private final static attribute vs private final attribute

Question

In Java  what s the difference between   private final static int NUMBER   10    and  private final int NUMBER   10    Both are private and final  the difference is the static attribute   What s better  And why

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If you mark this variable static then as you know, you would be requiring static methods to again access these values,this will be useful if you already think of using these variables only in static methods. If this is so then this would be the best one.

You can however make the variable now as public since no one can modify it just like "System.out", it again depends upon your intentions and what you want to achieve.

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Lets say if the class will not have more than one instance ever  then which one takes more memory   private static final int ID   250  or private final int ID   250   I ve understood that static will refer to the class type with only one copy in the memory and non static will be in a new memory location for each instance variable  However internally if we just compare 1 instance of the same class ever  i e  more than 1 instance would not be created   then is there any overhead in terms of space used by 1 static final variable

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As already Jon said  a static variable  also referred to as a class variable  is a variable which exists across instances of a class   I found an example of this here   public class StaticVariable     static int noOfInstances    StaticVariable           noOfInstances          public static void main String   args          StaticVariable sv1   new StaticVariable        System out println  No  of instances for sv1       sv1 noOfInstances        StaticVariable sv2   new StaticVariable        System out println  No  of instances for sv1        sv1 noOfInstances       System out println  No  of instances for st2        sv2 noOfInstances        StaticVariable sv3   new StaticVariable        System out println  No  of instances for sv1        sv1 noOfInstances       System out println  No  of instances for sv2        sv2 noOfInstances       System out println  No  of instances for sv3        sv3 noOfInstances           Output of the program is given below   As we can see in this example each object has its own copy of class variable   C  java gt java StaticVariable No  of instances for sv1   1 No  of instances for sv1   2 No  of instances for st2   2 No  of instances for sv1   3 No  of instances for sv2   3 No  of instances for sv3   3

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The static one is the same member on all of the class instances and the class itself  The non-static is one for every instance  object   so in your exact case it s a waste of memory if you don t put static

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very little  and static  There isn t much difference as they are both constants  For most class data objects  static would mean something associated with the class itself  there being only one copy no matter how many objects were created with new   Since it is a constant  it may not actually be stored in either the class or in an instance  but the compiler still isn t going to let you access instance objects from a static method  even if it knows what they would be  The existence of the reflection API may also require some pointless work if you don t make it static

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This might help  public class LengthDemo   public static void main String   args        Rectangle box   new Rectangle        System out println  Sending the value 10 0                  to the setLength method         box setLength 10 0       System out println  Done

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Since a variable in a class is declared as final AND initialised in the same command  there is absolutely no reason to not declare it as static  since it will have the same value no matter the instance  So  all instances can share the same memory address for a value  thus saving processing time by eliminating the need to create a new variable for each instance and saving memory by sharing 1 common address

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From the tests i have made  static final variables are not the same with final non-static  variables  Final non-static  variables can differ from object to object    But that s only if the initialization is made within the constructor   If it is not initialized from the constructor then it is only a waste of memory as it creates final variables for every object that is created that cannot be altered    For example   class A       final int f      static final int sf   5       A int num                this f   num             void show                 System out printf  About Object   s n Final   d n Static Final   d n n   this toString    this f  sf              public static void main String   args                A ob1   new A 14           ob1 show             A ob2   new A 21           ob2 show               What shows up on screen is   About Object  A addbf1  Final  14  Static Final  5  About Object  A 530daa  Final  21  Static Final  5  Anonymous 1st year IT student  Greece

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Static  keyword makes the variable property of the class rather than individual instances of the class  There will be one copy of that variable that is shared amongst all the instances of that class  Any change in the state of the static variable will be reflected across all the instances  Add final to static and we get a variable that has been initialised once and for all at the class loading time and cannot be changed later by any instance of the class  Static final variables need to be initialised at the declaration time else we have compile time error  As far as private instance field is concerned  it refers to the property  state of an object  instance of a class  Each instance  object of the class will have its own copy of instance variable  When instance variable is declared final  it means we cannot change its value for this instance  For this we need to initialise the final variable either at declaration or in the constructor If its not done in either  of them  compile time error will show  Once initialised  if you try to reassign a value you will get a compile time error  Use static final variables where the data will be shared across all the instances of the class and you want the data to be read only Use instance final variable if you want to represent some data that belongs to a each individual instance of the class but once stored cannot be changed  Usage of static and instance key word depends upon your design needs and what that data represents in the domain  If the data is used across the class instances then there is no need for individual copies memory references for each object

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For final  it can be assigned different values at runtime when initialized  For example class Test    public final int a     Test t1    new Test    t1 a   10  Test t2    new Test    t2 a   20    fixed  Thus each instance has different value of field a  For static final  all instances share the same value  and can t be altered after first initialized  class TestStatic        public static final int a   0     TestStatic t1    new TestStatic    t1 a   10     ERROR  CAN T BE ALTERED AFTER THE FIRST  TestStatic t2    new TestStatic    t1 a   20       ERROR  CAN T BE ALTERED AFTER THE FIRST INITIALIZATION

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Reading the answers I found no real test really getting to the point  Here are my 2 cents    public class ConstTest        private final int         value               10      private static final int  valueStatic         20      private final File        valueObject         new File          private static final File valueObjectStatic   new File           public void printAddresses               System out println  final int address                     ObjectUtils identityToString value            System out println  final static int address                     ObjectUtils identityToString valueStatic            System out println  final file address                      ObjectUtils identityToString valueObject            System out println  final static file address                      ObjectUtils identityToString valueObjectStatic                public static void main final String args                final ConstTest firstObj   new ConstTest            final ConstTest sndObj   new ConstTest             firstObj printAdresses            sndObj printAdresses               Results for first object    final int address java lang Integer 6d9efb05 final static int address java lang Integer 60723d7c final file address java io File 6c22c95b final static file address java io File 5fd1acd3   Results for 2nd object    final int address java lang Integer 6d9efb05 final static int address java lang Integer 60723d7c final file address java io File 3ea981ca final static file address java io File 5fd1acd3   Conclusion    As I thought java makes a difference between primitive and other types  Primitive types in Java are always  cached   same for strings literals  not new String objects   so no difference between static and non-static members   However there is a memory duplication for non-static members if they are not instance of a primitive type   Changing value of valueStatic to 10 will even go further as Java will give the same addresses to the two int variables

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If you use static the value of the variable will be the same throughout all of your instances  if changed in one instance the others will change too

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private static final will be considered as constant and the constant can be accessed within this class only  Since  the keyword static included  the value will be constant for all the objects of the class   private final variable value will be like constant per object   You can refer the java lang String or look for the example below   public final class Foo        private final int i      private static final int j 20       public Foo int val           this i val             public static void main String   args            Foo foo1  new Foo 10            Foo foo2  new Foo 40            System out println foo1 i           System out println foo2 i           System out println check j               Output   10 40 20

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A static variable stays in the memory for the entire lifetime of the application  and is initialised during class loading  A non-static variable is being initialised each time you construct a new object  It s generally better to use   private static final int NUMBER   10    Why  This reduces the memory footprint per instance  It possibly is also favourable for cache hits  And it just makes sense  static should be used for things that are shared across all instances  a k a  objects  of a certain type  a k a  class

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Furthermore to Jon s answer if you use static final it will behave as a kind-of  definition   Once you compile the class which uses it  it will be in the compiled  class file burnt  Check my thread about it here   For your main goal  If you don t use the NUMBER differently in the different instances of the class i would advise to use final and static   You just have to keep in mind to not to copy compiled class files without considering possible troubles like the one my case study describes  Most of the cases this does not occur  don t worry       To show you how to use different values in instances check this code   public class JustFinalAttr     public final int Number     public JustFinalAttr int a       Number a            System out println new JustFinalAttr 4  Number

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While the other answers seem to make it pretty clear that there is generally no reason to use non-static constants  I couldn t find anyone pointing out that it is possible to have various instances with different values on their constant variables   Consider the following example   public class TestClass       private final static double NUMBER   Math random         public TestClass              System out println NUMBER             Creating three instances of TestClass would print the same random value three times  since only one value is generated and stored into the static constant   However  when trying the following example instead   public class TestClass       private final double NUMBER   Math random         public TestClass              System out println NUMBER             Creating three instances of TestClass would now print three different random values  because each instance has its own randomly generated constant value   I can t think of any situation where it would be really useful to have different constant values on different instances  but I hope this helps pointing out that there is a clear difference between static and non-static finals

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Here is my two cents   final           String CENT 1   new Random   nextInt 2     0    HEADS     TAILS   final   static  String CENT 2   new Random   nextInt 2     0    HEADS     TAILS     Example   package test   public class Test        final long OBJECT ID   new Random   nextLong        final static long CLASSS ID   new Random   nextLong         public static void main String   args            Test   test   new Test 5           for  int i   0  i  lt  test length  i                 test i    new Test                System out println  Class id    test i  CLASSS ID     lt - Always the same value             System out println  Object id    test i  OBJECT ID     lt - Always different                     The key is that variables and functions can return different values Therefore final variables can be assigned with different values

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Just Another simple example to understand the usage of static  static final  final variables  Code comments have the proper explanation   public class City           base price that is always same for all objects For all cities       private static double iphone base price   10000          this is total price   iphone base price iphone diff      private double iphone citi price          extra price added to iphone base price  It is constant per city  Every        city has its own difference defined      private final double iphone diff       private String cityName               static final will be accessible everywhere within the class but cant be        changed once initialized      private static final String countryName    India        public City String cityName  double iphone diff            super            this iphone diff   iphone diff          iphone citi price   iphone base price   iphone diff          this cityName   cityName                         get phone price                 return             private double getPrice              return iphone citi price                        Get city name                 return             private String getCityName              return cityName             public static void main String   args                300 is the         City newyork   new City  Newyork   300           System out println newyork getPrice            newyork getCityName              City california   new City  California   800           System out println california getPrice            california getCityName                 We cant write below statement as a final variable can not be            reassigned            california iphone diff 1000                                         base price is defined for a class and not per instances             For any number of object creation  static variable s value would be the same            for all instances until and unless changed             Also it is accessible anywhere inside a class          iphone base price   9000           City delhi   new City  delhi   400           System out println delhi getPrice            delhi getCityName              City moscow   new City  delhi   500           System out println moscow getPrice            moscow getCityName                 Here countryName is accessible as it is static but we can not change it as it is final as well             Something are meant to be accessible with no permission to modify it             Try un-commenting below statements         System out println countryName               countryName  INDIA              System out println countryName

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Static variable belongs to the class  which means all the objects share that variable   Non static variable belongs to each objects   public class ExperimentFinal    private final int a  private static final int b   999    public ExperimentFinal int a        super        this a   a    public int getA         return a    public int getB         return b    public void print int a  int b        System out println  final int      a      nstatic final int      b     public static void main String   args        ExperimentFinal test   new ExperimentFinal 9       test print test getA    test getB           As you can see above example  for  final int  we can assign our variable for each instance  object  of the class  however for  static final int   we should assign a variable in the class  static variable belongs to the class

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static means  associated with the class   without it  the variable is associated with each instance of the class   If it s static  that means you ll have only one in memory  if not  you ll have one for each instance you create   static means the variable will remain in memory for as long as the class is loaded  without it  the variable can be gc d when its instance is

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In general  static means  associated with the type itself  rather than an instance of the type    That means you can reference a static variable without having ever created an instances of the type  and any code referring to the variable is referring to the exact same data  Compare this with an instance variable  in that case  there s one independent version of the variable per instance of the class  So for example   Test x   new Test    Test y   new Test    x instanceVariable   10  y instanceVariable   20  System out println x instanceVariable     prints out 10  y instanceVariable and x instanceVariable are separate  because x and y refer to different objects   You can refer to static members via references  although it s a bad idea to do so  If we did   Test x   new Test    Test y   new Test    x staticVariable   10  y staticVariable   20  System out println x staticVariable     then that would print out 20 - there s only one variable  not one per instance  It would have been clearer to write this as   Test x   new Test    Test y   new Test    Test staticVariable   10  Test staticVariable   20  System out println Test staticVariable     That makes the behaviour much more obvious  Modern IDEs will usually suggest changing the second listing into the third   There is no reason to have an inline declaration initializing the value like the following  as each instance will have its own NUMBER but always with the same value  is immutable and initialized with a literal   This is the same than to have only one final static variable for all instances   private final int NUMBER   10    Therefore if it cannot change  there is no point having one copy per instance   But  it makes sense if is initialized in a constructor like this      No initialization when is declared private final int number   public MyClass int n          The variable can be assigned in the constructor  but then       not modified later     number   n      Now  for each instance of MyClass  we can have a different but immutable value of number

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Final   Once a final variable has been assigned  it always contains the same value   wherever the variable is static or not  static  it will be only one variable for all instances initialized one time in Memory

[java] private final static attribute vs private final attribute

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