Why is the Java main method static

Question

The method signature of a Java main   method is   public static void main String   args               Is there a reason for this method to be static

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It s just a convention  but probably more convenient than the alternative  With a static main  all you need to know to invoke a Java program is the name and location of a class  If it weren t static  you d also have to know how to instantiate that class  or require that the class have an empty constructor

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because  a static members are not part of any specific class and that main method  not requires to create its Object  but can still refer to all other classes

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As the execution start of a program from main   and and java is purely object oriented program where the object is declared inside main   that means main   is called before object creation so if main   would non static then to call it there would be needed a object because static means no need of object

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The protoype public static void main String    is a convention defined in the JLS       The method main must be declared public  static  and void  It must specify a formal parameter    8 4 1  whose declared type is array of String     In the JVM specification 5 2  Virtual Machine Start-up  we can read      The Java virtual machine starts up by creating an initial class  which is specified in an implementation-dependent manner  using the bootstrap class loader    5 3 1   The Java virtual machine then links the initial class  initializes it  and invokes the public class method void main String     The invocation of this method drives all further execution  Execution of the Java virtual machine instructions constituting the main method may cause linking  and consequently creation  of additional classes and interfaces  as well as invocation of additional methods    Funny thing  in the JVM specification it s not mention that the main method has to be static  But the spec also says that the Java virtual machine perform 2 steps before     links the initial class  5 4  Linking  initializes it  5 5  Initialization       Initialization of a class or interface consists of executing its class or interface initialization method    In 2 9  Special Methods    A class or interface initialization method is defined       A class or interface has at most one class or interface initialization method and is initialized    5 5  by invoking that method  The initialization method of a class or interface has the special name  lt clinit gt   takes no arguments  and is void    And a class or interface initialization method is different from an instance initialization method defined as follow       At the level of the Java virtual machine  every constructor written in the Java programming language  JLS   8 8  appears as an instance initialization method that has the special name  lt init gt      So the JVM initialize a class or interface initialization method and not an instance initialization method that is actually a constructor  So they don t need to mention that the main method has to be static in the JVM spec because it s implied by the fact that no instance are created before calling the main method

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If it wasn t  which constructor should be used if there are more than one   There is more information on the initialization and execution of Java programs available in the Java Language Specification

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Any method declared as static in Java belongs to the class itself   Again  static method of a particular class can be accessed only by referring to the class like Class name method name     So a class need not to be instantiated before accessing a static method   So the main   method is declared as static so that it can be accessed without creating an object of that class   Since we save the program with the name of the class where the main method is present  or from where the program should begin its execution  applicable for classes without a main   method   Advanced Level    So by the above mentioned way   Class name method name      the main method can be accessed   In brief when the program is compiled it searches for the main   method having String arguments like  main String args    in the class mentioned i e  by the name of the program   and since at the the beginning it has no scope to instantiate that class  so the main   method is declared as static

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I don t know if the JVM calls the main method before the objects are instantiated    But there is a far more powerful reason why the main   method is static    When JVM calls the main method of the class  say  Person   it invokes it by  Person main     You see  the JVM invokes it by the class name  That is why the main   method is supposed to be static and public so that it can be accessed by the JVM   Hope it helped  If it did  let me know by commenting

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Recently  similar question has been posted at Programmers SE   Why a static main method in Java and C   rather than a constructor      Looking for a definitive answer from a primary or secondary source for why did  notably  Java and C  decide to have a static method as their entry point     rather than representing an application instance by an instance of an Application class  with the entry point being an appropriate constructor     TL DR part of the accepted answer is      In Java  the reason of public static void main String   args  is that         Gosling wanted   the code written by someone experienced in C  not in Java    to be executed by someone used to running PostScript on NeWS                        nbsp    For C   the reasoning is transitively similar so to speak  Language designers kept the program entry point syntax familiar for programmers coming from Java  As C  architect Anders Hejlsberg puts it               our approach with C  has simply been to offer an alternative    to Java programmers

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The static key word in the main method is used because there isn t any instantiation that take place in the main method   But object is constructed rather than invocation as a result we use the static key word in the main method  In jvm context memory is created when class loads into it And all static members are present in that memory  if we make the main static now it will be in memory and can be accessible to jvm  class main      so we can call the main method with out need of even need for heap been created

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Basically we make those DATA MEMBERS and MEMBER FUNCTIONS as STATIC which are not performing any task related to an object  And in case of main method  we are making it as an STATIC because it is nothing to do with object  as the main method always run whether we are creating an object or not

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Because otherwise  it would need an instance of the object to be executed  But it must be called from scratch  without constructing the object first  since it is usually the task of the main   function  bootstrap   to parse the arguments and construct the object  usually by using these arguments program parameters

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The true entry point to any application is a static method  If the Java language supported an instance method as the  entry point   then the runtime would need implement it internally as a static method which constructed an instance of the object followed by calling the instance method   With that out of the way  I ll examine the rationale for choosing a specific one of the following three options    A static void main   as we see it today  An instance method void main   called on a freshly constructed object  Using the constructor of a type as the entry point  e g   if the entry class was called Program  then the execution would effectively consist of new Program       Breakdown   static void main     Calls the static constructor of the enclosing class  Calls the static method main      void main     Calls the static constructor of the enclosing class  Constructs an instance of the enclosing class by effectively calling new ClassName    Calls the instance method main      new ClassName     Calls the static constructor of the enclosing class  Constructs an instance of the class  then does nothing with it and simply returns     Rationale   I ll go in reverse order for this one   Keep in mind that one of the design goals of Java was to emphasize  require when possible  good object-oriented programming practices  In this context  the constructor of an object initializes the object  but should not be responsible for the object s behavior  Therefore  a specification that gave an entry point of new ClassName   would confuse the situation for new Java developers by forcing an exception to the design of an  ideal  constructor on every application   By making main   an instance method  the above problem is certainly solved  However  it creates complexity by requiring the specification to list the signature of the entry class s constructor as well as the signature of the main   method   In summary  specifying a static void main   creates a specification with the least complexity while adhering to the principle of placing behavior into methods  Considering how straightforward it is to implement a main   method which itself constructs an instance of a class and calls an instance method  there is no real advantage to specifying main   as an instance method

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because  a static members are not part of any specific class and that main method  not requires to create its Object  but can still refer to all other classes

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The main   method in C    C  and Java are static Because they can then be invoked by the runtime engine without having to instantiate any objects then the code in the body of main   will do the rest

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This is just convention   In fact  even the name main    and the arguments passed in are purely convention   When you run java exe  or javaw exe on Windows   what is really happening is a couple of Java Native Interface  JNI  calls   These calls load the DLL that is really the JVM  that s right - java exe is NOT the JVM    JNI is the tool that we use when we have to bridge the virtual machine world  and the world of C  C    etc     The reverse is also true - it is not possible  at least to my knowledge  to actually get a JVM  running without using JNI   Basically  java exe is a super simple C application that parses the command line  creates a new String array in the JVM to hold those arguments  parses out the class name that you specified as containing main    uses JNI calls to find the main   method itself  then invokes the main   method  passing in the newly created string array as a parameter   This is very  very much like what you do when you use reflection from Java - it just uses confusingly named native function calls instead   It would be perfectly legal for you to write your own version of java exe  the source is distributed with the JDK  and have it do something entirely different   In fact  that s exactly what we do with all of our Java-based apps   Each of our Java apps has its own launcher   We primarily do this so we get our own icon and process name  but it has come in handy in other situations where we want to do something besides the regular main   call to get things going  For example  in one case we are doing COM interoperability  and we actually pass a COM handle into main   instead of a string array    So  long and short   the reason it is static is b c that s convenient   The reason it s called  main  is that it had to be something  and main   is what they did in the old days of C  and in those days  the name of the function was important    I suppose that java exe could have allowed you to just specify a fully qualified main method name  instead of just the class  java com mycompany Foo someSpecialMain  - but that just makes it harder on IDEs to auto-detect the  launchable  classes in a project

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Applets  midlets  servlets and beans of various kinds are constructed and then have lifecycle methods called on them  Invoking main is all that is ever done to the main class  so there is no need for a state to be held in an object that is called multiple times  It s quite normal to pin main on another class  although not a great idea   which would get in the way of using the class to create the main object

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The public keyword is an access modifier  which allows the programmer to control the visibility of class members  When a class member is preceded by public  then that member may be accessed by code outside the class in which it is declared   The opposite of public is private  which prevents a member from being used by code defined outside of its class   In this case  main   must be declared as public  since it must be called by code outside of its class when the program is started   The keyword static allows main   to be called without having to instantiate a particular instance of the class  This is necessary since main   is called by the Java interpreter before any objects are made   The keyword void simply tells the compiler that main   does not return a value

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From java sun com  there s more information on the site        The main method is static to give the Java VM interpreter a way to start the class without creating an instance of the control class first  Instances of the control class are created in the main method after the program starts    My understanding has always been simply that the main method  like any static method  can be called without creating an instance of the associated class  allowing it to run before anything else in the program   If it weren t static  you would have to instantiate an object before calling it-- which creates a  chicken and egg  problem  since the main method is generally what you use to instantiate objects at the beginning of the program

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Let s simply pretend  that static would not be required as the application entry point   An application class would then look like this   class MyApplication       public MyApplication               Some init code here           public void main String   args              real application code here           The distinction between constructor code and main method is necessary because in OO speak a constructor shall only make sure  that an instance is initialized properly  After initialization  the instance can be used for the intended  service   Putting the complete application code into the constructor would spoil that   So this approach would force three different contracts upon the application    There must be a default constructor  Otherwise  the JVM would not know which constructor to call and what parameters should be provided   There must be a main method1  Ok  this is not surprising  The class must not be abstract  Otherwise  the JVM could not instantiate it    The static approach on the other hand only requires one contract     There must be a main method1    Here neither abstract nor multiple constructors matters   Since Java was designed to be a simple language for the user it is not surprising that also the application entry point has been designed in a simple way using one contract and not in a complex way using three independent and brittle contracts   Please note  This argument is not about simplicity inside the JVM or inside the JRE  This argument is about simplicity for the user     1Here the complete signature counts as only one contract

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I think the keyword  static  makes the main method a class method  and class methods have only one copy of it and can be shared by all  and also  it does not require an object for reference  So when the driver class is compiled the main method can be invoked   I m just in alphabet level of java  sorry if I m wrong

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there is the simple reason behind it that is because object is not required to call static method   if It were non-static method  java virtual machine creates object first then call main   method that will lead to the problem of extra memory allocation

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static - When the JVM makes a call to the main method there is no object that exists for the class being called therefore it has to have static method to allow invocation from class

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main method always needs to be static because at RunTime JVM does not create any object to call main method and as we know in java static methods are the only methods which can be called using class name so main methods always needs to be static   for more information visit this video  https   www youtube com watch v Z7rPNwg-bfk amp feature youtu be

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main   is static because  at that point in the application s lifecycle  the application stack is procedural in nature due to there being no objects yet instantiated   It s a clean slate  Your application is running at this point  even without any objects being declared  remember  there s procedural AND OO coding patterns   You  as the developer  turn the application into an object-oriented solution by creating instances of your objects and depending upon the code compiled within   Object-oriented is great for millions of obvious reasons  However  gone are the days when most VB developers regularly used keywords like  goto  in their code   goto  is a procedural command in VB that is replaced by its OO counterpart  method invocation   You could also look at the static entry point  main  as pure liberty  Had Java been different enough to instantiate an object and present only that instance to you on run  you would have no choice BUT to write a procedural app  As unimaginable as it might sound for Java  it s possible there are many scenarios which call for procedural approaches   This is probably a very obscure reply  Remember   class  is only a collection of inter-related code   Instance  is an isolated  living and breathing autonomous generation of that class

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It is just a convention  The JVM could certainly deal with non-static main methods if that would have been the convention  After all  you can define a static initializer on your class  and instantiate a zillion objects before ever getting to your main   method

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When you execute the Java Virtual Machine  JVM  with the java command    java ClassName argument1 argument2       When you execute your application  you specify its class name as an argument to the java command  as above  the JVM attempts to invoke the main method of the class you specify         at this point  no objects of the class have been created       Declaring main as static allows the JVM to invoke main without creating   an instance of the class    let s back to the command   ClassName is a command-line argument to the JVM that tells it which class to execute  Following the ClassName  you can also specify a list of Strings  separated by spaces  as command-line arguments that the JVM will pass to your application  -Such arguments might be used to specify options  e g   a filename  to run the application- this is why there is a parameter called String   args in the main   References Java    How To Program  Early Objects   Tenth Edition

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Static methods don t require any object   It runs directly so main runs directly

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Let me explain these things in a much simpler way   public static void main String args      All Java applications  except applets  start their execution from main     The keyword public is an access modifier which allows the member to be called from outside the class   static is used because it allows main   to be called without having to instantiate a particular instance of that class   void indicates that main   does not return any value

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Why public static void main String   args    This is how Java Language is designed and Java Virtual Machine is designed and written  Oracle Java Language Specification Check out Chapter 12 Execution - Section 12 1 4 Invoke Test main   Finally  after completion of the initialization for class Test  during which other consequential loading  linking  and initializing may have occurred   the method main of Test is invoked  The method main must be declared public  static  and void  It must accept a single argument that is an array of strings  This method can be declared as either public static void main String   args   or public static void main String    args    Oracle Java Virtual Machine Specification Check out Chapter 2 Java Programming Language Concepts - Section 2 17 Execution   The Java virtual machine starts execution by invoking the method main of some specified class and passing it a single argument  which is an array of strings  This causes the specified class to be loaded    2 17 2   linked    2 17 3  to other types that it uses  and initialized    2 17 4   The method main must be declared public  static  and void   Oracle OpenJDK Source Download and extract the source jar and see how JVM is written  check out    launcher java c  which contains native C code behind command java  -options  class  args            Get the application s main class                 if  jarfile    0        mainClassName   GetMainClassName env  jarfile                 mainClass   LoadClass env  classname       if mainClass    NULL       exception occured                 Get the application s main method    mainID     env - gt GetStaticMethodID env  mainClass   quot main quot                                       quot   Ljava lang String  V quot                      Make sure the main method is public        jint mods      jmethodID mid      jobject obj     env - gt ToReflectedMethod env  mainClass                                              mainID  JNI TRUE                Build argument array    mainArgs   NewPlatformStringArray env  argv  argc   if  mainArgs    NULL        ReportExceptionDescription env       goto leave        Invoke main method       env - gt CallStaticVoidMethod env  mainClass  mainID  mainArgs

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If the main method would not be static  you would need to create an object of your main class from outside the program  How would you want to do that

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The main method of the program has the reserved word static which means it is allowed to be used in the static context  A context relates to the use of computer memory during the running of the program  When the virtual machine loads a program  it creates the static context for it  allocating computer memory to store the program and its data  etc   A dynamic context is certain kind of allocation of memory which is made later  during the running of the program  The program would not be able to start if the main method was not allowed to run in the static context

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The public static void keywords mean the Java virtual machine  JVM  interpreter can call the program s main method to start the program  public  without creating an instance of the class  static   and the program does not return data to the Java VM interpreter  void  when it ends    Source  Essentials  Part 1  Lesson 2  Building Applications

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static indicates that this method is class method and called without requirment of any object of class

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It is just a convention as we can see here      The method must be declared public and static  it must not return any   value  and it must accept a String array as a parameter  By default    the first non-option argument is the name of the class to be invoked    A fully-qualified class name should be used  If the -jar option is   specified  the first non-option argument is the name of a JAR archive   containing class and resource files for the application  with the   startup class indicated by the Main-Class manifest header    http   docs oracle com javase 1 4 2 docs tooldocs windows java html description

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The method is static because otherwise there would be ambiguity  which constructor should be called   Especially if your class looks like this   public class JavaClass    protected JavaClass int x      public void main String   args           Should the JVM call new JavaClass int    What should it pass for x   If not  should the JVM instantiate JavaClass without running any constructor method   I think it shouldn t  because that will special-case your entire class - sometimes you have an instance that hasn t been initialized  and you have to check for it in every method that could be called   There are just too many edge cases and ambiguities for it to make sense for the JVM to have to instantiate a class before the entry point is called   That s why main is static   I have no idea why main is always marked public though

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What is the meaning of public static void main String args       public is an access specifier meaning anyone can access invoke it such as JVM Java Virtual Machine  static allows main   to be called before an object of the class has been created  This is neccesary because main   is called by the JVM before any objects are made  Since it is static it can be directly invoked via the class   class demo           private int length      private static int breadth      void output            length 5          System out println length              static void staticOutput            breadth 10           System out println breadth              public static  void main String args             demo d1 new demo            d1 output       Note here output   function is not static so here            we need to create object         staticOutput       Note here staticOutput   function is  static so here            we needn t to create object Similar is the case with main            Although          demo staticOutput     Works fine         d1 staticOutput     Works fine              Similarly  we use static sometime for user defined methods so that we need not to make objects  void indicates that the main   method being declared  does not return a value  String   args specifies the only parameter in the main   method   args - a parameter which contains an array of objects of class type String

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Before the main method is called  no objects are instantiated  Having the static keyword means the method can be called without creating any objects first

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It s a frequently asked question why main   is static in Java   Answer  We know that in Java  execution starts from main   by JVM  When JVM executes  main   at that time  the class which contains main   is not instantiated so we can t call a nonstatic method without the reference of it s object  So to call it we made it static  due to which the class loader loads all the static methods in JVM context memory space from where JVM can directly call them

[java] Why is the Java main method static?

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