Java Access Modifiers

Java Access Modifiers

Java provides a number of access modifiers to set access levels for classes, variables, methods and constructors. The four access levels are:

·         Visible to the package. the default. No modifiers are needed.

·         Visible to the class only (private).

·         Visible to the world (public).

·         Visible to the package and all subclasses (protected).

Default Access Modifier - No keyword:

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

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.

Example:

Variables and methods can be declared without any modifiers, as in the following examples:

String version = "1.5.1";

boolean processOrder() {

   return true;

}

Private Access Modifier - private:

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

Private access modifier is the most restrictive access level. Class and interfaces cannot be private.

Variables that are declared private can be accessed outside the class if public getter methods are present in the class.

Using the private modifier is the main way that an object encapsulates itself and hide data from the outside world.

Example:

The following class uses private access control:

public class Logger {

   private String format;

   public String getFormat() {

      return this.format;

   }

   public void setFormat(String format) {

      this.format = format;

   }

}

Here, the format variable of the Logger class is private, so there's no way for other classes to retrieve or set its value directly.

So to make this variable available to the outside world, we defined two public methods: getFormat(), which returns the value of format, and setFormat(String), which sets its value.

Public Access Modifier - 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.

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.

Example:

The following function uses public access control:

public static void main(String[] arguments) {

   // ...

}

The main() method of an application has to be public. Otherwise, it could not be called by a Java interpreter (such as java) to run the class.

Protected Access Modifier - protected:

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.

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.

Protected access gives the subclass a chance to use the helper method or variable, while preventing a nonrelated class from trying to use it.

Example:

The following parent class uses protected access control, to allow its child class overrideopenSpeaker() method:

class AudioPlayer {

   protected boolean openSpeaker(Speaker sp) {

      // implementation details

   }

}

class StreamingAudioPlayer {

   boolean openSpeaker(Speaker sp) {

      // implementation details

   }

}

Here if we define openSpeaker() method as private then it would not be accessible from any other class other than AudioPlayer. If we define it as public then it would become accessible to all the outside world. But our intension is to expose this method to its subclass only, thats why we usedprotected modifier.

Access Control and Inheritance:

The following rules for inherited methods are enforced:

·         Methods declared public in a superclass also must be public in all subclasses.

·         Methods declared protected in a superclass must either be protected or public in subclasses; they cannot be private.

·         Methods declared without access control (no modifier was used) can be declared more private in subclasses.

·         Methods declared private are not inherited at all, so there is no rule for them.

Java provides a number of non-access modifiers to achieve many other functionality.

Java Non Access Modifiers

·         The static modifier for creating class methods and variables

·         The final modifier for finalizing the implementations of classes, methods, and variables.

·         The abstract modifier for creating abstract classes and methods.

·         The synchronized and volatile modifiers, which are used for threads.

The static Modifier:

Static Variables:

The static key word is used to create variables that will exist independently of any instances created for the class. Only one copy of the static variable exists regardless of the number of instances of the class.

Static variables are also known as class variables. Local variables cannot be declared static.

Static Methods:

The static key word is used to create methods that will exist independently of any instances created for the class.

Static methods do not use any instance variables of any object of the class they are defined in. Static methods take all the data from parameters and compute something from those parameters, with no reference to variables.

Class variables and methods can be accessed using the class name followed by a dot and the name of the variable or method.

Example:

The final Modifier:

final Variables:

A final variable can be explicitly initialized only once. A reference variable declared final can never be reassigned to refer to an different object.

However the data within the object can be changed. So the state of the object can be changed but not the reference.

With variables, the final modifier often is used with static to make the constant a class variable.

final Methods:

A final method cannot be overridden by any subclasses. As mentioned previously the final modifier prevents a method from being modified in a subclass.

The main intention of making a method final would be that the content of the method should not be changed by any outsider.

final Classes:

The main purpose of using a class being declared as final is to prevent the class from being subclassed. If a class is marked as final then no class can inherit any feature from the final class.

The abstract Modifier:

abstract Class:

An abstract class can never be instantiated. If a class is declared as abstract then the sole purpose is for the class to be extended.

A class cannot be both abstract and final. (since a final class cannot be extended). If a class contains abstract methods then the class should be declared abstract. Otherwise a compile error will be thrown.

An abstract class may contain both abstract methods as well normal methods.

Example:

abstract class Caravan{

   private double price;

   private String model;

   private String year;

   public abstract void goFast(); //an abstract method

   public abstract void changeColor();

}

abstract Methods:

An abstract method is a method declared with out any implementation. The methods body(implementation) is provided by the subclass. Abstract methods can never be final or strict.

Any class that extends an abstract class must implement all the abstract methods of the super class unless the subclass is also an abstract class.

If a class contains one or more abstract methods then the class must be declared abstract. An abstract class does not need to contain abstract methods.

The abstract method ends with a semicolon. Example: public abstract sample();

Example:

public abstract class SuperClass{

    abstract void m(); //abstract method

}

class SubClass extends SuperClass{

     // implements the abstract method

      void m(){

           .........

      }

}

The synchronized Modifier:

The synchronized key word used to indicate that a method can be accessed by only one thread at a time. The synchronized modifier can be applied with any of the four access level modifiers.

Example:

public synchronized void showDetails(){

.......

} 

The transient Modifier:

An instance variable is marked transient to indicate the JVM to skip the particular variable when serializing the object containing it.

This modifier is included in the statement that creates the variable, preceding the class or data type of the variable.

Example:

public transient int limit = 55;   // will not persist

public int b; // will persist

The volatile Modifier:

The volatile is used to let the JVM know that a thread accessing the variable must always merge its own private copy of the variable with the master copy in the memory.

Accessing a volatile variable synchronizes all the cached copied of the variables in the main memory. Volatile can only be applied to instance variables which are of type object or private. A volatile object reference can be null.

Example:

public class MyRunnable implements Runnable

{

    private volatile boolean active;

    public void run()

    {

        active = true;

        while (active) // line 1

        {

            // some code here

        }

    }

    public void stop()

    {

        active = false; // line 2

    }

}

Usually, run() is called in one thread (the one you start using the Runnable), and stop() is called from another thread. If in line 1 the cached value of active is used, the loop may not stop when you set active to false in line 2. That's when you want to use volatile.