What is the difference between public, protected,

Question

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?

Answer 1

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)

Answer 2

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.

Answer 3

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...).

Answer 4

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.

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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.

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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.

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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

• Overriding static methods in java

References links

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

Answer 5

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.

Answer 6

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.