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Java's Access Specifiers

By g+Xah Lee, 2005-02-28, …, 2011-12-26

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In Java code, class and variable and method and constructor declarations

can have “access specifiers”, that is one of: 「private」,

「protected」, 「public」. (or none.)

The purpose of access specifiers is to declare which entity can not be

accessed from where. Its effect is different when used on any of: {class,

class variable, class method, class's constructor}.

Access Specifiers for Class Variables and

Class Methods

Below is a table showing the effects of access specifiers for class

members (i.e. class variable and class methods).

◆ = Can Access. ◇ = No Access.

Specifier class subclass package world

private ◆ ◇ ◇ ◇

protected ◆ ◆ ◆ ◇

public ◆ ◆ ◆ ◆

(none) ◆ ◇ ◆ ◇

For example, if a variable is declared “protected”, then the class itself can

access it, its subclass can access it, and any class in the same package can

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also access it, but otherwise a class cannot access it.

If a class memeber doesn't have any access specifier (the “(none)” row in

above), its access level is sometimes known as “package”.

Here's a example.

class P { int x = 7;}public class A { public static void main(String[] args) { P p = new P(); System.out.println(p.x); }}

The code compiles and runs. But, if you add “private” in front of 「int x」,

then you'll get a compiler error: “x has private access in P”. This is because

when a member variable is private, it can only be accessed within that class.

Access Specifiers for Constructors

Constructors can have the same access specifiers used for variables and

methods. Their meaning is the same. For example, when a constructor has

“private” declared, then, only the class itself can create a instance of it (kind

of like self-reference). Other class in the same package can not create a

instance of that class. Nor any subclass of that class. Nor any other class

outside of this package.

(Note: constructors in Java are treated differently than methods. Class

members are made of 2 things: ① class's variables. ② class's methods.

Constructors are NOT considerd a class member.)

Here is a sample code.

class Q { public int x; private Q (int n) { x=n; System.out.println("i'm born!"); }}

public class A1 { public static void main(String[] args) { Q q = new Q(3); System.out.println(q.x);

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}}

In the above code, it won't compile because Q's contructor is “private” but it

is being created outside of itself. If you delete the “private” keyword in front of

Q's constructor, then it compiles.

Constructors in Same Class Can Have Different AccessSpecifiers

Remember that a class can have more than one constructors, each with

different parameters. Each constructor can have different access specifier.

In the following example, the class Q has two constructors, one takes a int

argument, the other takes a double argument. One is declared private, while

the other with no access specifier (default package level access).

class Q { Q (int n) { System.out.println("i'm born int!"); } private Q (double d) { System.out.println("i'm born double!"); }}

public class A2 { public static void main(String[] args) { Q q1 = new Q(3); //Q q2 = new Q(3.3); }}

The fact that there can be constructors with different access specifiers

means that in Java, the ability to create a object also depends on which

constructor is called to create the object.

Access Specifiers for Classes

For classes, only the “public” access specifier can be used on classes.

Basically, Java has this “One Class Per File” paradigm. That is, in every java

source code file, only one class in the file is public accessible, and that class

must have the same name as the file. (For Example, if the file is

〔xyz.java〕, then there must be a class named “xyz” in it, and that is the

class that's public.) Optionally, the class can be declared with “public”

keyword.

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(Note: By convention, classes names should start with capital letter. So, a

class named “xyz” really should be “Xyz”, with the file named “Xyz.java”.)

If you use any other access specifier on classes, or declare more than one

class “public” in a file, the compiler will complain. For detail, see Packages

in Java.

The Complexity of Access Specifiers in OOP

The rise of the elaborate access specifiers and their different consequences

in Java entities is a complexity out of OOP and its machinery.

Note that the concept of access specifiers such as “private” is not at all

related to issues of secrecy or the all-important aspect of cryptography in

computing. Compiled Java code can relatively easily be decompiled by

compiler specialists.