Multiple Types of Objects in Java containers

Question

I am learning about generics in Java, and was wondering if this would be considered poor coding.

If I declare an ArrayList<Object> I know that I can put any type of object into this list, as all objects descend from Object. I also know that when I call ArrayList.remove(index) the object that gets taken out of the array is of type Object, and that I need to typecast it to the type of object that I want to use.

Now suppose I have object of Dog, Cat, and Car. Would it be bad to put all three objects into the array, considering they are not similar classes?

Answer 1

Let us revise the definition of a Collection

A collection — sometimes called a container — is simply an object that groups multiple elements into a single unit. Collections are used to store, retrieve, manipulate, and communicate aggregate data. Typically, they represent data items that form a natural group....(continued).

Look at the bold words. That should give you your answer.

If you put objects in ArrayList which do not belong to same implementations, you are definitely inviting trouble in situations when you want to retrieve objects from the list and typecast them to appropriate types. So you should AVOID doing that.

Consider you store objects of Dog and Car in ArrayList. Then for each object stored, you want to call some method, say barkLoudly(). Now,this method will work fine with the Dog object. But when this is called on Car, boooom....An Exception will arise.

Answer 2

I think the answer is "usually, but not always." There's not a lot of hard and fast rules in coding; if you look hard enough you can always find an exception. But if you pull objects out of a collection that you then have to cast to the correct type, that's usually a design error.

Try to make code so that the objects you work with already have the correct type for what you want to do. If you don't need to cast, then Object can be fine. If you do need to cast, you should probably rethink your design.

Answer 3

The design will be really bad, to avoid all such kind of issues, Generics were introduced. But the same you can achive using below, provided you have same type of objects.

Cat and Dog extends Animal

Car and Bike extends Vehicle

But do not mix the the above two statements.

Animal class:

public abstract class Animal {
    protected String name;
}

Dog class:

public class Dog extends Animal {
    public Dog(String name) {
        this.name=name;
    }
}

Cat class:

public class Cat extends Animal {
    public Cat(String name) {
        this.name=name;
    }
}

Main class:

public class Main {
    public static void main(String[] args) {
        Cat Cat = new Cat("C");
        Dog Dog = new Dog("D");
        ArrayList<Animal> list = new ArrayList<Animal>();
        list.add(Cat);
        list.add(Dog);
   }
}