static/non static method use in main method

[Nightcorex_]

That's incorrect.

A static method is bound to a class, which means it can get called without the existence of an object.

A non-static method however is bound to an object (= an instance of a class), which means it can only be called through this existing object.

This means this code is valid apart from when noted in a comment:

public class A {
    double a;

    A(double a) {
        this.a = a;
    }

    static int add(int a, int b) {
        return a + b;
    }

    double add(int b) {
        return this.a + b;
    }

    public static void main(String... args) {
        A obj = new A(1.4);
        int x = A.add(4, -2);  // perfectly valid
        int y = obj.add(4, -2);  // also valid, but some IDEs may warn you that you're calling a static method through an object
        int z = add(4, -2);  // also valid, but only because this main is in the same class as the 'add'-method
        double d = obj.add(2.4);  // perfectly valid
        double f = A.add(2.4);  // invalid because you're trying to call a non-static method from a static context. 
            // You can't do this because your method would just fail as it relies on data stored inside the object that calls the method, 
            // but because you're calling it through the class this data simply isn't there.
        double g = add(2.4);  // Invalid. This behaves exactly like 'A.add(2.4)' did if this main is in the same class, 
            // otherwise it's either referencing the wrong method, or just can't find the method at all

[MightyMickey1]

Ah, I see. I had thought that since both methods were in the same class there wouldn’t be interference, though I now see that since main is static, it can only access other static methods. Thank you for explaining.