A {
// Outside of the package protected field can be accessed through inheritance
public void printUsingInheritance() {
System.out.println(a);
}
// In child class we can access protected field through instantiation of child class
// But should we do that ? .... No
public void printUsingInstantiation() {
B b = new B();
System.out.println(b.a);
// But not through instantiation of the class which contains the protected field
A a = new A();
System.out.println(a.a); // Compilation error “The field A.a is not visible”
}
}
And suppose there is one more class in the same package
package com.experiment;
public class C {
// We can not access protected field outside of the child class through instantiation
public void printUsingInstantiation() {
B b = new B();
System.out.println(b.a); // Compilation error “The field B.a is not visible”
}
}
And if same class 'C' extends 'B', Then again we will be able to access 'a' the same way we are able to access it in B class
package com.experiment;
public class C extends B {
// outside of the package protected field can only be accessed through inheritance
public void printUsingInheritance() {
System.out.println(a);
}
// In child class we can access protected field through instantiation as well
public void printUsingInstantiation() {
C c = new C();
System.out.println(c.a);
}
}
Because ‘a’ field is protected, we can access it in any way we want to inside the package but outside of the package ‘com.example’ it is only accessible through inheritance and because class ‘B’ is extending class ‘A’ we can use field ‘a’ inside class ‘B’ only as we are doing in printUsingInheritance() method. And we can not use ‘a’ outside of class ‘B’ without inheriting 'B' in another class.
Field ‘a’ will be accessible inside class B in through inheritance only but if we try to create an instance of class ‘B’ in ‘B’ class and then try to access ‘a’ from that instance we will able to use it in a similar manner we can use a private variable in the same class by instantiation of same class.
So If we are allowed to make a class protected then we can access it inside the package very easily but for accessing that class outside of the package we first need to extend that entity in which this class is defined which is again is its package.
And since a package can not be extended (can be imported) defining a class protected will again make it similar to defining it as default which we can already do. So again there is no benefit of defining a class protected.
Please feel free to reach me if you face any problem in understanding it or found any problem in the article.
Great Blog
ReplyDeletethanks buddy
DeleteSorry Jorge, It was a mistake, I have updated the article.
ReplyDeleteWe can access it within the same class through instantiation as well but not outside of the class.
Since class A is public it doesn't matter which modifier you give to members inside class A. Class A is visible to everyone
ReplyDeleteDefinitely if `A` is public, it will be accessible from everywhere but if we are allowed to make it protected or private because as discussed, it does make any sense.
ReplyDeleteTy for explaining so well!!🙂it's really helped...!!one doubt..since parent class object is also the object of child class so when u tri to access the parent class memeber a using parent class object a in its child class...as a.a y it's saying not visible ....but when child class object u created and accessed b.a ,it's compiled well...y!!(through inheritance parent class object is also the object of child class naa??!!)PLZZ clarify 🙂😐
ReplyDeleteIts because if we try to access a protected member outside of the package, it can accessed in the child class only and that is too using the inheritance only. Which means class `B` will have access to `a` because it inherited the same field from its parent `A` but if we try to make object of `A` and then try to access `a` from it, field `a` will not visible.
Deletethat was a great explanation
ReplyDeleteThanks Mohit
Deleteinterface never be private and methods of interface never be final or private.please change in private modifier.
ReplyDelete@Ashish I am not able to understand what are you trying to say, but I have not mentioned it anywhere that we can declare a private outer interface or its method can be final (we can declare a private inner interface).
DeleteA very good article! With a simple code you have managed to extract some key feature. Thanks!
ReplyDeleteThanks!
DeleteThanks
ReplyDeletewhy can't we make "protected class A " and use it within the same package in another class by creating the object of the class A?
ReplyDeleteprotected class A{
int a = 10;
}
class B{
public static void main(String args[])
{
A o = new A();
System.out.println(o.a);
}
}