Object type in Java and referencing arrays

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.

Answer 2

The runtime type of an object instance differs from the statically inferred type. The compiler will try to estimate what type each variable could be in the program, in order to catch certain types of errors early. In this case, a[0] will always be an array, but the compiler doesn't know this. Since you are getting an object out of an object array, all the compiler knows is that a[0] is an object. Therefore it raises an error.

In cases where you know something will always be a certain type but the compiler can't figure it out, you can get around this by inserting an explicit cast.

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