What is the use of a private static variable in Java

Question

If a variable is declared as public static varName   then I can access it from anywhere as ClassName varName  I am also aware that static members are shared by all instances of a class and are not reallocated in each instance   Is declaring a variable as private static varName  any different from declaring a variable private varName    In both cases it cannot be accessed as ClassName varName or as ClassInstance varName from any other class   Does declaring the variable as static give it other special properties

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In the following example  eye is changed by PersonB  while leg stays the same  This is because a private variable makes a copy of itself to the method  so that its original value stays the same  while a private static value only has one copy for all the methods to share  so editing its value will change its original value   public class test   private static int eye 2  private int leg 3   public test  int eyes  int legs       eye   eyes      leg leg     public test        public void print        System out println eye       System out println leg      public static void main String   args       test PersonA   new test              test PersonB   new test 14 8       PersonA print                      14   3

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Well  private static variables can be used to share data across instances of that class  While you are correct that we cannot access the private static variables using constructs like ClassName member or ClassInstance member but the member will always be visible from methods of that class or instances of that class  So in effect instances of that class will always be able to refer to member

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Static variables have a single value for all instances of a class   If you were to make something like   public class Person       private static int numberOfEyes      private String name      and then you wanted to change your name  that is fine  my name stays the same   If  however you wanted to change it so that you had 17 eyes then everyone in the world would also have 17 eyes

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private static variable will be shared in subclass as well  If you changed in one subclass and the other subclass will get the changed value  in which case  it may not what you expect   public class PrivateStatic    private static int var   10  public void setVar int newVal        var   newVal     public int getVar         return var     public static void main String    args        PrivateStatic p1   new Sub1        System out println PrivateStatic var       p1 setVar 200        PrivateStatic p2   new Sub2        System out println p2 getVar           class Sub1 extends PrivateStatic       class Sub2 extends PrivateStatic

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Of course it can be accessed as ClassName var name  but only from inside the class in which it is defined - that s because it is defined as private   public static or private static variables are often used for constants  For example  many people don t like to  hard-code  constants in their code  they like to make a public static or private static variable with a meaningful name and use that in their code  which should make the code more readable   You should also make such constants final    For example   public class Example       private final static String JDBC URL    jdbc mysql   localhost shopdb       private final static String JDBC USERNAME    username       private final static String JDBC PASSWORD    password        public static void main String   args            Connection conn   DriverManager getConnection JDBC URL                                           JDBC USERNAME  JDBC PASSWORD                             Whether you make it public or private depends on whether you want the variables to be visible outside the class or not

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I m new to Java  but one way I use static variables  as I m assuming many people do  is to count the number of instances of the class  e g    public Class Company       private static int numCompanies       public static int getNumCompanies            return numCompanies            Then you can sysout   Company getNumCompanies      You can also get access to numCompanies from each instance of the class  which I don t completely understand   but it won t be in a  static way   I have no idea if this is best practice or not  but it makes sense to me

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ThreadLocal variables are typically implemented as private static  In this way  they are not bound to the class and each thread has its own reference to its own  ThreadLocal  object

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What is the use of a private static class variable   Let s say you have a library book Class  Each time you create a new Book  you want to assign it a unique id  One way is to simply start at 0 and increment the id number  But  how do all the other books know the last created id number  Simple  save it as a static variable  Do patrons need to know that the actual internal id number is for each book  No  That information is private   public class Book       private static int numBooks   0      private int id      public String name       Book String name            id   numBooks            this name   name            This is a contrived example  but I m sure you can easily think of cases where you d want all class instances to have access to common information that should be kept private from everyone else  Or even if you can t  it is good programming practice to make things as private as possible  What if you accidentally made that numBooks field public  even though Book users were not supposed to do anything with it  Then someone could change the number of Books without creating a new Book    Very sneaky

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Static variables are those variables which are common for all the instances of a class  if one instance changes it   then value of static variable would be updated for all other instances

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Well you are right public static variables are used without making an instance of the class but private static variables are not  The main difference between them and where I use the private static variables is when you need to use a variable in a static function  For the static functions you can only use static variables  so you make them private to not access them from other classes  That is the only case I use private static for   Here is an example   Class test      public static String name    AA      private static String age      public static void setAge String yourAge             here if the age variable is not static you will get an error that you cannot access non static variables from static procedures so you have to make it static and private to not be accessed from other classes        age   yourAge

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If you use private static variables in your class  Static Inner classes in your class can reach your variables  This is perfectly good for context security

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If a variable is declared as private then it is not visible outside of the class this is called as datahiding     If a variable is declared as static then the value of the variable is same for all the instances and we no need to create an object to call that variable we can call that variable by simply  classname variablename

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Private static variables are useful in the same way that private instance variables are useful  they store state which is accessed only by code within the same class  The accessibility  private public etc  and the instance static nature of the variable are entirely orthogonal concepts   I would avoid thinking of static variables as being shared between  all instances  of the class - that suggests there has to be at least one instance for the state to be present  No - a static variable is associated with the type itself instead of any instances of the type   So any time you want some state which is associated with the type rather than any particular instance  and you want to keep that state private  perhaps allowing controlled access via properties  for example  it makes sense to have a private static variable   As an aside  I would strongly recommend that the only type of variables which you make public  or even non-private  are constants - static final variables of immutable types  Everything else should be private for the sake of separating API and implementation  amongst other things

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Is declaring a variable as private static varName  any different from   declaring a variable private varName     Yes  both are different  And the first one is called class variable because it holds single value for that class whereas the other one is called instance variable because it can hold different value for different instances Objects   The first one is created only once in jvm and other one is created once per instance i e if you have 10 instances then you will have 10 different private varName  in jvm       Does declaring the variable as static give it other special   properties    Yes  static variables gets some different properties than normal instance variables  I ve mentioned few already and let s see some here  class variables  instance variables which are declared as static  can be accessed directly by using class name like ClassName varName  And any object of that class can access and modify its value unlike instance variables are accessed by only its respective objects  Class variables can be used in static methods        What is the use of a private static variable in Java    Logically  private static variable is no different from public static variable rather the first one gives you more control  IMO  you can literally replace public static variable by private static variable with help of public static getter and setter methods    One widely used area of private static variable is in implementation of simple Singleton pattern where you will have only single instance of that class in whole world  Here static identifier plays crucial role to make that single instance is accessible by outside world Of course public static getter method also plays main role     public class Singleton       private static Singleton singletonInstance   new Singleton         private Singleton          public static Singleton getInstance            return Singleton singletonInstance

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For some people this makes more sense if they see it in a couple different languages so I wrote an example in Java  and PHP on my page where I explain some of these modifiers  You might be thinking about this incorrectly    You should look at my examples if it doesn t make sense below  Go here http   www siteconsortium com h D0000D php  The bottom line though is that it is pretty much exactly what it says it is  It s a static member variable that is private  For example if you wanted to create a Singleton object why would you want to make the SingletonExample instance variable public  If you did a person who was using the class could easily overwrite the value   That s all it is         public class SingletonExample         private static SingletonExample instance   null        private static int value   0        private SingletonExample               this value                public static SingletonExample getInstance             if instance  null          return instance          synchronized SingletonExample class            instance   new SingletonExample            return instance                          public void printValue             System out print  this value                   public static void main String    args            SingletonExample instance   getInstance            instance printValue            instance   getInstance            instance printValue

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The private keyword will allow the use for the variable access within the class and static means we can access the variable in a static method  You may need this cause a non-static reference variable cannot be accessible in a static method

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If a variable is defined as public static it can be accessed via its class name from any class   Usually functions are defined as public static which can be accessed just by calling the implementing class name   A very good example of it is the sleep   method in Thread class  Thread sleep 2500     If a variable is defined as private static it can be accessed only within that class so no class name is needed or you can still use the class name  upto you   The difference between private var name and private static var name is that private static variables can be accessed only by static methods of the class while private variables can be accessed by any method of that class except static methods   A very good example of it is while defining database connections or constants which require declaring variable as private static    Another common example is   private static int numberOfCars 10   public static int returnNumber     return numberOfCars

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Another perspective      A class and its instance are two different things at the runtime  A class info is  shared  by all the instances of that class  The non-static class variables belong to instances and the static variable belongs to class   Just like an instance variables can be private or public  static variables can also be private or public

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When in a static method you use a variable  the variable have to be static too as an example     private static int a 0    public static void testMethod               a 1

[java] What is the use of a private static variable in Java?

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