How to use static variables in Processing?

Processing Beginners
3

I’m until now don’t know what is the actual use of private and final.

I think final is like const in C/C++. And private maybe like static.

This works fine for final and can’t be changed:

void print_static(){
  final int a = 100;
  println(a);
}

When I change it to private I get this error:

void print_static(){
  private int a;
  println(a);
}

Error:

Illegal modifier for parameter a; only final is permitted

So I guess I have to use a class to be able to use private. But I’m having some problems calling functions from a class. I don’t know what I’m missing here. I tried this:

public class class_example {
  
  public void print_hi() {
    println("Hi there!");
  }
  
}

But output window freezes and I get error this error:

NullPointerException