Non-static variable cannot be referenced from a static context

Question

I ve written this test code   class MyProgram       int count   0      public static void main String   args                System out println count             But it gives the following error   Main java 6  error  non-static variable count cannot be referenced from a static context         System out println count                                  How do I get my methods to recognize my class variables

User · Answer

The first thing is to know the difference between an instance of a class, and the class itself. A class models certain properties, and the behaviour of the whole in the context of those properties. An instance will define specific values for those properties.
Anything bound to the static keyword is available in the context of the class rather than in the context of an instance of the class
As a corollary to the above
1. variables within a method can not be static
2. static fields, and methods must be invoked using the class-name e.g. MyProgram7.main(...)
The lifetime of a static field/method is equivalent to the lifetime of your application

E.g. Say, car has the property colour, and exhibits the behaviour 'motion'. An instance of the car would be a Red Volkswagen Beetle in motion at 25kmph.

Now a static property of the car would be the number of wheels (4) on the road, and this would apply to all cars.

HTH

User · Answer

This is bit diff to explain about static key word for all beginners  You wil get to know it clearly when you work more with Classes and Objects       Static   Static items can be called with Class Name  If you observe in codes  Some functions are directly called with Class names like  NamCls NamFnc     System out println      This is because NamFnc and println wil be declared using key word static before them       Non Static  Non Static items can be called with Class Variable If its not static  you need a variable of the class  put dot after the class variable and then call function   NamCls NamObjVar   new NamCls    NamObjVar NamFnc       Below code explains you neatly      Static and non Static function in class    public class NamCls       public static void main String   args                PlsPrnFnc  Tst Txt             NamCls NamObjVar   new NamCls            NamObjVar PrnFnc  Tst Txt               static void PlsPrnFnc String SrgPsgVal                System out println SrgPsgVal              void PrnFnc String SrgPsgVal                System out println SrgPsgVal                  Static and non Static Class inside a Class    public class NamCls       public static void main String   args                NamTicCls NamTicVaj   new NamTicCls            NamTicVaj PrnFnc  Tst Txt             NamCls NamObjVar   new NamCls            NamNicCls NamNicVar   NamObjVar new NamNicCls            NamNicVar PrnFnc  Tst Txt               static class NamTicCls               void PrnFnc String SrgPsgVal                        System out println SrgPsgVal                        class NamNicCls               void PrnFnc String SrgPsgVal                        System out println SrgPsgVal

User · Answer

The very basic thing is static variables or static methods are at class level  Class level variables or methods gets loaded prior to instance level methods or variables And obviously the thing which is not loaded can not be used  So java compiler not letting the things to be handled at run time resolves at compile time  That s why it is giving you error non-static things can not be referred from static context  You just need to read about Class Level Scope  Instance Level Scope and Local Scope

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You must understand the difference between a class and an instance of that class  If you see a car on the street  you know immediately that it s a car even if you can t see which model or type  This is because you compare what you see with the class  car   The class contains which is similar to all cars  Think of it as a template or an idea   At the same time  the car you see is an instance of the class  car  since it has all the properties which you expect  There is someone driving it  it has an engine  wheels   So the class says  all cars have a color  and the instance says  this specific car is red    In the OO world  you define the class and inside the class  you define a field of type Color  When the class is instantiated  when you create a specific instance   memory is reserved for the color and you can give this specific instance a color  Since these attributes are specific  they are non-static   Static fields and methods are shared with all instances  They are for values which are specific to the class and not a specific instance  For methods  this usually are global helper methods  like Integer parseInt     For fields  it s usually constants  like car types  i e  something where you have a limited set which doesn t change often    To solve your problem  you need to instantiate an instance  create an object  of your class so the runtime can reserve memory for the instance  otherwise  different instances would overwrite each other which you don t want    In your case  try this code as a starting block   public static void main  String   args        try               MyProgram7 obj   new MyProgram7             obj run  args             catch  Exception e                e printStackTrace                 instance variables here  public void run  String   args  throws Exception          put your code here     The new main   method creates an instance of the class it contains  sounds strange but since main   is created with the class instead of with the instance  it can do this  and then calls an instance method  run

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It is ClassLoader responsible to load the class files Let s see what happens when we write our own classes   Example 1     class StaticTest          static int a        int b        int c      Now we can see that class  StaticTest  has 3 fields But actually there is no existence of b c member variable But why      OK Lest s see  Here b c are instance variable Since instance variable gets the memory at the time of object creation  So here b c are not getting any memory yet  That s why there is no existence of b c  So There is only existence of a  For ClassLoader it has only one information about a  ClassLoader yet not recognize b c because it s object not instantiated yet   Let s see another example  Example 2   class StaticTest          public void display               System out println  Static Test                    public static void main String   cmd                  display                        Now if we try to compile this code compiler will give CE error  CE  non-static method display   cannot be referenced from a static context   Now For ClassLoader it looks like    class StaticTest          public static void main String   cmd                  display                        In Example 2 CE error is because we call non static method from a static context  So it is not possible for ClassLoader to recognize method display   at compile time So compile time error is occurred

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The static keyword modifies the lifecycle of a method or variable within a class  A static method or variable is created at the time a class is loaded  A method or variable that is not declared as static is created only when the class is instantiated as an object for example by using the new operator   The lifecycle of a class  in broad terms  is     the source code for the class is written creating a template or pattern or stamp which can then be used to create an object with the new operator using the class to make an instance of the class as an actual object and then when done with the object  destroy the object reclaiming the resources it is holding such as memory during garbage collection    In order to have an initial entry point for an application  Java has adopted the convention that the Java program must have a class that contains a method with an agreed upon or special name  This special method is called main    Since the method must exist whether the class containing the main method has been instantiated or not  the main   method must be declared with the static modifier so that as soon as the class is loaded  the main   method is available   The result is that when you start your Java application by a command line such as java helloworld a series of actions happen  First of all a Java Virtual Machine is started up and initialized  Next the helloworld class file containing the compiled Java code is loaded into the Java Virtual Machine  Then the Java Virtual Machine looks for a method in the helloworld class that is called main String    args   this method must be static so that it will exist even though the class has not actually been instantiated as an object  The Java Virtual Machine does not create an instance of the class by creating an object from the class  It just loads the class and starts execution at the main   method   So you need to create an instance of your class as an object and then you can access the methods and variables of the class that have not been declared with the static modifier  Once your Java program has started with the main   function you can then use any variables or methods that have the modifier of static since they exist as part of the class being loaded    However  those variables and methods of the class which are outside of the main   method which do not have the static modifier can not be used until an instance of the class has been created as an object within the main   method  After creating the object you can then use the variables and methods of the object  An attempt to use the variables and methods of the class which do not have the static modifier without going through an object of the class is caught by the Java compiler at compile time and flagged as an error   import java io     class HelloWorld       int myInt          this is a class variable that is unique to each object     static int myInt2      this is a class variable shared by all objects of this class      static void main  String    args               this is the main entry point for this Java application         System out println   Hello  World n            myInt2   14        able to access the static int         HelloWorld myWorld   new HelloWorld            myWorld myInt   32       able to access non-static through an object

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Static fields and methods are connected to the class itself and not its instances  If you have a class A  a  normal  method b  and a static method c  and you make an instance a of your class A  the calls to A c   and a b   are valid  Method c   has no idea which instance is connected  so it cannot use non-static fields   The solution for you is that you either make your fields static or your methods non-static  You main could look like this then   class Programm        public static void main String   args            Programm programm   new Programm            programm start               public void start                can now access non-static fields

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Now you can add use instances with in the method   public class Myprogram7      Scanner scan    int compareCount   0    int low   0    int high   0    int mid   0      int key   0      Scanner temp      int  list      String menu  outputString      int option   1      boolean found   false       private void readLine             private void findkey             private void printCount            public static void main String   args        Myprogram7 myprg new Myprogram7        myprg readLine        myprg findkey        myprg printCount

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Before you call an instance method or instance variable It needs a object Instance   When instance variable is called from static method compiler doesn t know which is the object this variable belongs to  Because static methods doesn t have an object  Only one copy always   When you call an instance variable or instance methods from instance method it refer the this object  It means the variable belongs to whatever object created and each object have it s own copy of instance methods and variables    Static variables are marked as static and instance variables doesn t have specific keyword

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To be able to access them from your static methods they need to be static member variables  like this   public class MyProgram7     static Scanner scan   new Scanner System in     static int compareCount   0    static int low   0    static int high   0    static int mid   0      static int key   0      static Scanner temp      static int  list      static String menu  outputString      static int option   1      static boolean found   false     public static void main  String  args  throws IOException

User · Answer

Let s analyze your program first   In your program  your first method is main    and keep it in mind it is the static method    Then you declare the local variable for that method  compareCount  low  high  etc     The scope of this variable is only the declared method  regardless of it being a static or non static method  So you can t use those variables outside that method  This is the basic error u made   Then we come to next point  You told static is killing you   It may be killing you but it only gives life to your program    First you must understand the basic thing   Static method calls only the static method and use only the static variable   Static variable or static method are not dependent on any instance of that class   i e  If you change any state of the static variable it will reflect in all objects of the class   Because of this you call it as a class variable or a class method  And a lot more is there about the  static  keyword  I hope now you get the idea  First change the scope of the variable and declare it as a static  to be able to use it in static methods    And the advice for you is  you misunderstood the idea of the scope of the variables and static functionalities  Get clear idea about that

User · Answer

I will try to explain the static thing to you  First of all static variables do not belong to any particular instance of the class  They are recognized with the name of the class  Static methods again do not belong again to any particular instance  They can access only static variables  Imagine you call MyClass myMethod   and myMethod is a static method  If you use non-static variables inside the method  how the hell on earth would it know which variables to use  That s why you can use from static methods only static variables  I repeat again they do NOT belong to any particular instance

[java] Non-static variable cannot be referenced from a static context

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