with - public private protected java




What is the difference between public, protected, package-private and private in Java? (16)

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?


Private

  • Methods,Variables and Constructors

Methods, Variables and Constructors that are declared private can only be accessed within the declared class itself.

  • Class and Interface

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.


Protected

  • Class and Interface

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.


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.

  • Different Packages

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 -No keyword:

Default access modifier means we do not explicitly declare an access modifier for a class, field, method, etc.

  • Within the same Packages

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.

Related Answers

References links

http://docs.oracle.com/javase/tutorial/java/javaOO/accesscontrol.html http://www.tutorialspoint.com/java/java_access_modifiers.htm


(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 :) )

Private

Like you'd think, only the class in which it is declared can see it.

Package Private

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).

Protected

Package Private + can be seen by subclasses or package member.

Public

Everyone can see it.

Published

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, 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.


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“. i.e 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. ....


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.


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:

  • (D)irect (invoke from a method inside the same class).
  • (R)eference (invoke a method using a reference to the class, or via "dot" syntax).
  • (I)nheritance (via subclassing).

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   |
+—————————————————+————————————+———————————+

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.


Here's a better version of the table. (Future proof with a column for modules.)

Explanations

  • A private member is only accessible within the same class as it is declared.

  • A member with no access modifier is only accessible within classes in the same package.

  • A protected member is accessible within all classes in the same package and within subclasses in other packages.

  • A public member is accessible to all classes (unless it resides in a module that does not export the package it is declared in).


Which modifier to choose?

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:

  • A field long internalCounter should probably be private since it's mutable and an implementation detail.
  • A class that should only be instantiated in a factory class (in the same package) should have a package restricted constructor, since it shouldn't be possible to call it directly from outside the package.
  • An internal void beforeRender() method called right before rendering and used as a hook in subclasses should be protected.
  • A void saveGame(File dst) method which is called from the GUI code should be public.

(*) What is Encapsulation exactly?


In very short

  • public: accessible from everywhere.
  • protected: accessible by the classes of the same package and the subclasses residing in any package.
  • default (no modifier specified): accessible by the classes of the same package.
  • private: accessible within the same class only.

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...).


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!


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.


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:

    1. 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();
          }
      }
      
    2. 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();
          }
      }
      
    3. 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();
          }
      }
      
    4. 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.

    1. 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
          }
      }
      
    2. 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
          }
      }
      

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.


                | 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         |       ✔       |     ✘     |       ✘       |   ✘    




access-modifiers