Global variables in Java

Question

How do you define Global variables in Java ?

Answer 1

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);