Java - Is it possible to subclass an array? And more questions about arrays in Java

Question

These questions are purely asked out of curiosity. I don't actually need to subclass an array, I'm just trying to figure out more about how they work in Java.

1. Where is the Javadoc API for arrays? I found one for the 'Arrays' class, but that class just contains utilities to use on Java arrays, and is not the actual array class. This leads me to my next question:

2. IS there an actual array class of which all arrays are subclasses?

3. Is Object[] a superclass of String[] (for example)? I'm guessing the answer here is no. Are these actual classes like any other class?

4. Is String[] a different class from String[][] ? Or String[][][] , etc?

5. As asked in the title, is it possible to subclass an array class (or to subclass THE array class? still not sure how it works as you can tell by my above questioning)? Could I create my own class, instances of which acted exactly like arrays, except they had more functionality?

Thanks.

Answer 1

The Java Language Specification answers all these questions:

The direct superclass of an array type is Object. Every array type implements the interfaces Cloneable and java.io.Serializable.

So no, there isn't a common base class for all arrays, and therefore no JavaDoc either. The members of arrays are defined by the spec instead.

Subtyping among array types is defined in section 4.10.3 - and yes, String[] is a subtype of Object[] . See also ArrayStoreException .

Yes, String[].class != String[][].class . (cf section 10.8 )

No, there is no syntax to subclass arrays. In particular, the extends clause must contain a class type (and array types are not class types).

Answer 2

There is no array class in Java.

Interestingly, arrays are objects (but not class instances):

An object is a class instance or an array.

More here: http://docs.oracle.com/javase/specs/jls/se7/html/jls-10.html

The classes implementing java.util.List provide a more object-oriented implementation of array-like structures.

Answer 3

You can't subclass arrays. Even though the syntax used with them is a bit different, they are objects (check with the JLS ). There's not much API to them - apart from just what Object has (with toString() not doing what you expect, use Arrays.deepToString() for that; equals() and hashCode() are similar) there's the length field. Additionally, arrays are cloneable. You can only cast array types if the target element type is a supertype of the source element type - you can cast String[] to Object[] but not the other way around. If you are sure the objects in the array are a specific type, you can cast each element individually. String[][] is an array of String[] , so it's a different type than String[] as its elements are arrays of String, not Strings. You can create classes which give similar functionality to arrays ( ArrayList does just that), but they will not be interchangeable with regular arrays.