java ArrayList contains different objects

Answer 1

Get use of polymorphism. Let's say you have a parent class Vehicle for Bus and Car.

ArrayList list = new ArrayList();

You can add objects of types Bus, Car or Vehicle to this list since Bus IS-A Vehicle, Car IS-A Vehicle and Vehicle IS-A Vehicle.

Retrieving an object from the list and operating based on its type:

Object obj = list.get(3);

if(obj instanceof Bus)
{
   Bus bus = (Bus) obj;
   bus.busMethod();
}
else if(obj instanceof Car)
{
   Car car = (Car) obj;
   car.carMethod();
}
else
{
   Vehicle vehicle = (Vehicle) obj;
   vehicle.vehicleMethod();
}