Last Time

Interfaces So far we know that an interface is a contract. It specifies methods by listing their headers inside of the interface body.

public interface Loser
{
    public void sellBackHouses();
    public void mortageDeeds();
    public boolean isConkingOut();
}

You sign the contract with the impements keyword.

public class Doofus implements Loser
{
    public void sellBackHouses()
    {
    }
    public void mortageDeeds()
    {
    }
    public boolean isConkingOut()
    {
        return true;
    }
}

In this case, both Loser and Doofus are types, and Doofus is a subtype of Loser.

We also saw the extends keyword in the context of interfaces.

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

public interface Polygon extends Shape
{
    public int numSides();    
}

In this case, Polygon is a subtype of Shape.

Triangle.java

Herron's Formula If a triangle has side lengths \(a\), \(b\), and \(c\), then its semiperimeter is defined as $$ s = {a + b + c\over 2},$$ and its area is $$ A = \sqrt{s(s-a)(s-b)(s-c)}.$$ Moreover, if the radicand is negative, then a triangle having sides of the specified lengths does not exist.

Inheritance and Subclassing

You have been subclassing all along! Every Java class, whether standard or user-created, extends Object.