Object

Class Object

• The root of the class hierarchy
• All types are subclasses of Object
• Used when the specific class is not known

Object use examples

We can create an ArrayList of Object

ArrayList<Object> myArray = new ArrayList<>();

Then we can add any class type to it

myArray.add("diamonds");
myArray.add("spades");
myArray.add(10);
myArray.add(10.5);

Java instanceof

instanceof operator is used to test whether the object is an instance of the specified type

for (Object o : myArray) {
     System.out.println(o instanceof String);
}

Java Type Casting

• Widening Casting or Upcasting
  • done automatically when a smaller type is assigned to a larger type size
  • byte -> short -> int -> long -> float -> double
• Narrowing Casting or Downcasting
  • needs to be specified by () when converting a larger type to a smaller size type
  • double -> float -> long -> int -> short -> byte

Upcasting will be more frequent than downcasting (especially if you are using interfaces and abstract classes)

Upcasting

Done automatically when a smaller type is assigned to a larger type size

int myInt = 13;
double myDouble = myInt;

String name = "Adriana";
Object oName = name;

Downcasting

double myDouble = 10.5;
int myInt = (int) myDouble;

Object oName = "Adriana";
String name = (String) oName;

Practice

Let’s go back to our Animal superclass and its subclasses.

Here’s the code in case you need it:

• Animal
• Cat
• Dog

Create an ArrayList of cats and dogs, iterate over the Arraylist printing each object’s string.

Solution

import java.util.ArrayList;

public class CreateAnimals {

    public static void main(String[] args) {
        ArrayList<Animal> zoo = new ArrayList<>();
        zoo.add(new Cat()); zoo.add(new Dog());
        zoo.add(new Cat()); zoo.add(new Dog());
        
        for (Animal a : zoo) {
            System.out.println(a.toString());
        }
    
    }

}