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up vote 7 down vote favorite
2

How do I make a multidimensional array of generic items in java?

Consider the class:

class A<T>
  {
    T t;
    public A(T t) { this.t = t; }
  }

When I try to create a multidimensional array: 

A<String>[][] array = new A<String>[2][3];

I get the following error: 

generic array creation
A<String>[][] array = new A<String>[2][3];
                      ^

I tried the following:

A<String>[][] array = (A<String>[][]) (new Object[2]3]);

But that just throws: java.lang.ClassCastException

What's the fix?

(I anticipate people recommending to use lists. Please explain how to achieve this using arrays.)

share|improve this question
2  
You might want to look at stackoverflow.com/questions/529085/… and stackoverflow.com/questions/3865946/… . –  Gareth McCaughan Mar 26 '11 at 21:37
2  
What is your reason for insisting on arrays rather than other collection types? –  Gareth McCaughan Mar 26 '11 at 21:38
    
You are casting an Object[][] to an A<String>[][], the 2 are not compatible. –  katsharp Mar 26 '11 at 21:43
    
@Gareth McCaughan -- Arrays are easier to index than lists. array[i][j] is more legible than array.get(i).get(j). When you're coding a complex algorithm, this is really helpful. –  dsg Mar 26 '11 at 22:01

7 Answers 7

up vote 4 down vote accepted

I was able to do something like this

    @SuppressWarnings("unchecked") 
    A<String>[][] array = (A<String>[][]) Array.newInstance(new A<String>("dummy").getClass(), 2, 3);

EDIT:

from @dsg's suggestion the following skips the creation of a temporary object.

    @SuppressWarnings("unchecked") 
    A<String>[][] array = (A<String>[][]) Array.newInstance(A.class, 2, 3);

or (from @irreputable's suggestion)

 @SuppressWarnings("unchecked")
 A<String>[][] array = new A[2][3];
share|improve this answer
    
Nice! Cleaner than mine. –  dsg Mar 26 '11 at 22:02
    
I believe the following also works: ` @SuppressWarnings("unchecked") A<String>[][] array = (A<String>[][]) java.lang.reflect.Array.newInstance(A.class,2,3);` –  dsg Mar 26 '11 at 22:15
    
@dsg thanks. I have added that to my edit. –  Bala R Mar 26 '11 at 22:18
2  
@dsg You guys are extremely funny. Array.newInstance(A.class, 2, 3) is exactly the same as new A[2][3] –  irreputable Mar 27 '11 at 0:01
    
@irreputable -- thanks! Shows how deep we know java :-P. –  dsg Mar 27 '11 at 0:06
up vote 1 down vote

Thanks to the comments I was able to piece together a solution.

As we saw, A<String>[][] array = new A<String>[2][3]; does not work.

Here how to construct a 2x3 array of A<String> objects that does work:

// get the class of the basic object
Class c = new A<String>("t").getClass();

// get the class of the inner array
A<String>[] a0 = (A<String>[]) java.lang.reflect.Array.newInstance(c, 0);

// construct the outer array
A<String>[][] array = (A<String>[][]) java.lang.reflect.Array.newInstance(a0.getClass(), 2); 

// fill it with instances of the inner array
for (int i = 0; i < 2; ++ i)
{   
  array[i] = (A<String>[]) java.lang.reflect.Array.newInstance(c, 3); 
}

A much cleaner version (Thanks, @Balla R):

@SuppressWarnings("unchecked")
A<String>[][] array = (A<String>[][]) java.lang.reflect.Array.newInstance(A.class,2,3);
share|improve this answer
    
Is there a way to get the class of the basic object without instantiating it? –  dsg Mar 26 '11 at 21:59
    
While I'm glad you figured this out, the burning question is still why go to all this trouble over using ArrayList<ArrayList<A<String>>> ? –  Brian Roach Mar 26 '11 at 22:05
    
no, that's the problem. Before you instantiate it, there's not a way to determine its type. See my answer, you can use A<?>[][] for your array. –  Brian Roach Mar 26 '11 at 22:24
up vote 2 down vote

You can't create an array of type-specific generic in simple way. 

List<String>[] list = new List<String>[2]; //Illegal
List<?> aa[] = new List<?>[2]; // OK
...
A<?>[][] array = new A<?>[2][3]; // OK
A[0][0] = new A<String>(...);

This is an interesting article about Java 1.5 generics, "Java theory and practice: Generics gotchas"

share|improve this answer
    
This is cool! I suppose you can do A<String>[][] array = (A<String>[][]) new A<?>[2][3]; –  dsg Mar 26 '11 at 22:42
    
yeah, but in this way you'll have unchecked cast warning, to prevent it add @SuppressWarnings("unchecked") –  smas Mar 26 '11 at 22:47
    
The link you provided is dead... –  Alerty Oct 28 '11 at 2:08
    
Fixed, thanks for notice this. –  smas Oct 28 '11 at 7:30
up vote 0 down vote

Hmm, I thought Java Arrays (as of Java 6) didn't support generics. One of my biggest "wtf" when I started programming with generics in java.

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up vote 1 down vote

Why don't you do something like this: (non-generic)

String[][] non_generic_array = new String[][];

And make a utility class to implement the functions you made in A<T> (as I suppose there are). Eg:

When you had this in A:

public class A<T>
{
    T obj;
    public A(T obj) { this.obj = obj; }

    public void someFunction() { ... }
}

You can make a utility class:

public class AUtils
{

    public static <T> void someFunction(T obj)
    {
        // Here your code, applied to obj
    }

}
share|improve this answer
up vote 1 down vote

new A[][] and cast it to A<String>[][]

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up vote 0 down vote 
class A<T> 
{
    T s;

    public A(T t)
    {
        s = t;
    }

    public String getType()
    {
        return s.getClass().toString();
    }

    public T getThing()
    {
        return s;
    }
}

public static void main(String[] args) 
{
    A<?>[][] a = new A<?>[2][3];
    a[0][1] = new A<String>("hi");

    System.out.println(a[0][1].getType());
    System.out.println(a[0][1].getThing());

    A<String> b = (A<String>) a[0][1];
}

output:

class java.lang.String

hi

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