为什么在子类类构造函数中添加super（）不会产生错误？

<pre lang="java">interface Superclass {
 
    void doSmth();
}
 
class Subclass implements Superclass {
 
    Subclass() {
        super();//line 1
    }
 
    @Override
    public void doSmth() {
        System.out.println("Do something");
    }
}

I learned from a book mentioning, "Interfaces do not have constructors". But if that's the case then why there isn't a run-time error, while adding super() in the subclass constructor? 

我的尝试：

What I have tried:

<pre><pre lang="java"><pre>interface Superclass {
 
    void doSmth();
    //Superclass()
    //{}
}
 
class Subclass implements Superclass {
 
    Subclass() {
        super();
        System.out.println("oca.Subclass.<init>()");
    }
 
    @Override
    public void doSmth() {
        System.out.println("Do something");
    }
}
public class Q1
{
    public static void main(String[] args) {
        Superclass sc=new Subclass();//line 2
        Subclass sb=new Subclass();//line 3
    }
}

输出

output

oca.Subclass.<init>()
oca.Subclass.<init>()

Also, I learned that whenever there is an instantiation of a subclass, then implicitly compiler will add super() in the subclass's default constructor which in turn invokes the parent class constructor. So in this case too why didn't it produce a runtime error even? 
Also, there's a compile-time error when I declare a constructor for the interface. 
Could anyone help me understand this ?