Question

My Assumptions:

1. Static method cannot cannot call non-static methods.
2. Constructors are kind of a method with no return type.

Given this example…

    public class Main {
        public static void main(String[] args) {
            Main p = new Main();  // constructor call
            k();  // [implicit] `this` reference
        }

        protected Main() {
            System.out.print("1234");
        }

        protected void k() {
        }
    }

• this line prints 1234: Main p = new Main()
• this line throws an Exception: k()

Why did the example code do those two things? Don’t they conflict with my above Assumptions? Are my Assumptions correct?

Answer 1

1 – Static method cannot cannot call non-static methods.

Sure they can, but they need an object to call the method on.

In a static method, there’s no this reference available, so foo() (which is equivalent to this.foo()) is illegal.

2 – Constructors are kind of a method with no return type.

If they should be compared to methods, I would say constructors are closer to non-static methods (since there is indeed a this reference inside a constructor).

Given this view, it should be clear to you why a static method can call a constructor without any problems.

---

So, to sum it up:

Main p = new Main();

is okay, since new Main() does not rely on any existing object.

k();

is not okay since it is equivalent to this.k() and this is not available in your (static) main method.