non-static global variable in java

Question

Is there any way to create a variable which i can change in class A and by these changes affect what will happen in another class B. Hope you understand.

Something like this:

class A{
public int var = 0;

}

And use value of variable var like this:

class B{
if(var == 0)
   {
    System.out.println("right now, var is equal 0");
   }
else if(var == 1)
   {
    System.out.println("right now, var is equal 1");
   }
}

Also as you can see, var can't be static because i need to change it's value during run of app.

Answer 1

I would do it by having a reference of class B in class A so that A could also change B when it needed to. For example:

public class A {

    private int var;
    private B b;

    public A(B b) {
        this.var = 0;
        this.b = b;
    }

    public void set(final int var) {
        this.var = var;
        b.set(var);
    }

}

public class B {

    private int var = 0;

    public void set(final int var) {
        this.var = var;
    }

}

Answer 2

You can use the observer-observed pattern.

public class A extends Observable {

    private int var = 0;

    public void setVar(int val) {
        this.var = val;
        notifyObservers();
    }

}

public class B implements Observer {

    public void init(A a) {
        a.addObserver(this);
    }

    @Override
    public void onUpdate(Observable obs, Object arg) {
        // Do something when A is updated
        if(var == 0) System.out.println("right now, var is equal 0");
        else if(var == 1) System.out.println("right now, var is equal 1");
    }

}

The way it works is that A becomes Observable , which means that other classes can be updated when something changes in A (the other classes are notified by A calling notifyObservers() ). The onUpdate() method in the observer is then called with the Observable (here A ) as the first argument. If you call notifyObservers() with an Object argument, the Object argument in onUpdate() will use that value.

Here is the control flow:

A : setVar -> A : notifyObservers -> B : onUpdate

Answer 3

Although Observer pattern suggested by other answer is actually a cleaner approach, given that OP seems don't even have proper understanding on basic concepts of Java as an OOP (eg what an object instance is), I believe what he is looking for is something even more basic, which is a reference to another object:

class A {
    private int value = 0;

    // getters and setters 
}

class B {
    private A a;

    // using constructor to have object reference populated
    // is only ONE OF THE WAYS
    public B(A a) {
        this.a = a;
    }

    public void foo() {
        System.out.println("my referred A value " + this.a.getValue();
    }
}

Of course you need to properly construct them. Somewhere in your code:

A a = new A();
B b = new B(a);
b.foo();
a.setValue(100);
b.foo();