What does the static keyword do in a class

Question

To be specific  I was trying this code   package hello   public class Hello        Clock clock   new Clock         public static void main String args              clock sayTime              But it gave the error      Cannot access non-static field in static method main   So I changed the declaration of clock to this   static Clock clock   new Clock      And it worked  What does it mean to put that keyword before the declaration  What exactly will it do and or restrict in terms of what can be done to that object

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Static means that you don t have to create an instance of the class to use the methods or variables associated with the class  In your example  you could call   Hello main new String        main      is declared as a static function in the Hello class   directly  instead of   Hello h   new Hello    h main new String         main      is a non-static function linked with the  h  variable   From inside a static method  which belongs to a class  you cannot access any members which are not static  since their values depend on your instantiation of the class  A non-static Clock object  which is an instance member  would have a different value reference for each instance of your Hello class  and therefore you could not access it from the static portion of the class

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Here is an example   public class StaticClass        static int version      public void printVersion              System out println version            public class MainClass        public static void main String args                StaticClass staticVar1   new StaticClass            staticVar1 version   10          staticVar1 printVersion      Output 10          StaticClass staticVar2   new StaticClass            staticVar2 printVersion      Output 10         staticVar2 version   20          staticVar2 printVersion      Output 20         staticVar1 printVersion      Output 20

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To add to existing answers  let me try with a picture   An interest rate of 2  is applied to ALL savings accounts  Hence it is static   A balance should be individual  so it is not static

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static members belong to the class instead of a specific instance   It means that only one instance of a static field exists 1  even if you create a million instances of the class or you don t create any  It will be shared by all instances   Since static methods also do not belong to a specific instance  they can t refer to instance members  In the example given  main does not know which instance of the Hello class  and therefore which instance of the Clock class  it should refer to  static members can only refer to static members  Instance members can  of course access static members   Side note  Of course  static members can access instance members through an object reference   Example   public class Example       private static boolean staticField      private boolean instanceField      public static void main String   args               a static method can access static fields         staticField   true              a static method can access instance fields through an object reference         Example instance   new Example            instance instanceField   true           1   Depending on the runtime characteristics  it can be one per ClassLoader or AppDomain or thread  but that is beside the point

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Static means that you don t have to create an instance of the class to use the methods or variables associated with the class  In your example  you could call   Hello main new String        main      is declared as a static function in the Hello class   directly  instead of   Hello h   new Hello    h main new String         main      is a non-static function linked with the  h  variable   From inside a static method  which belongs to a class  you cannot access any members which are not static  since their values depend on your instantiation of the class  A non-static Clock object  which is an instance member  would have a different value reference for each instance of your Hello class  and therefore you could not access it from the static portion of the class

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Static Variables Can only be accessed only in static methods  so when we declare the static variables those getter and setter methods will be static methods  static methods is a class level we can access using class name  The following is  example for Static Variables Getters And Setters   public class Static         private static String owner      private static int rent      private String car      public String getCar             return car            public void setCar String car            this car   car            public static int getRent             return rent            public static void setRent int rent            Static rent   rent            public static String getOwner             return owner             public static void setOwner String owner            Static owner   owner

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The static keyword in Java means that the variable or function is shared between all instances of that class as it belongs to the type  not the actual objects themselves   So if you have a variable  private static int i   0  and you increment it  i    in one instance  the change will be reflected in all instances  i will now be 1 in all instances   Static methods can be used without instantiating an object

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Can also think of static members not having a  this  pointer  They are shared among all instances

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A question was asked here about the choice of the word  static  for this concept   It was dup d to this question  but I don t think the etymology has been clearly addressed  So       It s due to keyword reuse  starting with C   Consider data declarations in C  inside a function body        void f             int foo   1          static int bar   2                     The variable foo is created on the stack when the function is entered  and destroyed when the function terminates    By contrast  bar is always there  so it s  static  in the sense of common English - it s not going anywhere   Java  and similar languages  have the same concept for data  Data can either be allocated per instance of the class  per object  or once for the entire class  Since Java aims to have familiar syntax for C C   programmers  the  static  keyword is appropriate here       class C           int foo   1          static int bar   2                     Lastly  we come to methods       class C           int foo                   static int bar                              There is  conceptually speaking  an instance of foo   for every instance of class C   There is only one instance of bar   for the entire class C  This is parallel to the case we discussed for data  and therefore using  static  is again a sensible choice  especially if you don t want to add more reserved keywords to your language

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Can also think of static members not having a  this  pointer  They are shared among all instances

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static methods don t use any instance variables of the class they are defined in   A very good explanation of the difference can be found on this page

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The static keyword means that something  a field  method or nested class  is related to the type rather than any particular instance of the type  So for example  one calls Math sin      without any instance of the Math class  and indeed you can t create an instance of the Math class   For more information  see the relevant bit of Oracle s Java Tutorial     Sidenote  Java unfortunately allows you to access static members as if they were instance members  e g      Bad code  Thread currentThread   sleep 5000   someOtherThread sleep 5000     That makes it look as if sleep is an instance method  but it s actually a static method - it always makes the current thread sleep  It s better practice to make this clear in the calling code      Clearer Thread sleep 5000

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In Java  the static keyword can be simply regarded as indicating the following       without regard or relationship to any particular instance    If you think of static in this way  it becomes easier to understand its use in the various contexts in which it is encountered    A static field is a field that belongs to the class rather than to any particular instance A static method is a method that has no notion of this  it is defined on the class and doesn t know about any particular instance of that class unless a reference is passed to it A static member class is a nested class without any notion or knowledge of an instance of its enclosing class  unless a reference to an enclosing class instance is passed to it

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Static in Java   Static is a Non Access Modifier  The static keyword belongs to the class than instance of the class  can be used to attach a Variable or Method to a Class   Static keyword CAN be used with   Method  Variable  Class nested within another Class  Initialization Block  CAN T be used with   Class  Not Nested   Constructor  Interfaces  Method Local Inner Class Difference then  nested class   Inner Class methods  Instance Variables  Local Variables  Example   Imagine the following example which has an instance variable named count which in incremented in the constructor   package pkg   class StaticExample       int count   0    will get memory when instance is created      StaticExample             count            System out println count              public static void main String args               StaticExample c1   new StaticExample            StaticExample c2   new StaticExample            StaticExample c3   new StaticExample               Output      1 1 1   Since instance variable gets the memory at the time of object creation  each object will have the copy of the instance variable  if it is incremented  it won t reflect to other objects    Now if we change the instance variable count to a static one then the program will produce different output   package pkg   class StaticExample       static int count   0    will get memory when instance is created      StaticExample             count            System out println count              public static void main String args               StaticExample c1   new StaticExample            StaticExample c2   new StaticExample            StaticExample c3   new StaticExample               Output      1 2 3   In this case static variable will get the memory only once  if any object changes the value of the static variable  it will retain its value   Static with Final   The global variable which is declared as final and static remains unchanged for the whole execution  Because  Static members are stored in the class memory and they are loaded only once in the whole execution  They are common to all objects of the class  If you declare static variables as final  any of the objects can   t change their value as it is final  Therefore  variables declared as final and static are sometimes referred to as Constants  All fields of interfaces are referred as constants  because they are final and static by default     Picture Resource   Final Static

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A field can be assigned to either the class or an instance of a class  By default fields are instance variables  By using static the field becomes a class variable  thus there is one and only one clock  If you make a changes in one place  it s visible everywhere  Instance varables are changed independently of one another

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Basic usage of static members     public class Hello          value   method     public static String staticValue      public String nonStaticValue     class A       Hello hello   new Hello        hello staticValue    abc       hello nonStaticValue    xyz      class B       Hello hello2   new Hello       here staticValue    abc      hello2 staticValue     will have value of  abc      hello2 nonStaticValue     will have value of null     That s how you can have values shared in all class members without sending class instance Hello to other class  And whit static you don t need to create class instance   Hello hello   new Hello    hello staticValue    abc     You can just call static values or methods by class name   Hello staticValue    abc

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Understanding Static concepts   public class StaticPractise1       public static void main String   args            StaticPractise2 staticPractise2   new StaticPractise2            staticPractise2 printUddhav      true         StaticPractise2 printUddhav       false  because printUddhav   is although inside StaticPractise2  but it is where exactly depends on PC program counter on runtime              StaticPractise2 printUddhavsStatic1      true         staticPractise2 printUddhavsStatic1      false  because  when staticPractise2 is blueprinted  it tracks everything other than static  things and it organizes in its own heap  So  class static methods  object can t reference               Second Class  public class StaticPractise2       public static void printUddhavsStatic1             System out println  Uddhav               public void printUddhav             System out println  Uddhav

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Can also think of static members not having a  this  pointer  They are shared among all instances

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Static means that you don t have to create an instance of the class to use the methods or variables associated with the class  In your example  you could call   Hello main new String        main      is declared as a static function in the Hello class   directly  instead of   Hello h   new Hello    h main new String         main      is a non-static function linked with the  h  variable   From inside a static method  which belongs to a class  you cannot access any members which are not static  since their values depend on your instantiation of the class  A non-static Clock object  which is an instance member  would have a different value reference for each instance of your Hello class  and therefore you could not access it from the static portion of the class

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static members belong to the class instead of a specific instance   It means that only one instance of a static field exists 1  even if you create a million instances of the class or you don t create any  It will be shared by all instances   Since static methods also do not belong to a specific instance  they can t refer to instance members  In the example given  main does not know which instance of the Hello class  and therefore which instance of the Clock class  it should refer to  static members can only refer to static members  Instance members can  of course access static members   Side note  Of course  static members can access instance members through an object reference   Example   public class Example       private static boolean staticField      private boolean instanceField      public static void main String   args               a static method can access static fields         staticField   true              a static method can access instance fields through an object reference         Example instance   new Example            instance instanceField   true           1   Depending on the runtime characteristics  it can be one per ClassLoader or AppDomain or thread  but that is beside the point

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static methods don t use any instance variables of the class they are defined in   A very good explanation of the difference can be found on this page

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A field can be assigned to either the class or an instance of a class  By default fields are instance variables  By using static the field becomes a class variable  thus there is one and only one clock  If you make a changes in one place  it s visible everywhere  Instance varables are changed independently of one another

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Here is an example   public class StaticClass        static int version      public void printVersion              System out println version            public class MainClass        public static void main String args                StaticClass staticVar1   new StaticClass            staticVar1 version   10          staticVar1 printVersion      Output 10          StaticClass staticVar2   new StaticClass            staticVar2 printVersion      Output 10         staticVar2 version   20          staticVar2 printVersion      Output 20         staticVar1 printVersion      Output 20

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Static makes the clock member a class member instead of an instance member  Without the static keyword you would need to create an instance of the Hello class  which has a clock member variable  - e g   Hello hello   new Hello    hello clock sayTime

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The static keyword in Java means that the variable or function is shared between all instances of that class as it belongs to the type  not the actual objects themselves   So if you have a variable  private static int i   0  and you increment it  i    in one instance  the change will be reflected in all instances  i will now be 1 in all instances   Static methods can be used without instantiating an object

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Static makes the clock member a class member instead of an instance member  Without the static keyword you would need to create an instance of the Hello class  which has a clock member variable  - e g   Hello hello   new Hello    hello clock sayTime

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This discussion has so far ignored classloader considerations  Strictly speaking  Java static fields are shared between all instances of a class for a given classloader

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Static means that you don t have to create an instance of the class to use the methods or variables associated with the class  In your example  you could call   Hello main new String        main      is declared as a static function in the Hello class   directly  instead of   Hello h   new Hello    h main new String         main      is a non-static function linked with the  h  variable   From inside a static method  which belongs to a class  you cannot access any members which are not static  since their values depend on your instantiation of the class  A non-static Clock object  which is an instance member  would have a different value reference for each instance of your Hello class  and therefore you could not access it from the static portion of the class

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A field can be assigned to either the class or an instance of a class  By default fields are instance variables  By using static the field becomes a class variable  thus there is one and only one clock  If you make a changes in one place  it s visible everywhere  Instance varables are changed independently of one another

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static members belong to the class instead of a specific instance   It means that only one instance of a static field exists 1  even if you create a million instances of the class or you don t create any  It will be shared by all instances   Since static methods also do not belong to a specific instance  they can t refer to instance members  In the example given  main does not know which instance of the Hello class  and therefore which instance of the Clock class  it should refer to  static members can only refer to static members  Instance members can  of course access static members   Side note  Of course  static members can access instance members through an object reference   Example   public class Example       private static boolean staticField      private boolean instanceField      public static void main String   args               a static method can access static fields         staticField   true              a static method can access instance fields through an object reference         Example instance   new Example            instance instanceField   true           1   Depending on the runtime characteristics  it can be one per ClassLoader or AppDomain or thread  but that is beside the point

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A question was asked here about the choice of the word  static  for this concept   It was dup d to this question  but I don t think the etymology has been clearly addressed  So       It s due to keyword reuse  starting with C   Consider data declarations in C  inside a function body        void f             int foo   1          static int bar   2                     The variable foo is created on the stack when the function is entered  and destroyed when the function terminates    By contrast  bar is always there  so it s  static  in the sense of common English - it s not going anywhere   Java  and similar languages  have the same concept for data  Data can either be allocated per instance of the class  per object  or once for the entire class  Since Java aims to have familiar syntax for C C   programmers  the  static  keyword is appropriate here       class C           int foo   1          static int bar   2                     Lastly  we come to methods       class C           int foo                   static int bar                              There is  conceptually speaking  an instance of foo   for every instance of class C   There is only one instance of bar   for the entire class C  This is parallel to the case we discussed for data  and therefore using  static  is again a sensible choice  especially if you don t want to add more reserved keywords to your language

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It means that there is only one instance of  clock  in Hello  not one per each separate instance of the  Hello  class  or more-so  it means that there will be one commonly shared  clock  reference among all instances of the  Hello  class   So if you were to do a  new Hello  anywhere in your code  A- in the first scenario  before the change  without using  static    it would make a new clock every time a  new Hello  is called  but  B- in the second scenario  after the change  using  static    every  new Hello  instance would still share and use the initial and same  clock  reference first created   Unless you needed  clock  somewhere outside of main  this would work just as well   package hello  public class Hello       public static void main String args                Clock clock new Clock          clock sayTime

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A field can be assigned to either the class or an instance of a class  By default fields are instance variables  By using static the field becomes a class variable  thus there is one and only one clock  If you make a changes in one place  it s visible everywhere  Instance varables are changed independently of one another

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This discussion has so far ignored classloader considerations  Strictly speaking  Java static fields are shared between all instances of a class for a given classloader

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Static in Java   Static is a Non Access Modifier  The static keyword belongs to the class than instance of the class  can be used to attach a Variable or Method to a Class   Static keyword CAN be used with   Method  Variable  Class nested within another Class  Initialization Block  CAN T be used with   Class  Not Nested   Constructor  Interfaces  Method Local Inner Class Difference then  nested class   Inner Class methods  Instance Variables  Local Variables  Example   Imagine the following example which has an instance variable named count which in incremented in the constructor   package pkg   class StaticExample       int count   0    will get memory when instance is created      StaticExample             count            System out println count              public static void main String args               StaticExample c1   new StaticExample            StaticExample c2   new StaticExample            StaticExample c3   new StaticExample               Output      1 1 1   Since instance variable gets the memory at the time of object creation  each object will have the copy of the instance variable  if it is incremented  it won t reflect to other objects    Now if we change the instance variable count to a static one then the program will produce different output   package pkg   class StaticExample       static int count   0    will get memory when instance is created      StaticExample             count            System out println count              public static void main String args               StaticExample c1   new StaticExample            StaticExample c2   new StaticExample            StaticExample c3   new StaticExample               Output      1 2 3   In this case static variable will get the memory only once  if any object changes the value of the static variable  it will retain its value   Static with Final   The global variable which is declared as final and static remains unchanged for the whole execution  Because  Static members are stored in the class memory and they are loaded only once in the whole execution  They are common to all objects of the class  If you declare static variables as final  any of the objects can   t change their value as it is final  Therefore  variables declared as final and static are sometimes referred to as Constants  All fields of interfaces are referred as constants  because they are final and static by default     Picture Resource   Final Static

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The static keyword means that something  a field  method or nested class  is related to the type rather than any particular instance of the type  So for example  one calls Math sin      without any instance of the Math class  and indeed you can t create an instance of the Math class   For more information  see the relevant bit of Oracle s Java Tutorial     Sidenote  Java unfortunately allows you to access static members as if they were instance members  e g      Bad code  Thread currentThread   sleep 5000   someOtherThread sleep 5000     That makes it look as if sleep is an instance method  but it s actually a static method - it always makes the current thread sleep  It s better practice to make this clear in the calling code      Clearer Thread sleep 5000

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Static makes the clock member a class member instead of an instance member  Without the static keyword you would need to create an instance of the Hello class  which has a clock member variable  - e g   Hello hello   new Hello    hello clock sayTime

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static members belong to the class instead of a specific instance   It means that only one instance of a static field exists 1  even if you create a million instances of the class or you don t create any  It will be shared by all instances   Since static methods also do not belong to a specific instance  they can t refer to instance members  In the example given  main does not know which instance of the Hello class  and therefore which instance of the Clock class  it should refer to  static members can only refer to static members  Instance members can  of course access static members   Side note  Of course  static members can access instance members through an object reference   Example   public class Example       private static boolean staticField      private boolean instanceField      public static void main String   args               a static method can access static fields         staticField   true              a static method can access instance fields through an object reference         Example instance   new Example            instance instanceField   true           1   Depending on the runtime characteristics  it can be one per ClassLoader or AppDomain or thread  but that is beside the point

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It means that there is only one instance of  clock  in Hello  not one per each separate instance of the  Hello  class  or more-so  it means that there will be one commonly shared  clock  reference among all instances of the  Hello  class   So if you were to do a  new Hello  anywhere in your code  A- in the first scenario  before the change  without using  static    it would make a new clock every time a  new Hello  is called  but  B- in the second scenario  after the change  using  static    every  new Hello  instance would still share and use the initial and same  clock  reference first created   Unless you needed  clock  somewhere outside of main  this would work just as well   package hello  public class Hello       public static void main String args                Clock clock new Clock          clock sayTime

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This discussion has so far ignored classloader considerations  Strictly speaking  Java static fields are shared between all instances of a class for a given classloader

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The keyword static is used to denote a field or a method as belonging to the class itself and not the instance   Using your code  if the object Clock is static  all of the instances of the Hello class will share this Clock data member  field  in common   If you make it non-static  each individual instance of Hello can have a unique Clock field  You added a main method to your class Hello so that you could run the code   The problem with that is that the main method is static and as such  it cannot refer to non-static fields or methods inside of it   You can resolve this in two ways   Make all fields and methods of the Hello class static so that they could be referred to inside the main method   This is really not a good thing to do  or the wrong reason to make a field and or a method static  Create an instance of your Hello class inside the main method and access all it s fields and methods the way they were intended to in the first place   For you  this means the following change to your code  package hello   public class Hello        private Clock clock   new Clock         public Clock getClock             return clock             public static void main String args              Hello hello   new Hello            hello getClock   sayTime

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In Java  the static keyword can be simply regarded as indicating the following       without regard or relationship to any particular instance    If you think of static in this way  it becomes easier to understand its use in the various contexts in which it is encountered    A static field is a field that belongs to the class rather than to any particular instance A static method is a method that has no notion of this  it is defined on the class and doesn t know about any particular instance of that class unless a reference is passed to it A static member class is a nested class without any notion or knowledge of an instance of its enclosing class  unless a reference to an enclosing class instance is passed to it

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main   is a static method which has two fundamental restrictions    The static method cannot use a non-static data member or directly call non-static method  this   and super   cannot be used in static context   class A         int a   40    non static     public static void main String args                System out println a                     Output  Compile Time Error

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main   is a static method which has two fundamental restrictions    The static method cannot use a non-static data member or directly call non-static method  this   and super   cannot be used in static context   class A         int a   40    non static     public static void main String args                System out println a                     Output  Compile Time Error

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Static Variables Can only be accessed only in static methods  so when we declare the static variables those getter and setter methods will be static methods  static methods is a class level we can access using class name  The following is  example for Static Variables Getters And Setters   public class Static         private static String owner      private static int rent      private String car      public String getCar             return car            public void setCar String car            this car   car            public static int getRent             return rent            public static void setRent int rent            Static rent   rent            public static String getOwner             return owner             public static void setOwner String owner            Static owner   owner

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To add to existing answers  let me try with a picture   An interest rate of 2  is applied to ALL savings accounts  Hence it is static   A balance should be individual  so it is not static

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This discussion has so far ignored classloader considerations  Strictly speaking  Java static fields are shared between all instances of a class for a given classloader

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I have developed a liking for static methods  only  if possible  in  helper  classes   The calling class need not create another member  instance  variable of the helper class  You just call the methods of the helper class  Also the helper class is improved because you no longer need a constructor  and you need no member  instance  variables   There are probably other advantages

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I have developed a liking for static methods  only  if possible  in  helper  classes   The calling class need not create another member  instance  variable of the helper class  You just call the methods of the helper class  Also the helper class is improved because you no longer need a constructor  and you need no member  instance  variables   There are probably other advantages

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Understanding Static concepts   public class StaticPractise1       public static void main String   args            StaticPractise2 staticPractise2   new StaticPractise2            staticPractise2 printUddhav      true         StaticPractise2 printUddhav       false  because printUddhav   is although inside StaticPractise2  but it is where exactly depends on PC program counter on runtime              StaticPractise2 printUddhavsStatic1      true         staticPractise2 printUddhavsStatic1      false  because  when staticPractise2 is blueprinted  it tracks everything other than static  things and it organizes in its own heap  So  class static methods  object can t reference               Second Class  public class StaticPractise2       public static void printUddhavsStatic1             System out println  Uddhav               public void printUddhav             System out println  Uddhav

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It means that there is only one instance of  clock  in Hello  not one per each separate instance of the  Hello  class  or more-so  it means that there will be one commonly shared  clock  reference among all instances of the  Hello  class   So if you were to do a  new Hello  anywhere in your code  A- in the first scenario  before the change  without using  static    it would make a new clock every time a  new Hello  is called  but  B- in the second scenario  after the change  using  static    every  new Hello  instance would still share and use the initial and same  clock  reference first created   Unless you needed  clock  somewhere outside of main  this would work just as well   package hello  public class Hello       public static void main String args                Clock clock new Clock          clock sayTime

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Can also think of static members not having a  this  pointer  They are shared among all instances

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static methods don t use any instance variables of the class they are defined in   A very good explanation of the difference can be found on this page

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Basic usage of static members     public class Hello          value   method     public static String staticValue      public String nonStaticValue     class A       Hello hello   new Hello        hello staticValue    abc       hello nonStaticValue    xyz      class B       Hello hello2   new Hello       here staticValue    abc      hello2 staticValue     will have value of  abc      hello2 nonStaticValue     will have value of null     That s how you can have values shared in all class members without sending class instance Hello to other class  And whit static you don t need to create class instance   Hello hello   new Hello    hello staticValue    abc     You can just call static values or methods by class name   Hello staticValue    abc

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It means that there is only one instance of  clock  in Hello  not one per each separate instance of the  Hello  class  or more-so  it means that there will be one commonly shared  clock  reference among all instances of the  Hello  class   So if you were to do a  new Hello  anywhere in your code  A- in the first scenario  before the change  without using  static    it would make a new clock every time a  new Hello  is called  but  B- in the second scenario  after the change  using  static    every  new Hello  instance would still share and use the initial and same  clock  reference first created   Unless you needed  clock  somewhere outside of main  this would work just as well   package hello  public class Hello       public static void main String args                Clock clock new Clock          clock sayTime

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static methods don t use any instance variables of the class they are defined in   A very good explanation of the difference can be found on this page

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The keyword static is used to denote a field or a method as belonging to the class itself and not the instance   Using your code  if the object Clock is static  all of the instances of the Hello class will share this Clock data member  field  in common   If you make it non-static  each individual instance of Hello can have a unique Clock field  You added a main method to your class Hello so that you could run the code   The problem with that is that the main method is static and as such  it cannot refer to non-static fields or methods inside of it   You can resolve this in two ways   Make all fields and methods of the Hello class static so that they could be referred to inside the main method   This is really not a good thing to do  or the wrong reason to make a field and or a method static  Create an instance of your Hello class inside the main method and access all it s fields and methods the way they were intended to in the first place   For you  this means the following change to your code  package hello   public class Hello        private Clock clock   new Clock         public Clock getClock             return clock             public static void main String args              Hello hello   new Hello            hello getClock   sayTime

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Static makes the clock member a class member instead of an instance member  Without the static keyword you would need to create an instance of the Hello class  which has a clock member variable  - e g   Hello hello   new Hello    hello clock sayTime

[java] What does the 'static' keyword do in a class?

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