//outside class
import java.util.ArrayList;

//in class, outside methods
private ArrayList<Food> eatenFood = new ArrayList<Food>(0);

//in act method
Food food = (Food) getOneIntersectingObject(Food.class);
if (food != null)
{
    eatenFood.add(food); //add the food object to the list
    getWorld().removeObject(food); //remove the food from the world
}

//method which returns the list
public ArrayList<Food> getEatenFood() {return eatenFood;}

public void eat(Minnow nemo)
    {
        Minnow food = (Minnow) getOneIntersectingObject(Minnow.class);
        if (getOneIntersectingObject(Minnow.class) != null)
        {
            eatenFood.add(food);
            getWorld().remove(food);
        }
    }
    
    public ArrayList<Minnow> getStomachContents()
    {
        return eatenFood;
    }

public void testGetStomachContents()
    {
        Minnow nemo = new Minnow();
        ocean.add(nemo, 50, 50);
        ocean.add(shark, 50, 50);
        
        shark.eat(nemo);

        assertEquals(0, ocean.getObjects(Minnow.class).size());
        assertEquals((nemo), shark.getStomachContents()); 
    }

expected: <Minnow at (50, 50)> but was: <[Minnow at (50, 50)]>

public void testGetStomachContents()
    {
        Minnow nemo = new Minnow();
        ocean.add(nemo, 50, 50);
        ocean.add(shark, 50, 50);
        
        shark.eat(nemo);

        assertEquals(0, ocean.getObjects(Minnow.class).size());
        assertEquals(nemo, shark.getStomachContents()); 
    }