In Java, are there clear rules on when to use each of access modifiers, namely the default (package private), public
, protected
and private
, while making class
and interface
and dealing with inheritance?
The official tutorial may be of some use to you.
Class | Package | Subclass (same pkg) |
Subclass (diff pkg) |
World | |
---|---|---|---|---|---|
public |
+ | + | + | + | + |
protected |
+ | + | + | + | |
no modifier | + | + | + | ||
private |
+ |
+ : accessible
blank : not accessible
(Caveat: I am not a Java programmer, I am a Perl programmer. Perl has no formal protections which is perhaps why I understand the problem so well :) )
Like you'd think, only the class in which it is declared can see it.
It can only be seen and used by the package in which it was declared. This is the default in Java (which some see as a mistake).
Package Private + can be seen by subclasses or package members.
Everyone can see it.
Visible outside the code I control. (While not Java syntax, it is important for this discussion).
C++ defines an additional level called "friend" and the less you know about that the better.
When should you use what? The whole idea is encapsulation to hide information. As much as possible you want to hide the detail of how something is done from your users. Why? Because then you can change them later and not break anybody's code. This lets you optimize, refactor, redesign, and fix bugs without worry that someone was using that code you just overhauled.
So, the rule of thumb is to make things only as visible as they have to be. Start with private and only add more visibility as needed. Only make public that which is absolutely necessary for the user to know, every detail you make public cramps your ability to redesign the system.
If you want users to be able to customize behaviors, rather than making internals public so they can override them, it's often a better idea to shove those guts into an object and make that interface public. That way they can simply plug in a new object. For example, if you were writing a CD player and wanted the "go find info about this CD" bit customizable, rather than make those methods public you'd put all that functionality into its own object and make just your object getter/setter public. In this way being stingy about exposing your guts encourages good composition and separation of concerns
Personally, I stick with just "private" and "public". Many OO languages just have that. "Protected" can be handy, but it's really a cheat. Once an interface is more than private it's outside of your control and you have to go looking in other people's code to find uses.
This is where the idea of "published" comes in. Changing an interface (refactoring it) requires that you find all the code which is using it and change that, too. If the interface is private, well no problem. If it's protected you have to go find all your subclasses. If it's public you have to go find all the code which uses your code. Sometimes this is possible, for example, if you're working on corporate code that's for internal use only it doesn't matter if an interface is public. You can grab all the code out of the corporate repository. But if an interface is "published", if there is code using it outside your control, then you're hosed. You must support that interface or risk breaking code. Even protected interfaces can be considered published (which is why I don't bother with protected).
Many languages find the hierarchical nature of public/protected/private to be too limiting and not in line with reality. To that end, there is the concept of a trait class , but that's another show.
Here's a better version of the table, that also includes a column for modules.
A private member ( i
) is only accessible within the same class as it is declared.
A member with no access modifier ( j
) is only accessible within classes in the same package.
A protected member ( k
) is accessible within all classes in the same package and within subclasses in other packages.
A public member ( l
) is accessible to all classes (unless it resides in a module that does not export the package it is declared in).
Access modifiers is a tool to help you to prevent accidentally breaking encapsulation (*) . Ask yourself if you intend the member to be something that's internal to the class, package, class hierarchy or not internal at all, and choose access level accordingly.
Examples:
long internalCounter
should probably be private since it's mutable and an implementation detail.void beforeRender()
method called right before rendering and used as a hook in subclasses should be protected.void saveGame(File dst)
method which is called from the GUI code should be public.____________________________________________________________________
| highest precedence <---------> lowest precedence
*———————————————+———————————————+———————————+———————————————+———————
\ xCanBeSeenBy | this | any class | this subclass | any
\__________ | class | in same | in another | class
\ | nonsubbed | package | package |
Modifier of x \ | | | |
————————————————*———————————————+———————————+———————————————+———————
public | ✔ | ✔ | ✔ | ✔
————————————————+———————————————+———————————+———————————————+———————
protected | ✔ | ✔ | ✔ | ✘
————————————————+———————————————+———————————+———————————————+———————
package-private | | | |
(no modifier) | ✔ | ✔ | ✘ | ✘
————————————————+———————————————+———————————+———————————————+———————
private | ✔ | ✘ | ✘ | ✘
____________________________________________________________________
Easy rule. Start with declaring everything private. And then progress towards the public as the needs arise and design warrants it.
When exposing members ask yourself if you are exposing representation choices or abstraction choices. The first is something you want to avoid as it will introduce too many dependencies on the actual representation rather than on its observable behavior.
As a general rule I try to avoid overriding method implementations by subclassing; it's too easy to screw up the logic. Declare abstract protected methods if you intend for it to be overridden.
Also, use the @Override annotation when overriding to keep things from breaking when you refactor.
It's actually a bit more complicated than a simple grid shows. The grid tells you whether an access is allowed, but what exactly constitutes an access? Also, access levels interact with nested classes and inheritance in complex ways.
The "default" access (specified by the absence of a keyword) is also called package-private . Exception: in an interface, no modifier means public access; modifiers other than public are forbidden. Enum constants are always public.
Is an access to a member with this access specifier allowed?
private
: Only if member is defined within the same class as calling code.protected
: Same package, or if member is defined in a superclass of the class containing the calling code. public
: Yes.Local variables and formal parameters cannot take access specifiers. Since they are inherently inaccessible to the outside according to scoping rules, they are effectively private.
For classes in the top scope, only public
and package-private are permitted. This design choice is presumably because protected
and private
would be redundant at the package level (there is no inheritance of packages).
All the access specifiers are possible on class members (constructors, methods and static member functions, nested classes).
Related: Java Class Accessibility
The access specifiers can be strictly ordered
public > protected > package-private > private
meaning that public
provides the most access, private
the least. Any reference possible on a private member is also valid for a package-private member; any reference to a package-private member is valid on a protected member, and so on. (Giving access to protected members to other classes in the same package was considered a mistake.)
private[this]
.) You also have to consider nested scopes, such as inner classes. An example of the complexity is that inner classes have members, which themselves can take access modifiers. So you can have a private inner class with a public member; can the member be accessed? (See below.) The general rule is to look at scope and think recursively to see whether you can access each level.
However, this is quite complicated, and for full details, consult the Java Language Specification . (Yes, there have been compiler bugs in the past.)
For a taste of how these interact, consider this example. It is possible to "leak" private inner classes; this is usually a warning:
class Test {
public static void main(final String ... args) {
System.out.println(Example.leakPrivateClass()); // OK
Example.leakPrivateClass().secretMethod(); // error
}
}
class Example {
private static class NestedClass {
public void secretMethod() {
System.out.println("Hello");
}
}
public static NestedClass leakPrivateClass() {
return new NestedClass();
}
}
Compiler output:
Test.java:4: secretMethod() in Example.NestedClass is defined in an inaccessible class or interface
Example.leakPrivateClass().secretMethod(); // error
^
1 error
Some related questions:
As a rule of thumb:
private
: class scope. default
(or package-private
): package scope. protected
: package scope + child
(like package, but we can subclass it from different packages). The protected modifier always keeps the "parent-child" relationship. public
: everywhere. As a result, if we divide access right into three rights:
then we have this simple table:
+—-———————————————+————————————+———————————+
| | Same | Different |
| | Package | Packages |
+—————————————————+————————————+———————————+
| private | D | |
+—————————————————+————————————+———————————+
| package-private | | |
| (no modifier) | D R I | |
+—————————————————+————————————+———————————+
| protected | D R I | I |
+—————————————————+————————————+———————————+
| public | D R I | R I |
+—————————————————+————————————+———————————+
In very short
public
: accessible from everywhere. protected
: accessible by the classes of the same package and the subclasses residing in any package. private
: accessible within the same class only. The most misunderstood access modifier in Java is protected
. We know that it's similar to the default modifier with one exception in which subclasses can see it. But how? Here is an example which hopefully clarifies the confusion:
Assume that we have 2 classes; Father
and Son
, each in its own package:
package fatherpackage; public class Father { } ------------------------------------------- package sonpackage; public class Son extends Father { }
Let's add a protected method foo()
to Father
.
package fatherpackage; public class Father { protected void foo(){} }
The method foo()
can be called in 4 contexts:
Inside a class that is located in the same package where foo()
is defined ( fatherpackage
):
package fatherpackage; public class SomeClass { public void someMethod(Father f, Son s) { f.foo(); s.foo(); } }
Inside a subclass, on the current instance via this
or super
:
package sonpackage; public class Son extends Father { public void sonMethod() { this.foo(); super.foo(); } }
On an reference whose type is the same class:
package fatherpackage; public class Father { public void fatherMethod(Father f) { f.foo(); // valid even if foo() is private } } ------------------------------------------- package sonpackage; public class Son extends Father { public void sonMethod(Son s) { s.foo(); } }
On an reference whose type is the parent class and it is inside the package where foo()
is defined ( fatherpackage
) [This can be included inside context no. 1]:
package fatherpackage; public class Son extends Father { public void sonMethod(Father f) { f.foo(); } }
The following situations are not valid.
On an reference whose type is the parent class and it is outside the package where foo()
is defined ( fatherpackage
):
package sonpackage; public class Son extends Father { public void sonMethod(Father f) { f.foo(); // compilation error } }
A non-subclass inside a package of a subclass (A subclass inherits the protected members from its parent, and it makes them private to non-subclasses):
package sonpackage; public class SomeClass { public void someMethod(Son s) throws Exception { s.foo(); // compilation error } }
Methods, Variables and Constructors that are declared private can only be accessed within the declared class itself.
Private access modifier is the most restrictive access level. Class and interfaces cannot be private.
Note
Variables that are declared private can be accessed outside the class if public getter methods are present in the class. 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.
The protected access modifier cannot be applied to class and interfaces.
Methods, fields can be declared protected, however methods and fields in a interface cannot be declared protected.
Note
Protected access gives the subclass a chance to use the helper method or variable, while preventing a nonrelated class from trying to use it.
A class, method, constructor, interface etc declared public can be accessed from any other class.
Therefore fields, methods, blocks declared inside a public class can be accessed from any class belonging to the Java Universe.
However if the public class we are trying to access is in a different package, then the public class still need to be imported.
Because of class inheritance, all public methods and variables of a class are inherited by its subclasses.
Default access modifier means we do not explicitly declare an access modifier for a class, field, method, etc.
A variable or method declared without any access control modifier is available to any other class in the same package. The fields in an interface are implicitly public static final and the methods in an interface are by default public.
Note
We cannot Override the Static fields.if you try to override it does not show any error but it doesnot work what we except.
http://docs.oracle.com/javase/tutorial/java/javaOO/accesscontrol.html http://www.tutorialspoint.com/java/java_access_modifiers.htm
The difference can be found in the links already provided but which one to use usually comes down to the "Principle of Least Knowledge". Only allow the least visibility that is needed.
Private : Limited access to class only
Default (no modifier) : Limited access to class and package
Protected : Limited access to class, package and subclasses (both inside and outside package)
Public : Accessible to class, package (all), and subclasses... In short, everywhere.
Access modifiers in Java.
Java access modifiers are used to provide access control in Java.
1. Default:
Accessible to the classes in the same package only.
For example,
// Saved in file A.java
package pack;
class A{
void msg(){System.out.println("Hello");}
}
// Saved in file B.java
package mypack;
import pack.*;
class B{
public static void main(String args[]){
A obj = new A(); // Compile Time Error
obj.msg(); // Compile Time Error
}
}
This access is more restricted than public and protected, but less restricted than private.
2. Public
Can be accessed from anywhere. (Global Access)
For example,
// Saved in file A.java
package pack;
public class A{
public void msg(){System.out.println("Hello");}
}
// Saved in file B.java
package mypack;
import pack.*;
class B{
public static void main(String args[]){
A obj = new A();
obj.msg();
}
}
Output:Hello
3. Private
Accessible only inside the same class.
If you try to access private members on one class in another will throw compile error. For example,
class A{
private int data = 40;
private void msg(){System.out.println("Hello java");}
}
public class Simple{
public static void main(String args[]){
A obj = new A();
System.out.println(obj.data); // Compile Time Error
obj.msg(); // Compile Time Error
}
}
4. Protected
Accessible only to the classes in the same package and to the subclasses
For example,
// Saved in file A.java
package pack;
public class A{
protected void msg(){System.out.println("Hello");}
}
// Saved in file B.java
package mypack;
import pack.*;
class B extends A{
public static void main(String args[]){
B obj = new B();
obj.msg();
}
}
Output: Hello
Access modifiers are there to restrict access at several levels.
Public: It is basically as simple as you can access from any class whether that is in same package or not.
To access if you are in same package you can access directly, but if you are in another package then you can create an object of the class.
Default: It is accessible in the same package from any of the class of package.
To access you can create an object of the class. But you can not access this variable outside of the package.
Protected: you can access variables in same package as well as subclass in any other package. so basically it is default + Inherited behavior.
To access protected field defined in base class you can create object of child class.
Private: it can be access in same class.
In non-static methods you can access directly because of this reference (also in constructors)but to access in static methods you need to create object of the class.
Java access modifies
Access modifier can be applicable for class
, field
[About] , method
. Try to access, subclass or override this.
field
or method
is through a class
.class
(subclass) access modifier can be any .method
(override) access modifier should be the same or expand itTop level class(first level scope) can be public
and default
. Nested class
[About] can have any of them
package
is not applying for package hierarchy
Visible to the package. The default. No modifiers are needed.
Visible to the class only ( private ).
Visible to the world ( public ).
Visible to the package and all subclasses ( protected ).
Variables and methods can be declared without any modifiers that are called. Default examples:
String name = "john";
public int age(){
return age;
}
Private access modifier - private:
Methods, variables and constructors that are declared private can only be accessed within the declared class itself. The private access modifier is the most restrictive access level. Class and interfaces cannot be private.
Variables that are declared private can be accessed outside the class if public getter methods are present in the class.
Using the private modifier is the main way that an object encapsulates itself and hides data from the outside world.
Examples:
Public class Details{
private String name;
public void setName(String n){
this.name = n;
}
public String getName(){
return this.name;
}
}
Public access modifier - public:
A class, method, constructor, interface, etc. declared public can be accessed from any other class. Therefore fields, methods, blocks declared inside a public class can be accessed from any class belonging to the Java universe.
However, if the public class we are trying to access is in a different package, then the public class still need to be imported.
Because of class inheritance, all public methods and variables of a class are inherited by its subclasses.
Example:
public void cal(){
}
Protected access modifier - protected:
Variables, methods and constructors which are declared protected in a superclass can be accessed only by the subclasses in another package or any class within the package of the protected members' class.
The protected access modifier cannot be applied to class and interfaces. Methods, fields can be declared protected, however methods and fields in a interface cannot be declared protected.
Protected access gives the subclass a chance to use the helper method or variable, while preventing a nonrelated class from trying to use it.
class Van{
protected boolean speed(){
}
}
class Car{
boolean speed(){
}
}
public - accessible from anywhere in the application.
default - accessible from package.
protected - accessible from package and sub-classes in other package. as well
private - accessible from its class only.
I just want to address a detail that is extremely commonly got wrong, including by most of the answers on this page. "Default" access (when no access modifier is present) is not always the same as package-private . It depends on what the thing is.
Non-member types (that is, classes, enums, interfaces, and annotation types not declared inside another type) are package-private by default. ( JLS §6.6.1 )
Class members and constructors are package-private by default. ( JLS §6.6.1 )
Enum constructors are private by default . (Indeed, enum contructors must be private, and it is an error to try to make them public or protected). Enum constants are public, and do not allow any access specifier. Other members of enums are package-private by default. ( JLS §8.9 )
All members of interfaces and annotation types are public by default . (Indeed, members of interfaces and annotation types must be public, and it is an error to try to make them private or protected.) ( JLS §9.3 to 9.5 )
This page writes well about the protected & default access modifier
.... Protected: Protected access modifier is the a little tricky and you can say is a superset of the default access modifier. Protected members are same as the default members as far as the access in the same package is concerned. The difference is that, the protected members are also accessible to the subclasses of the class in which the member is declared which are outside the package in which the parent class is present.
But these protected members are “accessible outside the package only through inheritance“. ie you can access a protected member of a class in its subclass present in some other package directly as if the member is present in the subclass itself. But that protected member will not be accessible in the subclass outside the package by using parent class's reference. ....
David's answer provides the meaning of each access modifier. As for when to use each, I'd suggest making public all classes and the methods of each class that are meant for external use (its API), and everything else private.
Over time you'll develop a sense for when to make some classes package-private and when to declare certain methods protected for use in subclasses.
Note: This is just a supplement for the accepted answer.
This is related to Java Access Modifiers .
From Java Access Modifiers :
A Java access modifier specifies which classes can access a given class and its fields, constructors and methods. Access modifiers can be specified separately for a class, its constructors, fields and methods. Java access modifiers are also sometimes referred to in daily speech as Java access specifiers, but the correct name is Java access modifiers. Classes, fields, constructors and methods can have one of four different Java access modifiers:
- List item
- private
- default (package)
- protected
- public
From Controlling Access to Members of a Class tutorials:
Access level modifiers determine whether other classes can use a particular field or invoke a particular method. There are two levels of access control:
- At the top level—public, or package-private (no explicit modifier).
- At the member level—public, private, protected, or package-private (no explicit modifier).
A class may be declared with the modifier public, in which case that class is visible to all classes everywhere. If a class has no modifier (the default, also known as package-private), it is visible only within its own package
The following table shows the access to members permitted by each modifier.
╔═════════════╦═══════╦═════════╦══════════╦═══════╗ ║ Modifier ║ Class ║ Package ║ Subclass ║ World ║ ╠═════════════╬═══════╬═════════╬══════════╬═══════╣ ║ public ║ Y ║ Y ║ Y ║ Y ║ ║ protected ║ Y ║ Y ║ Y ║ N ║ ║ no modifier ║ Y ║ Y ║ N ║ N ║ ║ private ║ Y ║ N ║ N ║ N ║ ╚═════════════╩═══════╩═════════╩══════════╩═══════╝
The first data column indicates whether the class itself has access to the member defined by the access level. As you can see, a class always has access to its own members. The second column indicates whether classes in the same package as the class (regardless of their parentage) have access to the member. The third column indicates whether subclasses of the class declared outside this package have access to the member. The fourth column indicates whether all classes have access to the member.
Access levels affect you in two ways. First, when you use classes that come from another source, such as the classes in the Java platform, access levels determine which members of those classes your own classes can use. Second, when you write a class, you need to decide what access level every member variable and every method in your class should have.
Often times I've realized that remembering the basic concepts of any language can made possible by creating real-world analogies. Here is my analogy for understanding access modifiers in Java:
Let's assume that you're a student at a university and you have a friend who's coming to visit you over the weekend. Suppose there exists a big statue of the university's founder in the middle of the campus.
When you bring him to the campus, the first thing that you and your friend sees is this statue. This means that anyone who walks in the campus can look at the statue without the university's permission. This makes the statue as PUBLIC .
Next, you want to take your friend to your dorm, but for that you need to register him as a visitor. This means that he gets an access pass (which is the same as yours) to get into various buildings on campus. This would make his access card as PROTECTED .
Your friend wants to login to the campus WiFi but doesn't have the any credentials to do so. The only way he can get online is if you share your login with him. (Remember, every student who goes to the university also possesses these login credentials). This would make your login credentials as NO MODIFIER .
Finally, your friend wants to read your progress report for the semester which is posted on the website. However, every student has their own personal login to access this section of the campus website. This would make these credentials as PRIVATE .
Hope this helps!
Public Protected Default and private are access modifiers.
They are meant for encapsulation, or hiding and showing contents of the class.
Private is not accessible outside the class Default is accessible only in the package. Protected in package as well as any class which extends it. Public is open for all.
Normally, member variables are defined private, but member methods are public.
When you are thinking of access modifiers just think of it in this way (applies to both variables and methods ):
public
--> accessible from every where private
--> accessible only within the same class where it is declared
Now the confusion arises when it comes to default
and protected
default
--> No access modifier keyword is present. This means it is available strictly within the package of the class. Nowhere outside that package it can be accessed.
protected
--> Slightly less stricter than default
and apart from the same package classes it can be accessed by sub classes outside the package it is declared.
I like Jenkov's article
The following table summarizes what Java constructs each Java access modifier can be applied to:
If a method or variable is marked as private (has the private access modifier assigned to it), then only code inside the same class can access the variable, or call the method. Code inside subclasses cannot access the variable or method, nor can code from any external class.
Classes cannot be marked with the private access modifier. Marking a class with the private access modifier would mean that no other class could access it, which means that you could not really use the class at all. Therefore the private access modifier is not allowed for classes.
The default Java access modifier is declared by not writing any access modifier at all. The default access modifier means that code inside the class itself as well as code inside classes in the same package as this class, can access the class, field, constructor or method which the default access modifier is assigned to. Therefore, the default access modifier is also sometimes referred to as the package access modifier.
The protected access modifier provides the same access as the default access modifier, with the addition that subclasses can access protected methods and member variables (fields) of the superclass. This is true even if the subclass is not located in the same package as the superclass.
The Java access modifier public means that all code can access the class, field, constructor or method, regardless of where the accessing code is located. The accessing code can be in a different class and different package.
Following block diagram explain how data members of base class are inherited when derived class access mode is private .
Note: Declaring data members with private access specifier is known as data hiding.
My two cents :)
private:
class -> a top level class cannot be private. inner classes can be private which are accessible from same class.
instance variable -> accessible only in the class. Cannot access outside the class.
package-private:
class -> a top level class can be package-private. It can only be accessible from same package. Not from sub package, not from outside package.
instance variable -> accessible from same package. Not from sub package, not from outside package.
protected:
class -> a top level class cannot be protected.
instance variable -> Only accessible in same package or subpackage. Can only be access outside the package while extending class.
public:
class -> accessible from package/subpackage/another package
instance variable -> accessible from package/subpackage/another package
Here is detailed answer
https://github.com/junto06/java-4-beginners/blob/master/basics/access-modifier.md
It is all about encapsulation (or as Joe Phillips stated, least knowledge ).
Start with the most restrictive (private) and see if you need less restrictive modifiers later on.
We all use method and member modifiers like private, public, ... but one thing too few developers do is use packages to organize code logically.
For example: You may put sensitive security methods in a 'security' package. Then put a public class which accesses some of the security related code in this package but keep other security classes package private . Thus other developers will only be able to use the publicly available class from outside of this package (unless they change the modifier). This is not a security feature, but will guide usage.
Outside world -> Package (SecurityEntryClass ---> Package private classes)
Another thing is that classes which depend a lot on each other may end up in the same package and could eventually be refactored or merged if the dependency is too strong.
If on the contrary you set everything as public it will not be clear what should or should not be accessed, which may lead to writing a lot of javadoc (which does not enforce anything via the compiler...).
public
If a class member is declared with public then it can be accessed from anywhere
protected
If a class member is declared with keyword protected then it can be accessed from same class members, outside class members within the same package and inherited class members. If a class member is protected then it can NOT be accessed from outside package class unless the outside packaged class is inherited ie extends the other package superclass. But a protected class member is always available to same package classes it does NOT matter weather the same package class is inherited or NOT
default
In Java default is NOT an access modifier keyword. If a class member is declared without any access modifier keyword then in this case it is considered as default member. The default class member is always available to same package class members. But outside package class member can NOT access default class members even if outside classes are subclasses unlike protected members
private
If a class member is declared with keyword protected then in this case it is available ONLY to same class members
Access Specifiers in Java: There are 4 access specifiers in java, namely private, package-private (default), protected and public in increasing access order.
Private : When you are developing some class and you want member of this class not to be exposed outside this class then you should declare it as private. private members can be accessed only in class where they are defined ie enclosing class. private members can be accessed on 'this' reference and also on other instances of class enclosing these members, but only within the definition of this class.
Package-private (default) : This access specifier will provide access specified by private access specifier in addition to access described below.
When you are developing some package and hence some class (say Class1) within it, you may use default (need not be mentioned explicitly) access specifier, to expose member within class, to other classes within your (same) package. In these other classes (within same package), you can access these default members on instance of Class1. Also you can access these default members within subclasses of Class1, say Class2 (on this reference or on instance of Class1 or on instance of Class2).
Basically, within same package you can access default members on instance of class directly or on 'this' reference in subclasses.
protected : This access specifier will provide access specified by package-private access specifier in addition to access described below.
When you are developing some package and hence some class (say Class1) within it, then you should use protected access specifier for data member within Class1 if you don't want this member to be accessed outside your package (say in package of consumer of your package ie client who is using your APIs) in general, but you want to make an exception and allow access to this member only if client writes class say Class2 that extends Class1. So, in general, protected members will be accessible on 'this' reference in derived classes ie Class2 and also on explicit instances of Class2.
Please note:
So bottom line is, protected members can be accessed in other packages, only if some class from this other package, extends class enclosing this protected member and protected member is accessed on 'this' reference or explicit instances of extended class, within definition of extended class.
public : This access specifier will provide access specified by protected access specifier in addition to access described below.
When you are developing some package and hence some class (say Class1) within it, then you should use public access specifier for data member within Class1 if you want this member to be accessible in other packages on instance of Class1 created in some class of other package. Basically this access specifier should be used when you intent to expose your data member to world without any condition.
There are two types of modifiers in Java: access modifiers and non-access modifiers.
The access modifiers in Java specifies the accessibility or scope of a field, method, constructor, or class. We can change the access level of fields, constructors, methods, and class by applying the access modifier on it.
There are four types of Java access modifiers:
The private access modifier is accessible only within the class.
Example of private access modifier
In this example, we have created two classes PrivateDemo and Main. PrivateDemo class contains private data member and private method. We are accessing these private members from outside the class, so there is a compile-time error.
class PrivateDemo {
private int data=999;
private void message(){System.out.println("I am private a method");}
}
public class Main{
public static void main(String args[]){
PrivateDemo obj=new PrivateDemo();
System.out.println(obj.data); //Compile Time Error
obj.message(); //Compile Time Error
}
}
NOTE If you make any class constructor private, you cannot create the instance of that class from outside the class.
The protected access modifier is accessible within package and outside the package but through inheritance only.
The protected access modifier can be applied on the data member, method and constructor. It can't be applied on the class.
It provides more accessibility than the default modifer.
Example of protected access modifier
In this example, we have created the two packages com.protectedone.ex and com.protectedtwo.ex. The ProtectedOne class of com.protectedone.ex package is public, so can be accessed from outside the package. But message method of this package is declared as protected, so it can be accessed from outside the class only through inheritance.
//save by ProtectedOne.java
package com.protectedone.ex;
public class ProtectedOne {
protected void message(){System.out.println("I am a protected method");}
}
//save by ProtecteTwo.java
package com.protectedtwo.ex;
import com.protectedone.ex.*;
class ProtectedTwo extends ProtectedOne{
public static void main(String args[]){
ProtectedTwo obj = new ProtectedTwo();
obj.message();
}
}
Output ::
I am a protected method
The public access modifier is accessible everywhere. It has the widest scope among all other modifiers.
Example of public access modifier
//save by PublicOne.java
package com.publicone.ex;
public class PublicOne {
public void message(){System.out.println("I am a public method");}
}
//save by PublicTwo.java
package com.publictwo.ex;
import com.publicone.ex.*;
class PublicTwo extends PublicOne{
public static void main(String args[]){
PublicOne obj = new PublicOne();
obj.message();
}
}
Output ::
I am a public method
If you don't use any modifier, it is treated as default by default. The default modifier is accessible only within package. It cannot be accessed from outside the package. It provides more accessibility than private. But, it is more restrictive than protected, and public.
Example of default access modifier
In this example, we have created two packages com.defaultone.ex and com.defaulttwo.ex. We are accessing the DefaultOne class from outside its package, since DefaultOne class is not public, so it cannot be accessed from outside the package.
//save by DefaultOne.java
package com.defaultone.ex;
public class DefaultOne {
void message(){System.out.println("I am a default method");}
}
//save by DefaultTwo.java
package com.defaulttwo.ex;
import com.defaultone.ex.*;
class PublicTwo {
public static void main(String args[]){
PublicOne obj = new PublicOne(); //Compile Time Error
obj.message(); //Compile Time Error
}
}
In the above example, the scope of class DefaultOne and its method message() is default so it cannot be accessed from outside the package.
Private : We can access private members within a class. They cannot be accessed outside of a class. We can declare a variable, method and constructor as private. But we cannot make a top-level class as private.
We can access default members only within a class and within a package. We cannot access them outside of a package.
We can access protected members within a class, within a package and in the subclass outside of a package.
We can access public members in any class or in any package. There is no restriction on the accessibility of a public member.
when talking about access modifiers we can easy to understand, very simple rules include them.
Private Access modifier use in : - Only same class
Default Access modifier use in : - Only same class / Same package subclasses
Protected Access modifier use in : - same class / Same package subclasses / Same package non - subclasses / Different package subclasses
Public Access modifier use in :- we can use anywhere (same class / Same package subclasses / Same package non - subclasses / Different package subclasses/ Different package non - subclasses)
For beginners considering this example can be helpful;
Consider I have developed MyClass
in foo
package and it has a fantastic method named print
which you are interested in calling it ( it could be a method
or property
)
package foo; // I am in foo
public class MyClass {
private void print() { //This is private
System.out.println("I can print!");
}
}
You have developed YourClass
in bar
package, and you are interested to use MyClass#print
package bar; \\You are not in same package as me
import foo.MyClass;
public class YourClass {
void test() {
MyClass myClass = new MyClass();
myClass.print();
}
}
Your code is not compile and you get error The method print() is undefined for the type MyClass
You come to me:
private
. Can you make it public
?others
use itnot others
use it.private
keyword. My Access modifier will be default
or private package
. As you are my friend you must be in same package as me. So you must
come to my package foo
. I mean exact package not even a sub package. Then MyClass
will be
package foo;
public class MyClass {
void print() { //No access modifier means default or package-private
System.out.println("I can print!");
}
}
YourClass
will be:
package foo;//You come to my package
public class YourClass {
void test() {
MyClass myClass = new MyClass();
myClass.print();
}
}
No consider this: Again you come to me
extend
me and I make print()
protected
, then you can use it whether you change you package or not. Here is MyClass
package foo;
protected class MyClass { // it is now protected
protected void print() {
System.out.println("I can print!");
}
}
Here is YourClass
package bar; // You are on your own package
import foo.MyClass;
public class YourClass extends MyClass {
void test() {
//You initiate yourself! But as you extend me you can call my print()
YourClass yourClass = new YourClass();
yourClass.print();
}
}
use private when you don't want your variables/methods/classes to be visible outside that class. use protected when you want that only the children of this class can use that variables. use public when you want that the variable/class/method should be accessible from anywhere. use package-private when you want your variable/classes/methods to be used inside that package only.
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