Cast Java array of arrays to Java 2D array

Question

I have an object that is of type Object[]. All elements of the array are actually Object[] objects. So essentially it looks like this

Object[] oneD = {objectArray1, objectArray2, objectArray3, ...}

I want to cast this to an Object[][], like this:

Object[][] twoD = (Object[][])oneD;

but I get compiler errors and ClassCastException's.

Is there a (correct) way to do this?

I'm not declaring it; it's being returned from a method. For more context, I have a method that operates on 1D arrays and I want to pass in a couple of 2D arrays and just have it operate on their first dimension. — nullromo
– nullromo, Commented Feb 7, 2017 at 1:03
Don't. Create a new method instead, that handles 2d arrays, and have it pass the 1st dimension to the original method. — Nir Alfasi
– Nir Alfasi, Commented Feb 7, 2017 at 1:05
Yes, I could copy and paste the 1D method into a second method that handles 2D arrays and move on. But what I'm looking for is a better solution than that. — nullromo
– nullromo, Commented Feb 7, 2017 at 1:07
This is a better solution than what you're suggesting. You're trying to force the same method to handle two types of arrays. That's a bad pattern! — Nir Alfasi
– Nir Alfasi, Commented Feb 7, 2017 at 1:15

Bohemian · Accepted Answer · 2017-02-07 01:09:48Z

2

You can't cast between array types - java ain't C, but you can transform one pretty easily:

Object[] oneD = {new Object[]{}, new Object[]{}, ...};
Object[][] twoD = Arrays.stream(oneD).map(Object[].class::cast).toArray(Object[][]::new);

answered Feb 7, 2017 at 1:09

Bohemian♦

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1 Comment

nullromo Over a year ago

Thanks! This is exactly what I needed. Take all the elements of asdf, cast them to Object[] objects, and then put them all into an Object[][]. Brilliant solution.

Scary Wombat · Accepted Answer · 2017-02-07 01:02:23Z

1

Simply as

    String [] a = {"a", "b"}; 
    String [] b = {"c", "d"}; 

    String [][] ab = {a,b};

answered Feb 7, 2017 at 1:02

Scary Wombat

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4 Comments

Bohemian Over a year ago

I think OP has an Object[] with an arbitrary number of elements, each of which is Object[], handed to him and wants to address it as a Object[][]. So he can't code it up like this.

Scary Wombat Over a year ago

so I should vote to close this question as unclear

nullromo Over a year ago

I'm asking about casting or converting an Object[] to an Object[][], not instantiating an Object[][].

Bohemian Over a year ago

I think it is clear, and can be answered

VHS · Accepted Answer · 2017-02-07 01:10:44Z

0

Assuming that the inner Object arrays have the same size, you can do it this way:

    Object[] asdf = {new Object[3], new Object[3], new Object[3]};
    Object[][] newAsdf = {{asdf[0]}, {asdf[1]}, {asdf[2]}};

answered Feb 7, 2017 at 1:10

VHS

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1 Comment

Bohemian Over a year ago

Except OP doesn't know how many elements are in asdf, so this code isn't useful.

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