如何解决“静态方法应以静态方式访问”警告

我正在翻阅这本书 Just Java 2 但我显然缺少一些基本的东西。这是两个独立的项目。我为第二个生成了一个JAR,并将其添加到第一个的构建路径中。将打印正确的区域,但编译器会生成这些警告。如何解决这些问题?

// -----------------------------------------------------------
// Testing.java
// -----------------------------------------------------------
public class Testing {
    public static void main(String[] args) {
        RectangleDFC r = new RectangleDFC(3, 4);
        System.out.println(r.Area());
            // WARNING: The static method Area() from the type RectangleDFC
            //          should be accessed in a static way
        r.SetSides (10, 10);
            // WARNING: The static method SetSides(int, int) from the type
            //          RectangleDFC should be accessed in a static way
        System.out.println(r.Area());
            // WARNING: The static method Area() from the type RectangleDFC
            //          should be accessed in a static way
    }
}
// -----------------------------------------------------------
// RectangleDFC.java
// -----------------------------------------------------------
public class RectangleDFC {
    int side1;
    int side2;
    RectangleDFC(int s1, int s2) {
        SetSides(s1, s2);
    }
    public void SetSides(int s1, int s2) {
        side1 = s1;
        side2 = s2;
    }
    public int Area() {
        return side1 * side2;
    }
}