Answer 1

Extracting from https://docs.oracle.com/javase/tutorial/java/IandI/abstract.html :

An abstract class is a class that is declared abstract—it may or may not include abstract methods. Abstract classes cannot be instantiated, but they can be subclassed.

An abstract method is a method that is declared without an implementation (without braces, and followed by a semicolon), like this:

abstract void moveTo(double deltaX, double deltaY);

If a class includes abstract methods, then the class itself must be declared abstract, as in:

public abstract class GraphicObject {
// declare fields
// declare nonabstract methods abstract void draw(); }