Calling a method from main method

Question

When I am calling a method directly from main method, it is not allowed. However, when I am calling the same method from the constructor of a class, it is allowed.

The allowed version;

public class App {

Integer firstVariable;
Integer secondVariable;

public static void main(String[] args) {
    App obj = new App(3, 2);
}

public App(Integer firstVariable, Integer secondVariable) {
    this.firstVariable = firstVariable;
    this.secondVariable = secondVariable;

    this.calculate(firstVariable, secondVariable);
}

public int calculate(int a, int b) {
    int result = a + b;

    return result;
}
}

The disallowed version;

public class App {

Integer firstVariable;
Integer secondVariable;

public static void main(String[] args) {
    App obj = new App(3, 2);

    obj.calculate(firstVariable, secondVariable);
}

public App(Integer firstVariable, Integer secondVariable) {
    this.firstVariable = firstVariable;
    this.secondVariable = secondVariable;

}

public int calculate(int a, int b) {
    int result = a + b;
    return result;
}
}

I know it is "Cannot make a static reference to the non-static field firstVariable" error. My question is; In both code blocks, the same thing is done but what is the difference between them?

Answer 1

The issue isn't your method. The issue is that your variables (the arguments that you're trying to pass) are being referenced from a static context.