Abstract Classes

We can stop instantiations of certain superclasses by making them abstract:

classDiagram
class Shape
<<abstract>> Shape
Shape : +colour String
Shape <|-- Circle
class Circle{
	+radius double
	+area() double
}

The name of this class should be in italics. If it is abstract.

I have used an annotation here which is also acceptable.

Code Example

public abstract class Shape {
	public String colour;
}

public class Circle extends Shape {
	public double radius;
	public double area() {
		return (radius * radius) * Math.PI;
	}
}

Shape s = new Circle();
Shape s = new Shape(); // not possible as Shape is abstract

public abstract class Shape {
	public String colour;
	public abstract double area();
	public String report() {
		return "My area is " + area();
	}
}

By stating abstract on the area we say that this method must be overrode by subclasses.

You can also call abstract methods before they are defined.

public class Circle extends Shape {
	public double radius;
	public double area() {
		return (radius * radius) * Math.PI;
	}
}

Shape s = new Circle();
double a = s.area();
double a = ((Circle) s).area();
String r = s.report();

If we implement an abstract method for Shape.area then the second line will work. If not we will have to type-cast like in the third line.

There are two good reasons for making a class `abstract: