Answer 1

I've understood interfaces are rather used to help developers maintain the classes, they offer good view for developers to check what methods a class should have, class self it can look like:
class NewMath {
public static double sin(double x) {
//100 lines of code
}
public static double sqrt(double x) {
//Another 100 lines of code
}
}
But interface:
interface NewMath {
public double sin(double x);
public double sqrt(double x);
}
It makes it easier for developer to check what member do a class have.

Answer 2

Here are some posts about interfaces, abstract classes and abstraction.
https://www.sololearn.com/Discuss/1782228/?ref=app
https://www.sololearn.com/Discuss/1780241/?ref=app
https://www.sololearn.com/Discuss/2105357/?ref=app
https://www.sololearn.com/Discuss/1470391/?ref=app
https://www.sololearn.com/Discuss/1101782/?ref=app
https://www.sololearn.com/Discuss/1747/?ref=app
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