Why isn't there a java.lang.Array class? If a java array is an Object, shouldn't it extend Object?

Question

Here is the java package-tree: http://docs.oracle.com/javase/7/docs/api/java/lang/package-tree.html

I read a tutorial on Java which stated that in Java arrays are objects.

Where is the array class? How comes we can make arrays like this:

byte[] byteArr = new byte[];
char[] charArr = new char[];
int[] intArr = new int[];

and the arrays will inherit methods from Object; for example:

    byte thisByte = 1;
    byte thatByte = 2;
    byte[] theseBytes = new byte[] {thisByte, thatByte};
    int inheritance = theseBytes.length; //inherited 'length' field and some methods
    int wasntInWill = thatByte.length; //error

What's going on here?

EDIT:

As per the answers, I now know it is a final class in java.lang.reflect package.

I have now created a package java.lang.reflect in my Android project and have added a class called Array.java to it. To confirm this is in the way of the original class, Eclipse gave me the error "... already exists in path/to/android.jar"

If I write out the same class as java.lang.reflect.Array but change the toString() method... this should work within my application right?

Answer 1

From JLS:

Every array has an associated Class object, shared with all other arrays with the same component type. [This] acts as if: the direct superclass of an array type is Object [and] every array type implements the interfaces Cloneable and java.io.Serializable.

This is shown by the following example code:

class Test {
    public static void main(String[] args) {
        int[] ia = new int[3];
        System.out.println(ia.getClass());
        System.out.println(ia.getClass().getSuperclass());
    }
}

which prints:

class [I
class java.lang.Object

where the string "[I" is the run-time type signature for the class object "array with component type int".

And yes, since array types effectively extend Object, you can invoke toString() on arrayObject also see the above example

int arr[] = new arr[2];
arr.toString();

Answer 2

Arrays are a language feature - they have a specific syntax for declaring and accessing. And their class definition is hidden from you.

They have a representation in the refleciton API - java.lang.reflect.Array

Btw, the length field is not inherited from Object. The .getClass(), .toString(), etc. methods are inherited.

Answer 3

Slight elaboration of the above code segment:

public class ClassForName {
    public static void main(String[] argv) throws ClassNotFoundException {
        Class theClass = Class.forName("[I");
        System.out.println(theClass.getName());
        Class superClass = theClass.getSuperclass();
        System.out.println(superClass.getName());
    }
}

Results:

C:\JavaTools>java ClassForName
[I
java.lang.Object

As can be seen, "[I" is the name of the class which we would call, in English, "array of int". The class is a "full citizenship" Java class, which responds to all the methods of Object. The only difference is that the new syntax is different and it doesn't support the newInstance() method of Class.

(The classes "[I", "[C", et al, are "pre-defined" in the JVM -- there are no .class files corresponding to them. Java also will implicitly create, on the fly, the "[MyJavaClass;" class if you have an array of "MyJavaClass" in your program.)

Answer 4

Why isn't there an array class?

There are array classes, one per element type used in the program. Both the JLS and the JVM Spec state clearly that a class object for an array is dynamically created. JLS #10.8 'Class Objects for Arrays' and JVM Spec #5.3.3 'Creating Array Classes'.

If a java array is an Object, shouldn't it extend Object?

It does extend java.lang.Object. JLS #10.8 states clearly that 'the direct superclass of every array type is Object.'