2. Understanding contracts

Contract is an interface: what is to be done, not how.

Interface is like a class with a set of public operations but no attributes and no implementation.

A UML diagram of a Java interface:

public interface Nameable {
    String getName();
    void setName (String aName);
}

Compare to:

public abstract class Nameable
{
    public abstract  String getName();
    public abstract void setName (String aName);

}