Java program compilation failed - class

Question

While studying interface I came along this weird behaviour
When I am running this
int num=20;
public void sound();
public void eat();
}
class Dog implements Animal{
public void sound(){
System.out.println("Wooof!!!!!!!");
}
public void eat(){
System.out.println("Food");
}
}
public class Main{
public static void main(String[]args){
Dog dog=new Dog();
dog.sound();
dog.eat();
System.out.println(Dog.num);
//System.out.println(Dog.num1);
}
}
It runs fine while If I declare a no static variable with same name i.e. num as of the one in interface like this
interface Animal{
int num=20;
public void sound();
public void eat();
}
class Dog implements Animal{
int num=10;
public void sound(){
System.out.println("Wooof!!!!!!!");
}
public void eat(){
System.out.println("Food");
}
}
public class Main{
public static void main(String[]args){
Dog dog=new Dog();
dog.sound();
dog.eat();
System.out.println(Dog.num);
//System.out.println(Dog.num1);
}
}
It gives this error Main.java:22: error: non-static variable num cannot be referenced from a static context
My question was since one from interface is static and is of class level why the child class i.e. Dog compilation fails when I declare a non static instance level variable.

Answer 1

The problem is your are trying to access Dog class into System.out.println(). Not the variable dog where the class has been initialized.
If variable num is not static is impossible to read it without initialize the class, so if you want to do Dog.num, that variable has to be static.
If you don't add the variable into Dog class, the value will be declared in the interface, but if the variable exists in the class, the compiler will try read that.