Java language has a wide variety of modifier, including the following:
Java Access Modifiers
Non Access Modifiers
To use a modifier, you include its keyword in the definition of a class,method or variable.The modifier preceds the rest of the statement, as in the following examples(Italic ones)
public class className{
//...
}
private boolean myFlag;
static final double weeks=9.5;
protected static final int BOXWIDTH=42;
public static void main(Strting[] arguments)
{
//body of the main method
}
Access Control 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 package, the default.No modifiers are needed.
Visible to class only(Private)
Visible to word(Public)
Visible to the package and all subclasses(Protected)
Non Access Modifiers:
Java provides a number of non access modifiers toa chieve many other functionality.
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.
Access Control Modifiers:
Default access modifier – No keyword:
Default access modifier means we don’t 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:
variable and methods can be declared without any modifiers,as in the following examples:
String version="1.5.2"
boolean processOrder()
{
}
Private Access Modifier – private:
Methods, Variables and Constructors that are private can only be accessed within the declared class itself.
Private access modifier is the most restricitve 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 of the world.The following class uses private access control:
public class Logger{
private String format;
public String getFormat()
{
return this.format;
}
public void setFormat(String formatter)
{
this.format=formatter;
}
}
here format variables 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 at the outside world, we defined two public methods: getFormat(), which returns the value fo format,a nd setFormat(String), which set it’s value.
Public:
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.
Example:
The following function uses public access control:
public static void main(String[] arguments)
{
//....
}
Prtotected 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 protected member 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, wehile preventing a nonrelated class from trying to use it.
example:
public class AudioPlayer {
protected boolean openSpeaker(Speaker sp)
{
//implementation details
}
}
class streamingAudioPlayer{
boolean openSpeaker(Speaker sp)
{
//implementation details
}
}
The above parent class use protected access control, to allow its child class override openSpeaker() method.
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 intensio is to expose this method to its subclass only, thats why we used protected modifier đ .
Access control and Inheritance:
The following rules for inherited method are enforced.
-Method declared public in a superclass also must be public in all subclasses.
– Method declares protected in a superclass must either be protected or public in subclasses; they cannot be priavte.
– method declared without access control(no modifier was used) can be declared more private in subclasses.
– method declared private are not inherited at all, so there is no rule for them
Java Non Access Modifiers:
Java provides a number of non access modifiers to achieve many other functionality.
– The static modifier for creating class methods and variables
– The final modifier for finalizing the implementation of classes,methods and variables.
– The abstract modifier for creating abstarct classes and methods.
– The synchronized and volatile modifiers, which are used for threads.
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 during the class name followed by a dot and the name of the variable or method.
example:
The static modifier is used to create class methods and variables as in the following example:
public class InstanceCounter {
private static int numInstances=0;
protected static int getCount()
{
return numInstances;
}
private static void addInstance(){
numInstances++;
}
InstanceCounter(){
InstanceCounter.addInstance();
}
public static void main(String[] args)
{
System.out.println("Starting with "+InstanceCounter.getCount()+" instances");
for(int i=0;i<500;i++)
{
new InstanceCounter();
}
System.out.println("Created "+InstanceCounter.getCount()+" instances");
}
}
result would be :
Starting with 0 instances Created 500 instances
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.
final variables with the final modifier often is used with static to make the constant a class variable.
public class TestFinal {
final int value=10;
//the following are the exaples of declaring contsants:
public static final int BOXERDOIRGHO=6;
static final String TITLE="Manageri";
public void changeValue(){
value=10;//wil give an error
}
}
Final Methods:
A final method cannot be overriden 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.
example:
You declasre methods using final modifier in the class declaration, as in the following example.
public class Test{
public final void changeName()
{
//body of the method
}
}
final Classes:
The main purposes of using a clas sbeing 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.
example:
public final class Test
{
//body of class
}
The abstract Modifier:
abstract class:
An abstract class can nver 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 conatin both abstract methods as well as 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 is provided by the subclass.Abstract methodsa can never be final or strict.
Any class that extendsan abstract class must implement all the abstarct methods of the super class unless the subclass is also an abstract class.
If a class contains one or moree 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();
public abstract class SuperClass{
abstract void mango();//abstract method
}
class SubClass extends SuperClass{
//implements the abstract method
void mango();
}
The Synchronized Modifier:
synchronized modifier used to indicate that a method can be accessed by only one thread at a time.This modifier can be applied with any of the four access level modifiers.
Example:
public synchronized void showDetails()
{
//body
}
The transient Modifier:
An instance variable is marked transient to indicate ther JVM to skip the particular variable when serializing the object containing it.
This modifier is included in the statement that creates the variable,preceing 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:
You can see this link for detailed and perfect explanation
reference:Â http://www.tutorialspoint.com/java/java_nonaccess_modifiers.htm
public cass 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
}
}
