I just started reading a Java book and wondered; which access specifier is the default one, if none is specified?
The default visibility is known as “package-private” (though you can't use this explicitly), which means the field will be accessible from inside the same package to which the class belongs.
As mdma pointed out, it isn't true for interface members though, for which the default is "public".
See Java's Access Specifiers
The default specifier depends upon context.
For classes, and interface declarations, the default is package private. This falls between protected and private, allowing only classes in the same package access. (protected is like this, but also allowing access to subclasses outside of the package.)
class MyClass // package private
{
int field; // package private field
void calc() { // package private method
}
}
For interface members (fields and methods), the default access is public. But note that the interface declaration itself defaults to package private.
interface MyInterface // package private
{
int field1; // static final public
void method1(); // public abstract
}
If we then have the declaration
public interface MyInterface2 extends MyInterface
{
}
Classes using MyInterface2 can then see field1 and method1 from the super interface, because they are public, even though they cannot see the declaration of MyInterface itself.
If no access specifier is given, it's package-level access (there is no explicit specifier for this) for classes and class members. Interface methods are implicitly public.
The default visibility (no keyword) is package which means that it will be available to every class that is located in the same package.
Interesting side note is that protected doesn't limit visibility to the subclasses but also to the other classes in the same package
It depends on what the thing is.
Top-level types (that is, classes, enums, interfaces, and annotation types not declared inside another type) are package-private by default. (JLS §6.6.1)
In classes, all members (that means fields, methods, and nested type declarations) and constructors are package-private by default. (JLS §6.6.1)
When a class has no explicitly declared constructor, the compiler inserts a default zero-argument constructor which has the same access specifier as the class. (JLS §8.8.9) The default constructor is commonly misstated as always being public, but in rare cases that's not equivalent.
In enums, constructors are private by default. Indeed, enum contructors must be private, and it is an error to specify them as public or protected. Enum constants are always public, and do not permit any access specifier. Other members of enums are package-private by default. (JLS §8.9)
In interfaces and annotation types, all members (again, that means fields, methods, and nested type declarations) are public by default. Indeed, members of interfaces and annotation types must be public, and it is an error to specify them as private or protected. (JLS §9.3 to 9.5)
Local classes are named classes declared inside a method, constructor, or initializer block. They are scoped to the {..} block in which they are declared and do not permit any access specifier. (JLS §14.3) Using reflection, you can instantiate local classes from elsewhere, and they are package-private, although I'm not sure if that detail is in the JLS.
Anonymous classes are custom classes created with new which specify a class body directly in the expression. (JLS §15.9.5) Their syntax does not permit any access specifier. Using reflection, you can instantiate anonymous classes from elsewhere, and both they and their generated constructors are are package-private, although I'm not sure if that detail is in the JLS.
Instance and static initializer blocks do not have access specifiers at the language level (JLS §8.6 & 8.7), but static initializer blocks are implemented as a method named <clinit> (JVMS §2.9), so the method must, internally, have some access specifier. I examined classes compiled by javac and by Eclipse's compiler using a hex editor and found that both generate the method as package-private. However, you can't call <clinit>() within the language because the < and > characters are invalid in a method name, and the reflection methods are hardwired to deny its existence, so effectively its access specifier is no access. The method can only be called by the VM, during class initialization. Instance initializer blocks are not compiled as separate methods; their code is copied into each constructor, so they can't be accessed individually, even by reflection.
default is a keyword that is used as an access modifier for methods and variables.
Using this access modifier will make your class, variable, method or constructor acessible from own class or package, it will be also is set if no access modifier is present.
Access Levels
Modifier Class Package Subclass EveryWhere
public Y Y Y Y
protected Y Y Y N
default Y Y N N
private Y N N N
if you use a default in a interface you will be able to implement a method there like this exemple
public interface Computer {
default void Start() {
throw new UnsupportedOperationException("Error");
}
}
However it will only works from the 8 Java version
Official Documentation
Access Modifiers in Java
See here for more details. The default is none of private/public/protected, but a completely different access specification. It's not widely used, and I prefer to be much more specific in my access definitions.
the default access specifier is package.Classes can access the members of other classes in the same package.but outside the package it appears as private
Here is a quote about package level visibility from an interview with James Gosling, the creator of Java:
Bill Venners: Java has four access levels. The default is package. I
have always wondered if making package access default was convenient
because the three keywords that people from C++ already knew about
were private, protected, and public. Or if you had some particular
reason that you felt package access should be the default.
James Gosling: A package is generally a set of things that are kind of
written together. So generically I could have done one of two things.
One was force you always to put in a keyword that gives you the
domain. Or I could have had a default value. And then the question is,
what makes a sensible default? And I tend to go for what is the least
dangerous thing.
So public would have been a really bad thing to make the default.
Private would probably have been a bad thing to make a default, if
only because people actually don't write private methods that often.
And same thing with protected. And in looking at a bunch of code that
I had, I decided that the most common thing that was reasonably safe
was in the package. And C++ didn't have a keyword for that, because
they didn't have a notion of packages.
But I liked it rather than the friends notion, because with friends
you kind of have to enumerate who all of your friends are, and so if
you add a new class to a package, then you generally end up having to
go to all of the classes in that package and update their friends,
which I had always found to be a complete pain in the butt.
But the friends list itself causes sort of a versioning problem. And
so there was this notion of a friendly class. And the nice thing that
I was making that the default -- I'll solve the problem so what should
the keyword be?
For a while there actually was a friendly keyword. But because all the
others start with "P," it was "phriendly" with a "PH." But that was
only in there for maybe a day.
http://www.artima.com/intv/gosling2P.html
Update Java 8 usage of default keyword:
As many others have noted The default visibility (no keyword)
the field will be accessible from inside the same package to which the
class belongs.
Not to be confused with the new Java 8 feature (Default Methods) that allows an interface to provide an implementation when its labeled with the default keyword.
See: Access modifiers
There is an access modifier called "default" in JAVA, which allows direct instance creation of that entity only within that package.
Here is a useful link:
Java Access Modifiers/Specifiers
First of all let me say one thing there is no such term as "Access specifier" in java. We should call everything as "Modifiers". As we know that final, static, synchronised, volatile.... are called as modifiers, even Public, private, protected, default, abstract should also be called as modifiers . Default is such a modifiers where physical existence is not there but no modifiers is placed then it should be treated as default modifiers.
To justify this take one example:
public class Simple{
public static void main(String args[]){
System.out.println("Hello Java");
}
}
Output will be: Hello Java
Now change public to private and see what compiler error you get:
It says "Modifier private is not allowed here"
What conclusion is someone can be wrong or some tutorial can be wrong but compiler cannot be wrong.
So we can say there is no term access specifier in java everything is modifiers.
Related
This question already has answers here:
What is the difference between public, protected, package-private and private in Java?
(30 answers)
Closed 7 years ago.
I just though about it:
if you have class like this:
public class a
{
int x = 5;
//setter & getter
}
you can't access to x from another class unless it is public.
so, that i know, but if it is like that, why do i need to use private?
the x variable cannot be accessed outside of the class without public or in other cases static reserved words.
so, the question(s):
what do i need the private for?
do i have to use the private or it doesn't really matter?
No access specifier is not the same as private.
When you have no access specifier, as in your example, the variable x is accessible in class a, but also in all other classes that are in the same package as class a.
When it is private, it is only accessible in class a, and not in any other class.
See Controlling Access to Members of a Class in Oracle's Java Tutorials.
By leaving out the access modifier, you are relying on the language's default.
For example, in C# class variables default to private.
The most used access modifiers are undoubtedly public and private.
public means you expose your field or method to every other class. This is useful for methods that are necessary to let the class perform its major function(s) and, in some cases, for constants that need to be visible.
private means you hide the member from every other class. This is useful for internal state and functions which other classes must not know about. This enforces the concept of information hiding.
The other modifiers protected and default (when no modifier is mentioned) are intermediate levels of access which can be useful in specific cases.
For example when sub classes need access to a method in the parent (or base) class and you don't want other classes to be able to access it, you can make it protected.
The default access modifier is more restricting than protected and can be used to allow access only from within the same package. This can be useful if a package contains a group of tightly related classes which need to interact among each other via methods which you don't want to expose elsewhere, not even to sub classes outside this package.
Maybe you will also benefit from this article.
this question is already answered in stackoverflow in this link.
There is 4 types of access restrictions in java:
public: the attribute is accessible for all clients
private: the attribute is only accessible from the class, not subclasses.
protected: the attribute is accessible from package and subclasses.
no modifier: the attribute is accessible from the class and package.
It's necessary to declare the scope of accessibility to encapsulate the functionality correctly.
I'm looking for documents about access modifiers in Kotlin.
But its unfound.
I want to know and check access modifier for package function and difference protected modifier in Kotlin and in Java.
[edited]
I'm looking for public document by Jetbrians.
I found thread about access modifier.
http://devnet.jetbrains.com/thread/451236?tstart=0
http://kotlinlang.org/docs/reference/visibility-modifiers.html
Classes, objects, traits, constructors, functions, properties and their setters can have visibility modifiers. (Getters always have the same visibility as the property.) There are four visibility modifiers in Kotlin:
private — visible only in the declaring scope and its subscopes (inside the same module);
protected — (applicable only to class/trait
members) like private, but also visible in subclasses;
internal —
visible everywhere within the same module (if the
owner of declaring scope is visible);
public — (used by default) visible everywhere (if
the owner of declaring scope is visible).
NOTE: Functions with expression bodies and all properties declared
public must always specify return types explicitly. This is required
so that one has no chance of accidentally changing a type that is a
part of a public API by merely altering the implementation.
Are classes private or public by default in Java and C++?
Java:
By default, the classes visibility is package private, i.e. only visible for classes in the same package.
C++:
The class has no visibility defined like in Java. They are visible if you included them to the compilation unit.
In Java, a top-level class is either public or non-public. There is no "private". You can only use the public keyword or leave it off. If you leave it off it is non-public, i.e., visible only to other classes in the same package.
A nested class, that is, a class inside of another class, can be made public, package-private, protected, or private, just like any other class member. The default (i.e., the one with no modifier) is package-private, visible only to classes in the same package.
EDIT: Forgot the C++ answer, so see (and upvote) #zeller's answer. :)
According to §6.6.1 of the JLS,
If a top level class or interface type is not declared public, then it may be accessed only from within the package in which it is declared.
So, a Java class is by default package-private.
This doesn't apply to C++, however. A class lacks visibility -- only its members can have access control. See §11 of the C++11 standard for information on member access control. Here's an excerpt from ¶1...
A member of a class can be
private; that is, its name can be used only by members and friends of the class in which it is declared.
protected; that is, its name can be used only by members and friends of the class in which it is
declared, by classes derived from that class, and by their friends (see 11.4).
public; that is, its name can be used anywhere without access restriction.
Java
By default Java class is package-private.
C++
In C++ there is no visibility defined like Java.
In java, do we really have to do that in order our variable or method to be public? For example:
void aaa() {
...
}
or
public void aaa() {
...
}
If it is a must, why?
Well that's not a variable, that's a method - but yes, you have to do that to make a method public. If you don't, it has the default package access, and won't be accessible from other packages. You should judge for yourself whether any particular method you write should be accessible only within the same class (make it private) to subclasses (make it protected), to the package (leave the default; you can't state package access explicitly, unfortunately) or to everything (make it public).
(This is a slight simplification of the access modifiers, but it's a start.)
As for why this is the case - typically you should limit visibility so that your type only exposes the methods which really make sense for the concept it's trying to encapsulate. You may have many more private methods which are implementation-specific, and which the outside world shouldn't know or care about.
You can ommmit access level modifier anywhere except near main method. There it must be set to public.
Not putting a modifier is actually different than public, private, or protected.
With no modifier, only the class itself and any class in the same package can access the attribute.
Learn more here: In Java, difference between default, public, protected, and private
The two methods are different: the first has package visibility, while the second one is public.
The difference is that package-visible methods appear public only to methods inside the same package, while public methods are visible to all methods, inside and outside the package.
So the answer to your question depends on your intent: if your method is truly part of your component's interface, make it private; if it is designed for use only inside the package, keep it at the default package visibility.
In your first example, the method will be package scoped -- i.e. only things in the same package can use it. So yes, you do need to declare public fields/methods/classes as such, otherwise they will have package scope.
The default is package private.
If you want public or anything else, you must specify it.
Method can be accessed by the class itself, other classes within the same package, but not outside of the package, and not by sub-classes.
See chart at bottom:
http://docs.oracle.com/javase/tutorial/java/javaOO/accesscontrol.html
I think it is assuming public in Java, so it is not really required.If you start having public, protected, and private throw in the mix in a single class, it is easier to see what scope the method may have.
How do access modifiers internally work in JVM?
The Java Virtual Machine Specification covers this in detail. In particular, section 2.7.4 Qualified Names and Access Control states:
A field, method, or constructor of a class may be declared using at most one of the public, private, or protected keywords. A public member may be accessed by any class or interface. A private member may be accessed only from within the class that contains its declaration. A member that is not declared public, protected, or private is said to have default access and may be accessed from, and only from, anywhere in the package in which it is declared.