Java casting Object/String arrays

Question

I'm playing with Java to see how it works but I have some doubts about some kind of castings. Consider the following piece of code:

String[][] s = null;
Object[] o = null;

o = (Object[][]) s; // compile-time correct

Now, consider the following example:

Object[][] o = null;
String[] s = null;

s = (String[]) o; // compile-time error: Cannot cast from Object[] to String[]

Why does that happen? I'm confused.

You're not casting from Object[] to String[]. You're casting from Object[][] to String[] — Rabbit Guy
– Rabbit Guy, Commented Jul 12, 2016 at 20:56
The first example works because an Object[] is an Object (everything is an Object). — RaminS
– RaminS, Commented Jul 12, 2016 at 20:58
Please explain why you want to cast arrays of different dimension. This is obviously not very meaningful. — martinhh
– martinhh, Commented Jul 12, 2016 at 20:59
You can't make a 2-dimensional array into a 1-dimensional array. — Chris Gong
– Chris Gong, Commented Jul 12, 2016 at 20:59
An Object[] is not necessarily one-dimensional. It could be any dimension you like. An Object[][][][][] is an Object, so your Object[] can contain several Object[][][][][], making it six-dimensional. — RaminS
– RaminS, Commented Jul 12, 2016 at 21:22

Hamza Abbad · Accepted Answer · 2016-07-12 21:01:43Z

2

Notice that this doesn't give your compile error:

Object[] o = null;
String[] s = null;

s = (String[]) o;

Object[][] to String[] will give incompatible type error.
Object[] to String[] will work normally.

edited Jul 12, 2016 at 21:01

Hamza Abbad

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answered Jul 12, 2016 at 20:59

Steven Whaley

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Comments

Taylor · Accepted Answer · 2016-07-12 21:02:03Z

1

It's failing because it's deterministically (i.e. always and can only ever be) wrong.

o necessarily contains arrays of objects. These can NEVER be Strings.

The first sample you have, a String array can be typed as an object.

We can illustrate this if we remove an "array nesting". Consider:

String[] myStringArray = null;  //instantiation not important
Object someObj = myStringArray; //no problem since arrays are Objects

What you're doing in the second example amounts to

Object[] myObjectArray = null;  //instantiation not important
String someString = myObjectArray; //compile time error, since an Object[] is never a String

answered Jul 12, 2016 at 21:02

Taylor

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