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Object a[] = new Object[2];
a[0] = "asd";
a[1] = 1;

I am seeking an explanation to how this is possible in Java. I did look at the implementation of Object and I must ask for some help to understand it.

Creating a variable that can hold any type in e.g. Python is built-in but in Java we need the Object class.

What is it in the implementation of Object that allows it to have any type, and please whats the explanation?

Daniel
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2 Answers2

7

Let's go step by step:

  • Object is the root of all reference types in Java. Anything that is a reference, is also instanceof Object!
  • "Obviously", a[0] = "asd"; assigns a String. Strings are Objects by default.
  • and finally, a[1] = 1; leads to auto-boxing. You are assigning an Integer object to that array slot, not a primitive int value of 1.

And of course, covariance is also worth mentioning here.

GhostCat
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  • Thanks for your answer and reply. What if I were to implement my own "any type" class. Its another angle of approach, it would help to understand it. – Daniel Aug 27 '19 at 10:46
  • Unclear question in fact but you have my vote anyway. – davidxxx Aug 27 '19 at 10:48
  • Maybe unclear but very relevant to ask and start from the top. Hard to know how or what to ask for if the thing I seek is abstract in my head. What do you suggest? – Daniel Aug 27 '19 at 10:49
  • I guess I have to accept this answer. It can take me closer to an answer for understanding. – Daniel Aug 27 '19 at 10:52
  • @Daniel Probably the base is understanding how strong typed OOP languages work. The `Object` class is a standard. Besides understanding how `Object[]` work is not simple since this brings another concept : covariance. Things that is not always allowed in generics and that could do things unclear in your general understanding. – davidxxx Aug 27 '19 at 10:54
  • @Daniel You can't "insert" something into the Java type hierarchy. You can only subclass Object, and then all subclasses of that subclass ... are of your type, but also Objects. – GhostCat Aug 27 '19 at 10:54
  • @Daniel : it could help : https://docs.oracle.com/javase/tutorial/java/IandI/objectclass.html : "The `Object` class, in the `java.lang package`, sits at the top of the class hierarchy tree. Every class is a descendant, direct or indirect, of the `Object` class. Every class you use or write inherits the instance methods of `Object`. You need not use any of these methods, but, if you choose to do so..." – davidxxx Aug 27 '19 at 11:00
0

Imagine, you have a class animal 

class Animal {
    ...
}

and further classes which extend the animal class, for example a class dog and another class cat:

class Dog extends Animal {
    ...
}

class Cat extends Animal {
    ...
}

In a container (array, list ...) you now want to store different objects. The following things are probably obvious:

Dog[] dogs = new Dog[2];
dogs[0]    = new Dog("Bobby");
dogs[1]    = new Dog("Jack");

Cat[] cats = new Cat[2];
cats[0]    = new Cat("Cathy");
cats[1]    = new Cat("Jenny");

What is not possible is to store a dog in a cat array or vice versa. So the following does not work:

cats[1]    = new Dog("Tommy"); or dogs[1]  = new Cat("Tammy");

But if you want to have different animals in an array, this must be of type a superclass of all the animals that are to be stored in it

Animal[] pets = new Animal[3];
pets[0]  = new Dog("Bobby");
pets[1]  = new Cat("Cathy");
pets[2]  = new Fish("Nemo");

As already mentioned in the comments and the above answer, Object is the supper class of all classes in java. Even if you write your own class, it extends the object class.

The following things are equivalent:

class MyOwnClass { ... }

class MyOwnClass extends Object { ... }

This means even if it does not explicitly state so, each class extends the object class. So if object is the superclass of all other classes, you can do in an array of type object the same as you did with your animal array. That is, store different animal types in it. Therfore (since all classes inherit from the object), the following applies, even if it does not make much sense

Object[] objects = new Object[6];
objects [0]  = "Some String";
objects [1]  = 42;
objects [2]  = Double.NEGATIVE_INFINITY;;
objects [3]  = new Dog("Bobby");
objects [4]  = new Cat("Cathy");
objects [5]  = new File("C:\\temp\\test.txt");
Eritrean
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