How does a static method access instance variables?

Static methods can not directly access any instance variables or methods. But they can access them by using their object reference. Static methods may even access private instance variables via a object reference.

public class Program {
  private int count;
    public Program(int ballcount){
    count=ballcount;
  }

  public static void main(String argv[]){
    Program s = new Program(99);
    //System.out.println(count); //compile time error
    //add(10);  //compile time error
    System.out.println(s.count);
    s.add(10);
    System.out.println(s.count);
  }

  private void add(int num) {
    count += num;
  }
}

The output result is

99
109

All private instance variables are private to its class, they can be accessed by static methods or non-static method of the class.