Global variables in Java

Question

How do you define Global variables in Java

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There is no such thing as a truly global variable in Java. Every static variable must belong to some class (like System.out), but when you have decided which class it will go in, you can refer to it from everywhere loaded by the same classloader.

Note that static variables should always be protected when updating to avoid race conditions.

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Lots of good answers  but I want  to give this example as it s considered the more proper way to access variables of a class by another class  using getters and setters   The reason why you use getters and setters this way instead of just making the variable public is as follows  Lets say your var is going to be a global parameter that you NEVER want someone to change during the execution of your program  in the case when you are developing code with a team   something like maybe the URL for a website  In theory this could change and may be used many times in your program  so you want to use a global var to be able to update it all at once  But you do not want someone else to go in and change this var  possibly without realizing how important it is   In that case you simply do not include a setter method  and only include the getter method   public class Global      private static int var   5       public static int getVar            return Global var               If you do not want to change the var ever then do not include this     public static void setVar int var           Global var   var

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You are better off using dependency injection   public class Globals       public int a      public int b     public class UsesGlobals       private final Globals globals      public UsesGlobals Globals globals            this globals   globals

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In general  Java doesn t have any global variables  Other than local variables  all variables comes under the scope of any class defined in the program  We can have static variables to have the scope of global variables

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Get the access of global while retaining priveleges     You can access variables in one class from another  with provisions     The primitive must be protected or no modifier  seen in example       the first class public class farm     int eggs     an integer to be set by constructor   fox afox     declaration of a fox object       the constructor inits   farm        eggs   4      afox   new fox       an instance of a fox object         show count of eggs before the fox arrives     System out println  Count of eggs before      eggs           call class fox  afox method  pass myFarm as a reference     afox stealEgg this           show the farm class  myFarm  primitive value     System out println  Count of eggs after       eggs           end constructor    public static void main String   args           instance of a farm class object     farm myFarm   new farm             end main       end class     the second class public class fox        theFarm is the myFarm object instance      any public  protected  or  no modifier  variable is accessible   void stealEgg farm theFarm   --theFarm eggs          end class

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Creating an independent file  eg  Example java to use the 1st solution  is just fine  You can do that also within the app  if e g  the global variables are special to your current app  etc    Create a class at the beginning and declare your variables in there   class Globals     static int month number    static String month name      You can then access these variables -- using them as  Globals month number   etc  -- from averywhere in your app

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Generally Global variable  I assume you are comparing it with C Cpp  define as public static final  like  class GlobalConstant      public static final String CODE     cd       ENUMs are also useful in such scenario    For Example  Calendar JANUARY

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Nothing should be global  except for constants   public class MyMainClass       public final static boolean DEBUGMODE true      Put this within your main class   In other  java files  use it through   if MyMainClass DEBUGMODE  System out println  Some debugging info      Make sure when you move your code off the cutting room floor and into release you remove or comment out this functionality   If you have a workhorse method  like a randomizer  I suggest creating a  Toolbox  package   All coders should have one  then whenever you want to use it in a  java  just import it

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To define Global Variable you can make use of static Keyword  public class Example       public static int a      public static int b      now you can access a and b from anywhere by calling  Example a   Example b

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If you need to update global property  a simple getter setter wrapper class can be used as global variable  A typical example is shown below   public class GlobalHolder        private static final GlobalHolder INSTANCE   new GlobalHolder         private volatile int globalProperty       public static GlobalHolder getInstance             return INSTANCE             public int getGlobalProperty             return globalProperty             public void setGlobalProperty int globalProperty            this globalProperty   globalProperty             public static void main String   args            GlobalHolder getInstance   setGlobalProperty 10           System out println GlobalHolder getInstance   getGlobalProperty

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You don t  That s by design  You shouldn t do it even if you could    That being said you could create a set of public static members in a class named Globals    public class Globals      public static int globalInt   0             but you really shouldn t     Seriously    don t do it

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public class GlobalImpl        public static int global   5       you can call anywhere you want   GlobalImpl global    5

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There is no global variable in Java  Nevertheless  what we do have is a static keyword and that is all we need  Nothing exists outside of class in Java  The static keyword represents a class variable that  contrary to instance variable  only has one copy and that transcends across all the instances of that class created  which means that its value can be changed and accessed across all instances at any point   If you need a global variable which can be accessed beyond scopes  then this is the variable that you need  but its scope exists only where the class is  and that will be all

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Going by the concept  global variables  also known as instance variable are the class level variables i e   they are defined inside a class but outside methods  In order to make them available completely and use them directly provide the static keyword   So if i am writing a program for simple arithmetical operation and it requires a number pair then two instance variables are defined as such   public class Add       static int a      static int b       static int c       public static void main String arg              c sum            System out println  Sum is    c               static int sum            a 20         b 30         return a b              Output  Sum is  50   Moreover using static keyword prior to the instance variables enable us not to specify datatypes for same variables again and again  Just write the variable directly

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Another way is to create an interface like this   public interface GlobalConstants     String name    Chilly Billy     String address    10 Chicken head Lane       Any class that needs to use them only has to implement the interface   public class GlobalImpl implements GlobalConstants     public GlobalImpl            System out println name

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To define Global Variable you can make use of static Keyword  public final class Tools     public static int a    public static int b      now you can access a and b from anywhere by calling  Tools a  Tools b    Yoy are right   specially in J2ME      You can avoid NullPointerException by putting inside your MidLet constructor  proggy initialization  this line of code   new Tools      This ensures that Tools will be allocated before any instruction that uses it   That s it

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As you probably guess from the answer there is no global variables in Java and the only thing you can do is to create a class with static members   public class Global       public static int a      You can use it with Global a elsewhere  However if you use Java 1 5 or better you can use the import static magic to make it look even more as a real global variable   import static test Global     public class UseGlobal       public void foo             int i   a            And voil     Now this is far from a best practice so as you can see in the commercials  don t do this at home

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very simple  class UseOfGlobal   private static int a  private static int b     but it is always good to have local variables defined inside method blocks where ever possible

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There are no global variables in Java  but there are global classes with public fields  You can use static import feature of java 5 to make it look almost like global variables

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Truly speaking there is not a concept of  GLOBAL  in a java OO program  Nevertheless there is some truth behind your question because there will be some cases where you want to run a method at any part of the program  For example---random   method in Phrase-O-Matic app it is a method should be callable from anywhere of a program   So in order to satisfy the things like Above  We need to have Global-like variables and methods   TO DECLARE A VARIABLE AS GLOBAL    1 Mark the variable as public static final While declaring    TO DECLARE A METHOD AS GLOBAL    1  Mark the method as public static While declaring    Because I declared global variables and method as static you can call them anywhere you wish by simply with the help of following code  ClassName X  NOTE  X can be either method name or variable name as per the requirement and ClassName is the name of the class in which you declared them

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without static this is possible too   class Main     String globalVar    Global Value      class Class1       Class1           System out println  Class1    globalVar         globalVar      - changed           class Class2       Class2           System out println  Class2    globalVar            public static void main String   args          Main m   new Main        m mainCode          void mainCode         Class1 o1   new Class1        Class2 o2   new Class2              Output  Class1  Global Value Class2  Global Value - changed

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Understanding the problem  I consider the qualification of global variable as a variable that could be accessed and changed anywhere in the code without caring about static instance call or passing any reference from one class to another   Usually if you have class A  public class A       private int myVar       public A int myVar            this myVar   myVar             public int getMyVar             return myVar             public void setMyVar int mewVar            this myVar   newVar            and want to access and update myvar in a class B   public class B       private A a       public void passA A a           this a   a             public void changeMyVar int newVar           a setMyvar newVar             you will need to have a reference of an instance of the class A and update the value in the class B like this   int initialValue   2  int newValue   3  A a   new A initialValue   B b   new B    b passA a   b changeMyVar newValue   assertEquals a getMyVar   newValue      true   Solution  So my solution to this   even if i m not sure if it s a good practice   is to use a singleton    public class Globals       private static Globals globalsInstance   new Globals         public static Globals getInstance             return globalsInstance             private int myVar   2       private Globals                public int getMyVar             return myVar             public void setMyVar int myVar            this myVar   myVar            Now you can get the Global unique instance anywhere with   Globals globals   Globals getInstance       and read and write to myVar with the getter and setter like  int myVar   globals getMyVar    global setMyVar 3

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Object-Oriented Programming is built with the understanding that the scope of variables is closely exclusive to the class object that encapsulates those variables    The problem with creating  global variables  is that it s not industry standard for Java  It s not industry standard because it allows multiple classes to manipulate data asyncronized  if you re running a multi-threaded application  this gets a little more complicated and dangerous in terms of thread-safety  There are various other reasons why using global variables are ineffective  but if you want to avoid that  I suggest you resort to Aspect-Oriented Programming    Aspect-Oriented Programming negates this problem by putting the parent class in charge of the scope through something called  advices   which adds additional behavior to the code without actually modifying it  It offers solutions to cross-cutting concerns  or global variable usage    Spring is a Java framework that utilizes AOP  and while it is traditionally used for web-applications  the core application can be used universally throughout the Java framework  8 0 included   This might be a direction you want to explore more

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public class GlobalClass        public static int x   37       public static String s    aaa       This way you can access them with GlobalClass x and GlobalClass s

[java] Global variables in Java

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