Is there a time when you use Interface instead of an abstract class? And is there the opposite?

Abstract classes can share states or functions. The interface only serves to provide status or functionality. I mean, an abstract class is an abstract class if you have any abstract method. So it's possible to implement the method in an abstract class. The interface, on the other hand, provides only form without implementation, and the implementation must be done in the inherited class.

A good abstract class can share features or states, reducing the amount of code that needs to be rewritten.

If you take Bulgogi's explanation as an example,

The abstract class can pre-implement the method like Bird below.

abstract class Bird{
        abstract void sing();

        void fly(){
            System.out.println ("Flying")
        }
    }

class Duck extends Bird{
        @Override
        void sing() {
            System.out.println ("quack!!");
        }
    }

To make this an interface, all implementations must be done by Duck. It's different that we can't implement a common method like fly in advance.

interface Bird{
        void sing();
        void fly();
    }

class Duck implements Bird{
        void fly(){
            System.out.println ("Flying")
        }

        void sing() {
            System.out.println ("quack!!");
        }
    }