Question about static context when using classes

Question

hey there.
i'm pretty new to java an here's my problem:
public class kreise {
     public static void main (String[] args) {
         //creating the circles
         create a = new create(1,4,3);
         create b = new create(7,2,5);
         System.out.println(a);
         System.out.println(b);
         //diameters...unimportant right now
         getDiameter a_ = new getDiameter(a.radius);
         getDiameter b_ = new getDiameter(b.radius);
         System.out.println(a_);
         System.out.println(b_);
         //moving the circles
         double x = 2;
         double y = 9;
         double z = 3;
         a = create.move();
         System.out.println(a);
}creating a circle makes use of the class create which looks like this:
public class create {
    public double k1;
    public double k2;
    public double radius;
    public create(double x,double y,double r) {
        k1 = x;
        k2 = y;
        radius = r;
    public create move() {
        k1 = 1;
        k2 = 1;
        radius = 3;
        return new create (k1,k2,radius);
    public String toString() {
    return "Koordinaten: "+k1+" / "+k2+". Radius: "+radius;
}now that's all totally fine, but when i try to usw create.move to change the circles coordinates the compiler says that the non-static method move() can't be referenced from a static context. so far i've seen that my main() funktion MUST be static. when declaring the doubles k1, k2, and radius in create() static it works, but then of course when having created the second circle it overwrites the first one.
i pretty much have the feeling this is very much a standard beginner problem, but searching for the topic never really brought up my problem exactly. thanks in advance!

Answer 1

You can't access a non-static method from within a static context. So, you have to call move() from outside of the main method. main has to be static because, in short, at least one method has to be static because there haven't been any objects initialized when the program is started. There are more fundamental problems than with just the static context issue.
I'm confused by your code though. You call create.move(), but this would only be possible if move() was static, and from what I see, it's not. Now that's just one part of it. My second issue is that the logic behind the move() method is very messy. You shouldn't return a newly instantiated object, instead it should just change the fields of the current object. Also as a general rule, instance fields should be private; you have them as public, and this would be problematic because anything can access it.
Have you heard of getters and setters? That would be what I recommend.
Now, also, when you are "moving" it, you are basically creating a new circle with completely differently properties; in light of this, I've renamed it change(). Here would be my version of your code:
public class CircleTester {
    public static void main (String[] args)
         //Bad way to do it, but here's one way around it:
         new CircleTester().moveCircle();
    private void moveCircle()     //really a bad method, but for now, it'll do
        Circle a = new Circle(1,4,3);
        Circle b = new Circle(7,2,5);
        System.out.println(a);
        System.out.println(b);
        //diameters. Don't need to have a new getDiameter class
        double a_ = a.getRadius() * 2;     //Instead of doing * 2 each time, you could have a method that just returns the radius * 2
        double b_ = b.getRadius() * 2;
        System.out.println(a_);
        System.out.println(b_);
        //move the circle
        a.change(2,9,3);
        System.out.println(a);
public class Circle {
    private double k1;
    private double k2;
    private double radius;
    public Circle(double x,double y,double r)
        k1 = x;
        k2 = y;
        radius = r;
    public void change(int x, int y, int r)
        k1 = x;
        k2 = y;
        radius = r;
    public String toString()
         return "Koordinaten: "+k1+" / "+k2+". Radius: "+radius;
    public double getRadius()
         return radius;
}On another note, there is already a ellipse class: http://java.sun.com/j2se/1.5.0/docs/api/java/awt/geom/Ellipse2D.html