Classes - The Java Language Specification - page 180

void move(int dx, int dy) {
x += dx;
y += dy;
alert();
}
abstract void alert();
}
abstract class ColoredPoint extends Point {
int color;
}
class SimplePoint extends Point {
void alert() { }
}
Here, a class Point is declared that must be declared abstract , because it contains a de-
claration of an abstract method named alert . The subclass of Point named ColoredPoint in-
herits the abstract method alert , so it must also be declared abstract . On the other hand,
the subclass of Point named SimplePoint provides an implementation of alert , so it need
not be abstract .
The statement:
Point p = new Point();
would result in a compile-time error; the class Point cannot be instantiated because it
is abstract . However, a Point variable could correctly be initialized with a reference to
any subclass of Point , and the class SimplePoint is not abstract , so the statement:
Point p = new SimplePoint();
would be correct. Instantiation of a SimplePoint causes the default constructor and field
initializers for x and y of Point to be executed.
Example 8.1.1.1-2. Abstract Class Declaration that Prohibits Subclasses
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interface Colorable {
void setColor(int color);
}
abstract class Colored implements Colorable {
public abstract int setColor(int color);
}
These declarations result in a compile-time error: it would be impossible for any sub-
class of class Colored to provide an implementation of a method named setColor , taking