Question

public class RefMix {
  public static void main(String[] args) {
    Object[] a = {null, "foo"};
    Object[] b = {"bar", b};
    a[0] = b;

    System.out.println(a[0][0]);
  }
}

My understanding is that arrays are objects in Java, and therefore a subclass of the Object type. My further understanding is that a 2-dim array is implemented as an array of references to arrays. Therefore I don’t understand why my a[0][0] does not produce bar in the code above. Instead it doesn’t compile:

RefMix.java:7: array required, but java.lang.Object found

Answer 1

My understanding is that arrays are objects in Java, and therefore a subclass of the Object type. My further understanding is that a 2-dim array is implemented as an array of references to arrays.

This is all correct and explains why you can do the assignment

a[0] = b;

without any complaints from the compiler.

Therefore I don’t understand why my a[0][0] does not produce bar in the code above.

Okay, let’s take a look at the types in this expression:

• a is a Object[] — that is an array of Objects
• a[0] is an Object
• a[0][0] — Now you are trying to use an array subscript on an Object. The compiler does not know that the Object is in fact an array, so it complains.