Create array of arrays (2D) without initializing inner arrays in Java

Question

I want to build a 2D array from N (one-dimensional) arrays. The only problem is, as far as I can tell, the only way to declare a multi-dimensional array in Java requires you to specify all inner dimension sizes, and those sub-arrays get created. In this case, this seems wasteful, since they will be overwritten

In C++, I could do this:

Object** arr = new Object*[n];

for(int i=0;i<n; ++i)
{
      arr[i] = getObjArr();
}

In Java, you have to say this:

Object[][] arr = new Object[n][0]; //inner dimension is required

Each of those n zero-length arrays will be created, only to get overwritten as the array is populated. Is there a way to declare a multi-dimensional array (similar to the pointer to pointer(s) syntax of C) without allocating the inner arrays?

stackoverflow.com/a/4781111/212870 suggests you can omit the 0. — Alan Stokes
– Alan Stokes, Commented Apr 14, 2015 at 4:36

BadZen · Accepted Answer · 2015-04-14 04:32:03Z

2

Sure. I don't know why you think you have to do that!

   double[][] foo = new double[2][];

is perfectly fine. The "inner" arrays will be null.

answered Apr 14, 2015 at 4:32

BadZen

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BadZen Over a year ago

Perhaps you're switching indexes wrt "row" vs. "column" major (for lack of a better thing to call that)?

ajb · Accepted Answer · 2015-04-14 04:32:58Z

1

Inner dimensions do not have to be specified. This is legal in Java:

Object[][] arr = new Object[n][];

This creates an array of n elements, where each element has type Object[]. Since you did not specify an inner dimension, those n elements are all initialized to null. You can then assign any Object[] you want to any of those n array elements.

For higher-dimensional arrays, the rule (if you don't have an initializer) is that you can say new T followed by one or more specified dimensions ([some-value]), followed by zero or more [] for unspecified dimensions. You can't have [] followed by [value].

answered Apr 14, 2015 at 4:32

ajb

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3 Comments

stands2reason Over a year ago

Well I thought I had already tried that, but sure enough, it does work!

ajb Over a year ago

Maybe you tried new Object[][n]? That's not legal.

stands2reason Over a year ago

No, I understand that Java is row-major, just like C. I think I got confused by an unrelated error.

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