Global variables in Java

Question

How do you define Global variables in Java ?

Answer 1

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;

Answer 2

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.

Answer 3

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);
  }
}

Answer 4

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;
    }
}

Answer 5

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;
    }
}

Answer 6

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.

Answer 7

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.

Answer 8

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!

Answer 9

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.

Answer 10

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);

Answer 11

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

Answer 12

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());
    }
}

Answer 13

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.

Answer 14

public class GlobalImpl {   

 public static int global = 5;

}

you can call anywhere you want:

GlobalImpl.global // 5

Answer 15

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 voilà!

Now this is far from a best practice so as you can see in the commercials: don't do this at home

Answer 16

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.

Answer 17

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)

Answer 18

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.

Answer 19

To allow an unqualified access to static members of another class, you can also do a static import:

import static my.package.GlobalConstants;

Now, instead of print(GlobalConstants.MY_PASSWORD);
you can use the Constant directly: print(MY_PASSWORD);

See What does the "static" modifier after "import" mean? to decide about.

And consider the answer of Evan Lévesque about interfaces to carry the Constants.

Answer 20

// 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

Answer 21

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.

Answer 22

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.

Answer 23

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
*/

Answer 24

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.

Answer 25

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!