abstract Thing

	abstract Thing initialThing()
	abstract Thing finalThing()
	abstract boolean isSameThingAs(Thing otherThing)

	boolean hasChanged( return not initialThing().isSameThingAs(finalThing()))

abstract ThingInTime extends Thing

        abstract Thing thingAtTime(Time time)
        abstract Time initialTime()
        abstract Time finalTime()

        Thing initialThing( return thingAtTime(initialTime())
        Thing finalThing( return thingAtTime(finalTime())

ThingInTimeImplementation implements ThingInTime

        Thing thingAtTime(Time time)
        Time initialTime()
        Time finalTime()
        boolean isSameThingAs(Thing otherThing)

package reality;

import junit.framework.Assert;
import org.junit.Test;

public class TestReality  {

    private Self self = new DefaultSelf();

    @Test
    public void testThatSelfExists() {
        Assert.assertTrue(self.exists());
    }
}

TestReality.java:[8,12] cannot find symbol
symbol  : class Self
location: class reality.TestReality

TestReality.java:[8,28] cannot find symbol
symbol  : class DefaultSelf
location: class reality.TestReality

package reality;

public interface Self {

    public boolean exists();
}

package reality;

public class DefaultSelf implements Self {

    public boolean exists() {
        return thinks();
    }

    private boolean thinks() {
        return true;
    }

    private void think() {
        // whew!!!
    }
}