What is the reason behind non-static method cannot be referenced from a static context

Question

The very common beginner mistake is when you try to use a class property  statically  without making an instance of that class  It leaves you with the mentioned error message      You can either make the non static method static or make an instance of that class to use its properties    Why  I am not asking for solutions  I would be grateful to know what is the reason behind it  The very core reason   private java util List lt String gt  someMethod           Some Code        return someList                 public static void main String   strArgs                     The following statement causes the error  You know why       java util List lt String gt  someList   someMethod

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The answers so far describe why  but here is a something else you might want to consider    You can can call a method from an instantiable class by appending a method call to its constructor    Object instance   new Constuctor   methodCall      or  primitive name   new Constuctor   methodCall      This is useful it you only wish to use a method of an instantiable class once within a single scope   If you are calling multiple methods from an instantiable class within a single scope  definitely create a referable instance

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The answers so far describe why  but here is a something else you might want to consider    You can can call a method from an instantiable class by appending a method call to its constructor    Object instance   new Constuctor   methodCall      or  primitive name   new Constuctor   methodCall      This is useful it you only wish to use a method of an instantiable class once within a single scope   If you are calling multiple methods from an instantiable class within a single scope  definitely create a referable instance

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A non-static method is dependent on the object  It is recognized by the program once the object is created     Static methods can be called even before the creation of an object  Static methods are great for doing comparisons or operations that aren t dependent on the actual objects you plan to work with

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The compiler actually adds an argument to non-static methods  It adds a this pointer reference  This is also the reason why a static method can not use this  because there is no object

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You can t call something that doesn t exist   Since you haven t created an object  the non-static method doesn t exist yet   A static method  by definition  always exists

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The essence of object oriented programming is encapsulating logic together with the data it operates on    Instance methods are the logic  instance fields are the data  Together  they form an object   public class Foo       private String foo      public Foo String foo   this foo   foo        public getFoo    return this foo         public static void main String   args           System out println  getFoo                What could possibly be the result of running the above program   Without an object  there is no instance data  and while the instance methods exist as part of the class definition  they need an object instance to provide data for them   In theory  an instance method that does not access any instance data could work in a static context  but then there isn t really any reason for it to be an instance method  It s a language design decision to allow it anyway rather than making up an extra rule to forbid it

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The simple reason behind this is that Static data members of parent class can be accessed  only if they are not overridden  but for instance non-static  data members or methods we need their reference and so they can only be called through an object

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if a method is not static  that  tells  the compiler that the method requires access to instance-level data in the class   like a non-static field    This data would not be available unless an instance of the class has been created   So the compiler throws an error if you try to call the method from a static method   If in fact the method does NOT reference any non-static member of the class  make the method static   In Resharper  for example  just creating a non-static method that does NOT reference any static member of the class generates a warning message  This method can be made static

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So you are asking for a very core reason    Well  since you are developing in Java  the compiler generates an object code that the Java Virtual Machine can interpret  The JVM anyway is a binary program that run in machine language  probably the JVM   s version specific for your operating system and hardware was previously compiled by another programming language like C in order to get a machine code that can run in your processor   At the end  any code is translated to machine code  So  create an object  an instance of a class  is equivalent to reserve a memory space  memory registers that will be processor registers when the CPU scheduler of the operating system put your program at the top of the queue in order to execute it  to have a data storage place that can be able to read and write data  If you don   t have an instance of a class  which happens on a static context   then you don   t have that memory space to read or write the data  In fact  like other people had said  the data don   t exist  because from the begin you never had written neither had reserved the memory space to store it    Sorry for my english  I m latin

User · Answer

The answers so far describe why  but here is a something else you might want to consider    You can can call a method from an instantiable class by appending a method call to its constructor    Object instance   new Constuctor   methodCall      or  primitive name   new Constuctor   methodCall      This is useful it you only wish to use a method of an instantiable class once within a single scope   If you are calling multiple methods from an instantiable class within a single scope  definitely create a referable instance

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I think it is worth pointing out that by the rules of the Java language the Java compiler inserts the equivalent of  this   when it notices that you re accessing instance methods or instance fields without an explicit instance   Of course  the compiler knows that it can only do this from within an instance method  which has a  this  variable  as static methods don t   Which means that when you re in an instance method the following are equivalent   instanceMethod    this instanceMethod      and these are also equivalent         instanceField        this instanceField    The compiler is effectively inserting the  this   when you don t supply a specific instance     This  pun intended  bit of  magic help  by the compiler can confuse novices  it means that instance calls and static calls sometimes appear to have the same syntax while in reality are calls of different types and underlying mechanisms   The instance method call is sometimes referred to as a method invocation or dispatch because of the behaviors of virtual methods supporting polymorphism  dispatching behavior happens regardless of whether you wrote an explicit object instance to use or the compiler inserted a  this     The static method call mechanism is simpler  like a function call in a non-OOP language   Personally  I think the error message is misleading  it could read  non-static method cannot be referenced from a static context without specifying an explicit object instance        What the compiler is complaining about is that it cannot simply insert the standard  this   as it does within instance methods  because this code is within a static method  however  maybe the author merely forgot to supply the instance of interest for this invocation   8212  say  an instance possibly supplied to the static method as parameter  or created within this static method     In short  you most certainly can call instance methods from within a static method  you just need to have and specify an explicit instance object for the invocation

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So you are asking for a very core reason    Well  since you are developing in Java  the compiler generates an object code that the Java Virtual Machine can interpret  The JVM anyway is a binary program that run in machine language  probably the JVM   s version specific for your operating system and hardware was previously compiled by another programming language like C in order to get a machine code that can run in your processor   At the end  any code is translated to machine code  So  create an object  an instance of a class  is equivalent to reserve a memory space  memory registers that will be processor registers when the CPU scheduler of the operating system put your program at the top of the queue in order to execute it  to have a data storage place that can be able to read and write data  If you don   t have an instance of a class  which happens on a static context   then you don   t have that memory space to read or write the data  In fact  like other people had said  the data don   t exist  because from the begin you never had written neither had reserved the memory space to store it    Sorry for my english  I m latin

User · Answer

A non-static method is dependent on the object  It is recognized by the program once the object is created     Static methods can be called even before the creation of an object  Static methods are great for doing comparisons or operations that aren t dependent on the actual objects you plan to work with

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If we try to access an instance method from a static context   the compiler has no way to guess which instance method   variable for which object    you are referring to  Though  you can always access it using an object reference

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I think it is worth pointing out that by the rules of the Java language the Java compiler inserts the equivalent of  this   when it notices that you re accessing instance methods or instance fields without an explicit instance   Of course  the compiler knows that it can only do this from within an instance method  which has a  this  variable  as static methods don t   Which means that when you re in an instance method the following are equivalent   instanceMethod    this instanceMethod      and these are also equivalent         instanceField        this instanceField    The compiler is effectively inserting the  this   when you don t supply a specific instance     This  pun intended  bit of  magic help  by the compiler can confuse novices  it means that instance calls and static calls sometimes appear to have the same syntax while in reality are calls of different types and underlying mechanisms   The instance method call is sometimes referred to as a method invocation or dispatch because of the behaviors of virtual methods supporting polymorphism  dispatching behavior happens regardless of whether you wrote an explicit object instance to use or the compiler inserted a  this     The static method call mechanism is simpler  like a function call in a non-OOP language   Personally  I think the error message is misleading  it could read  non-static method cannot be referenced from a static context without specifying an explicit object instance        What the compiler is complaining about is that it cannot simply insert the standard  this   as it does within instance methods  because this code is within a static method  however  maybe the author merely forgot to supply the instance of interest for this invocation   8212  say  an instance possibly supplied to the static method as parameter  or created within this static method     In short  you most certainly can call instance methods from within a static method  you just need to have and specify an explicit instance object for the invocation

User · Answer

You can t call something that doesn t exist   Since you haven t created an object  the non-static method doesn t exist yet   A static method  by definition  always exists

User · Answer

if a method is not static  that  tells  the compiler that the method requires access to instance-level data in the class   like a non-static field    This data would not be available unless an instance of the class has been created   So the compiler throws an error if you try to call the method from a static method   If in fact the method does NOT reference any non-static member of the class  make the method static   In Resharper  for example  just creating a non-static method that does NOT reference any static member of the class generates a warning message  This method can be made static

User · Answer

You can t call something that doesn t exist   Since you haven t created an object  the non-static method doesn t exist yet   A static method  by definition  always exists

User · Answer

I just realized  I think people shouldn t be exposed to the concept of  static  very early   Static methods should probably be the exception rather than the norm  Especially early on anyways if you want to learn OOP   Why start with an exception to the rule   That s very counter-pedagogical of Java  that the  first  thing you should learn is the public static void main thing   Few real Java applications have their own main methods anyways

User · Answer

The method you are trying to call is an instance-level method  you do not have an instance   static methods belong to the class  non-static methods belong to instances of the class

User · Answer

If we try to access an instance method from a static context   the compiler has no way to guess which instance method   variable for which object    you are referring to  Though  you can always access it using an object reference

User · Answer

I just realized  I think people shouldn t be exposed to the concept of  static  very early   Static methods should probably be the exception rather than the norm  Especially early on anyways if you want to learn OOP   Why start with an exception to the rule   That s very counter-pedagogical of Java  that the  first  thing you should learn is the public static void main thing   Few real Java applications have their own main methods anyways

User · Answer

The compiler actually adds an argument to non-static methods  It adds a this pointer reference  This is also the reason why a static method can not use this  because there is no object

User · Answer

if a method is not static  that  tells  the compiler that the method requires access to instance-level data in the class   like a non-static field    This data would not be available unless an instance of the class has been created   So the compiler throws an error if you try to call the method from a static method   If in fact the method does NOT reference any non-static member of the class  make the method static   In Resharper  for example  just creating a non-static method that does NOT reference any static member of the class generates a warning message  This method can be made static

User · Answer

You can t call something that doesn t exist   Since you haven t created an object  the non-static method doesn t exist yet   A static method  by definition  always exists

User · Answer

A static method relates an action to a type of object  whereas the non static method relates an action to an instance of that type of object   Typically it is a method that does something with relation to the instance   Ex   class Car might have a wash method  which would indicate washing a particular car  whereas a static method would apply to the type car

User · Answer

The method you are trying to call is an instance-level method  you do not have an instance   static methods belong to the class  non-static methods belong to instances of the class

User · Answer

if a method is not static  that  tells  the compiler that the method requires access to instance-level data in the class   like a non-static field    This data would not be available unless an instance of the class has been created   So the compiler throws an error if you try to call the method from a static method   If in fact the method does NOT reference any non-static member of the class  make the method static   In Resharper  for example  just creating a non-static method that does NOT reference any static member of the class generates a warning message  This method can be made static

User · Answer

The answers so far describe why  but here is a something else you might want to consider    You can can call a method from an instantiable class by appending a method call to its constructor    Object instance   new Constuctor   methodCall      or  primitive name   new Constuctor   methodCall      This is useful it you only wish to use a method of an instantiable class once within a single scope   If you are calling multiple methods from an instantiable class within a single scope  definitely create a referable instance

User · Answer

A static method relates an action to a type of object  whereas the non static method relates an action to an instance of that type of object   Typically it is a method that does something with relation to the instance   Ex   class Car might have a wash method  which would indicate washing a particular car  whereas a static method would apply to the type car

User · Answer

The simple reason behind this is that Static data members of parent class can be accessed  only if they are not overridden  but for instance non-static  data members or methods we need their reference and so they can only be called through an object

User · Answer

The method you are trying to call is an instance-level method  you do not have an instance   static methods belong to the class  non-static methods belong to instances of the class

User · Answer

The essence of object oriented programming is encapsulating logic together with the data it operates on    Instance methods are the logic  instance fields are the data  Together  they form an object   public class Foo       private String foo      public Foo String foo   this foo   foo        public getFoo    return this foo         public static void main String   args           System out println  getFoo                What could possibly be the result of running the above program   Without an object  there is no instance data  and while the instance methods exist as part of the class definition  they need an object instance to provide data for them   In theory  an instance method that does not access any instance data could work in a static context  but then there isn t really any reason for it to be an instance method  It s a language design decision to allow it anyway rather than making up an extra rule to forbid it

User · Answer

A static method relates an action to a type of object  whereas the non static method relates an action to an instance of that type of object   Typically it is a method that does something with relation to the instance   Ex   class Car might have a wash method  which would indicate washing a particular car  whereas a static method would apply to the type car

User · Answer

I just realized  I think people shouldn t be exposed to the concept of  static  very early   Static methods should probably be the exception rather than the norm  Especially early on anyways if you want to learn OOP   Why start with an exception to the rule   That s very counter-pedagogical of Java  that the  first  thing you should learn is the public static void main thing   Few real Java applications have their own main methods anyways

User · Answer

A static method relates an action to a type of object  whereas the non static method relates an action to an instance of that type of object   Typically it is a method that does something with relation to the instance   Ex   class Car might have a wash method  which would indicate washing a particular car  whereas a static method would apply to the type car

User · Answer

The method you are trying to call is an instance-level method  you do not have an instance   static methods belong to the class  non-static methods belong to instances of the class

User · Answer

I just realized  I think people shouldn t be exposed to the concept of  static  very early   Static methods should probably be the exception rather than the norm  Especially early on anyways if you want to learn OOP   Why start with an exception to the rule   That s very counter-pedagogical of Java  that the  first  thing you should learn is the public static void main thing   Few real Java applications have their own main methods anyways

[java] What is the reason behind "non-static method cannot be referenced from a static context"?

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