CODEWITHSUNDEEP
javaarraysmultidimensional-array
skystar7
skystar7

Reputation: 4597

Multidimensional arrays in Java extends which class?

I need to know which class multidimensional arrays in Java extends exactly?

When we assign

Object[] ref=new int[]{1,2,3};

the compiler complains that the objects are of different types. So it seems that one dimensional arrays extend Object; I know that already.

But when we assign

Object[] ref2=new int[][]{{1,2,3},{4,5,6}};

the compiler will not complain. So it seems that two dimensional arrays extend Object[].

But when I print its superclass name:

System.out.println(ref2.getClass().getSuperclass().getName());

I got java.lang.Object.

So can anyone explain what's going on here?

Upvotes: 7

Views: 2999

Answers (4)

Cephalopod
Cephalopod

Reputation: 15145

When we assign

Object[] ref=new int[]{1,2,3};

the compiler complains

That's because int is not a subtype of Object, int.class.getSuperclass() returns null. Remember that in Java, primitive values (ints, longs, doubles, ...) are not objects.

So it seems that two dimensional arrays extend Object[].
But when I print its superclass name:

 System.out.println(ref2.getClass().getSuperclass().getName());

I got java.lang.Object.

Arrays are more like interfaces, as they do multiple inheritance. But they are no real interfaces, in the sense of a Java interface.

class A {}
interface I {}
class B extends A implements I {}


B[][] bArray = new B[1][1];
System.out.println(bArray instanceof A[][]);
System.out.println(bArray instanceof I[][]);
System.out.println(bArray instanceof Object[]);
System.out.println(bArray.getClass().getSuperclass());
for (Class i: bArray.getClass().getInterfaces())
    System.out.println(i);
System.out.println(I[][].class.isAssignableFrom(bArray.getClass()));
System.out.println(I[][].class.isInstance(bArray));

Output:

true
true
true
class java.lang.Object
interface java.lang.Cloneable
interface java.io.Serializable
true
true

Furthermore, Java violates the Liskov substitution principle, because

B[] bArray = new B[1];
A[] aArray = bArray;
// bArray[0] = new A(); // causes a compile error
aArray[0] = new A(); // compiles, but causes runtime exception

Upvotes: 1

Ben Lings
Ben Lings

Reputation: 29423

Arrays in Java are covariant. This means that TSub[] is a subtype of TSuper[] if TSub is a subtype of TSuper.

You have int[][] which is an array of int[]. Now, as others have pointed out, any array in Java is a subtype of Object, so int[] is a subtype of Object. So, due to array covariance, int[][] is a subtype of Object[] (substitute TSub = int[] and TSuper = Object in the above definition of covariance).

Edit - To make it clear why covariance is important here, consider that doing the same thing with List<T> wouldn't work:

List<Object> ref2 = new List<int[]>()

Upvotes: 2

Thilo
Thilo

Reputation: 262724

A multidimensional array in Java is really just an array of arrays (of arrays)* .

Also, arrays are considered subclasses of Object.

So, your int[][] is an Object[] (with component type int[]), and also an Object (because all arrays are objects)

An int[] however is not an Object[] (but it is still an Object).

So it seems that two dimensional arrays extend Object[]

I am not sure if "extend" is the proper word here. Arrays have a special place in the Java type system, and work a little different from other objects. A two dimensional array is definitely an Object[]. But if you are asking about superclasses, the only superclass that any kind of array has is Object. All arrays are also Cloneable and Serializable.

Upvotes: 13

Bob Cross
Bob Cross

Reputation: 22292

Your inheritance tree looks something like this:

  1. ref2 is-a int[][]
  2. ref2 is-a Object[]
  3. ref2 is-a Object

Here's a code fragment that illustrates what I mean:

Object ref2 = new int[][]{{1,2,3}, {4,5,6}};
System.err.println("ref2: " + (ref2 instanceof int[][]) + 
  " " + (ref2 instanceof Object[]));

You should see something like:

ref2: true true

Upvotes: 3

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