Member access modifiers java

Member access modifiers java

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Member access modifiers java

1) Private

The private access modifier is accessible only within the class.

Simple example of private access modifier

In this example, we have created two classes A and Simple. A 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.

Role of Private Constructor

If you make any class constructor private, you cannot create the instance of that class from outside the class. For example:

Note: A class cannot be private or protected except nested class.

2) Default

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 pack and mypack. We are accessing the A class from outside its package, since A class is not public, so it cannot be accessed from outside the package.

In the above example, the scope of class A and its method msg() is default so it cannot be accessed from outside the package.

3) Protected

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 pack and mypack. The A class of pack package is public, so can be accessed from outside the package. But msg method of this package is declared as protected, so it can be accessed from outside the class only through inheritance.

4) Public

The public access modifier is accessible everywhere. It has the widest scope among all other modifiers.

Example of public access modifier

Java Access Modifiers with Method Overriding

If you are overriding any method, overridden method (i.e. declared in subclass) must not be more restrictive.

The default modifier is more restrictive than protected. That is why, there is a compile-time error.

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Controlling Access to Members of a Class

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 (packages are named groups of related classes — you will learn about them in a later lesson.)

At the member level, you can also use the public modifier or no modifier (package-private) just as with top-level classes, and with the same meaning. For members, there are two additional access modifiers: private and protected . The private modifier specifies that the member can only be accessed in its own class. The protected modifier specifies that the member can only be accessed within its own package (as with package-private) and, in addition, by a subclass of its class in another package.

The following table shows the access to members permitted by each modifier.

Access Levels

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.

Let’s look at a collection of classes and see how access levels affect visibility. The following figure shows the four classes in this example and how they are related.

Classes and Packages of the Example Used to Illustrate Access Levels

The following table shows where the members of the Alpha class are visible for each of the access modifiers that can be applied to them.

Visibility

Modifier	Alpha	Beta	Alphasub	Gamma
public	Y	Y	Y	Y
protected	Y	Y	Y	N
no modifier	Y	Y	N	N
private	Y	N	N	N

If other programmers use your class, you want to ensure that errors from misuse cannot happen. Access levels can help you do this.

• Use the most restrictive access level that makes sense for a particular member. Use private unless you have a good reason not to.
• Avoid public fields except for constants. (Many of the examples in the tutorial use public fields. This may help to illustrate some points concisely, but is not recommended for production code.) Public fields tend to link you to a particular implementation and limit your flexibility in changing your code.

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Class Member Access Modifiers

The components of a class, such as its instance variables or methods are called the members of a class or class members. A class member is declared with an access modifier to specify how it is accessed by the other classes in Java. A Java class member can take any of the access modifiers, such as — public, protected, default and private.

Public access modifier

A class member, be it an instance variable or a method, can be declared with the public keyword. A class member declared with a public access modifier is accessible and visible to all the classes, as long as its class is visible to other classes. Let us see an example —

Output is :

Program Analysis

In this code, we have created two classes, A(in A.java) and B(in B.java). Class A has a method, m() which is declared public and that’s why it is visible and accessible in class B, by using an object of class A. Hence, there is no compile error when B.java is compiled.

Private access modifier

Output is —

A7.java:6: error: i has private access in A System.out.println(ob.i); ^ 1 error

Program Analysis

The compiler shows a compile error which describes that an int member variable of A class, which has private access, cannot be used outside its class.

Default access modifier

Let’s understand this by an example —

Accessing a class member with default access

package pack1; public class A < int num=10; //a is declared with no access modifier, hence, it has default access modifier > 

Output is -:

Program Analysis

Class A and class B exist in the same package, pack1, hence, when the member variable, num(which has default access) of class A was accessed in the code of B class, we have received no compile error in the compilation of B.java.

Protected access modifier

• All the classes in the same package.
• All the subclasses in the different packages.
  Let’s understand this by an example —

package pack1; public class A < protected void m() < System.out.println("A's method is called"); >> 

package pack2; import pack1.A; class B extends A < public static void main(String. ar) < B ob = new B(); ob.m(); >>

After compiling :

Program Analysis

Class A is declared in the package, pack1, while class B is declared in the package, pack2. Class B has imported and extended the class A, and has further called the protected method, m() of class A, on the object of class B, which is completely within the rules, hence no compile error is thrown.

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