The Shape Hierarchy

public interface Shape
{
    public double area();
    public double diameter();
    public double perimeter();
}

Here we are inking the contract offered by the interface Shape.

public class Rectangle implements Shape
{
    private double width;
    private double height;
    public Rectangle(double width, double height)
    {   
        this.width = width;
        this.height = height;
    }   
    public double area()
    {   
        return width*height;
    }   
    public double diameter()
    {   
        return Math.hypot(width,  height);
    }   
    public double perimeter()
    {   
        return 2*(width + height);
    }   
}

This is no surprise.

jshell> /open Shape.java

jshell> /open Rectangle.java

jshell> Rectangle r = new Rectangle(6,8);
r ==> Rectangle@1d56ce6a

jshell> r.diameter()
$4 ==> 10.0

jshell> r.perimeter()
$5 ==> 28.0

jshell> r.area()
$6 ==> 48.0

Since a Shape is a supertype of Rectangle, a variable of Shape type can point at an object of type Rectangle.

jshell> Shape s = new Rectangle(3,4);
s ==> Rectangle@238e0d81

jshell> s.area()
$8 ==> 12.0

The Object type is the root type of the Java object type system. Anything of object type is a subtype of Object. Here we see an Object pointing at a Rectangle!

jshell> Object o = new Rectangle(6,8);
o ==> Rectangle@728938a9

jshell> o
o ==> Rectangle@728938a9


Uh oh.  Look at this.

jshell> o.area()
|  Error:
|  cannot find symbol
|    symbol:   method area()
|  o.area()
|  ^----^


Visibility Principle  The type of a variable
dictates what methods are visible.  An Object variable
can only see Object methods. A Shape
variable is required to be able to see area, 
diameter, and perimeter.

We now exit jshell.


jshell> /exit
|  Goodbye


Now let's add a toString method to our Rectangle


@Override 
public String toString()
{
    return String.format("Rectangle(%s, %s)", width, height);
}


We open a fresh jshell and do this.


jshell> /open Shape.java

jshell> /open Rectangle.java

jshell> Object o = new Rectangle(3,4);
o ==> Rectangle(3.0, 4.0)

jshell> o.toString()
$4 ==> "Rectangle(3.0, 4.0)"

jshell> /exit
|  Goodbye


Delegation Prinicple  Method calls are delegated to the
object that is pointed at.  Notice we have an Object
variable pointing at a Rectangle, but the Rectangle's
toString() method was called.