What is wrong in the Item class?

public class Item extends A implements B {
    private int x;
    public Item(int x, double z) {
        super(z);
        this.x = x;
    }
    
    public void start() {
        go();
    }
    
    @Override
    public String toString() {
        System.out.println(x);
    }
}
    
    public abstract class A {
        private int y;
        public A(int y) {
            this.y = y;
        }
        
        public void go() {
            System.out.println("Go!");
        }
        
        abstract void run();
    }
    
    public interface B {
        public void stop();
    }
}

I figured this problems:
toString() only prints but should return.
There is no implementation of method stop required by interface.