是否可以从另一个类访问非静态类的实例

Question

lets say I have classA and classB.

I know that I cannot just call a non static variable or method of classA from classB because the system doesn't know which instance of classA I want to use. but is there a way to specify which instance?

something like this: in class AI declare a static variable which whould hold the some sort of ID or context to the specific instance of the class

class classA{
  static Instance instance 

  onCreate(){
    instance = thisInstance();
  }

  Method1(){
   }
}

then in class BI would refer to that instance like this:

  ClassA.instance.method1();

is something like this possible? if so, what is the exact syntax?

[Bonus]: if no, what is the simplest way invoke a method in a class from another class? I assume some sort of event handling would be required. (I come from the embedded c world)

Answer 1

Declare a static member in ClassA

public class ClassA {
    public static ClassA object = new ClassA();

    public void doStuff() {
        // do stuff
    }
}

Then in ClassB

public void someMethod() {
     ClassA.object.doStuff();
}

Answer 2

In class B, you can define:

Class B {
  private static ClassA instanceA = null; // By making it null, you can later confirm that the instance was successfully passed by making sure instanceA != null.

  /**
  * This method allows you to pass the instance of ClassA into B so you can use its non-static methods.
  */
  public static void setInstanceA(ClassA instance) {
    instanceA = instance;
  }
}