Question

I have two classes:

public class A{
   int money;
   ArrayList<Item> boughtItems;
   .
   .
   public void addBoughtItem(Item item){
      .
      .
   }
   public void adjustMoney(int adjustAmount){
      money = money + adjustamount;
   }
}

and another class :

public class B{
  ArrayList<Item> itemsForSale;
  .
  .
  public Item sellMeItem(Item item){
    .
    .
    return itemsForSale.get(...);
  }
}

Now in the main, I start an instance of class A, and an instance of class B.
inside class B, whenever I sell them an item, I want to update the purchasedItems and adjust the money of the class A instance in realtime (that means, inside the sellMeItem function of class B.)
Therefore I think I need the A instance to be global.
How can I reach the A object that is in the main from inside the class B sellMeItem function? (and class B instance is also in the main)

Answer 1

Therefore I think I need the A instance to be global.

Not really, you could simply pass a reference to your instance of A to B, for example via a constructor:

class B {

    private A a;

    public B(A a) {
        this.a = a;
    }
}

Then you can access a in B. And in your main:

A a = new A();
B b = new B(a);

However, the fact that the 2 classes are so intimately related probably means that the way you have split responsibilities among them is not optimal.