Instance variables or methods are ones that are dependent on a specific instance of a class. For example, every circle has its own radius, so the radius is dependent on a specific circle. Therefore, radius is an instance variable of the Circle class. None of the methods in the Math class, such as random, pow, sin, and cos, are dependent on a specific instance, therefore they are static methods. An example:
public class F {
int i;
static String s;
}
F f = new F();
System.out.println(f.i); //valid
System.out.println(f.s); //valid
System.out.println(F.i); //invalid
System.out.println(F.s); //valid
