如何从另一个类更新一个类中的变量

Question

I realize this is probably a really basic question, but I can't figure it out.

Say I have this main class

public class Main{

    public static void main(String[] args){
        int a = 0;
        AddSomething.addOne(a);
        System.out.println("Value of a is: "+String.valueOf(a));
    }
}

Here is AddSomething class and addOne() method

public class AddSomething{

    public static void addOne(int a){

        a++;

    }
}

The addOne method is not adding anything

System.out.println("Value of a is: "+String.valueOf(a));
// Prints 0 not 1

How can I make Add class update variable a in Main class?

Answer 1

addOne receives a copy of a , so it can't change the a variable of your main method.

The only way to change that variable is to return a value from the method and assign it back to a :

a = Add.addOne(a);

...

public int addOne(int a){
    return ++a;
}

Answer 2

Thats becouse primitive types in java pass to methods by value. Only one way to do what you want is reassign variable, like :

public class Main{

    public static void main(String[] args){
        int a = 0;
        a = Add.addOne(a);
        System.out.println("Value of a is: "+String.valueOf(a));
    }
}

and

public class AddSomething{

    public static int addOne(int a){

    return a++;

    }
}

Answer 3

I know, Eran's answer is what you all need. But just to show another way, posting this answer.

public class Main
{
  static int a = 0;
  public static void main(String[] args)
  {
    AddSomething.addOne();
    System.out.println("Value of a is: "+a);
  }
}

And in AddSomething class..

public class AddSomething
{
    public static void addOne(){ Main.a++ };
}

AddSomething must be in same package as Main class, as int a has default access modifier.