Convert List<String[]> to Object[][] 2d array in Java

Question

I am trying to transform a List of string arrays to 2D object arrays Object[][].

Here is what I have so far. Is this correct?

private static Object[][] listToObject2dConvert(List<String[]> convertor) {
    Object[][] array = new Object[convertor.size()][];

    for (int i = 0; i < array.length; i++) {
        array[i] = new Object[convertor.get(i).length];
    }

    for (int i = 0; i < convertor.size(); i++){
        for (int j = 0; j < convertor.get(i).length; j++) {
            array[i][j] = convertor.get(i).length;
        }
    }
    return array;
}

Why don't you just run this piece of code and after that print the result instead of asking the community is this code correct. If you are looking better solution as given in the answers you have to ask about it, but not ask about verification. We are not a compilator;) — Eugene Troyanskii
– Eugene Troyanskii, Commented Feb 19, 2020 at 17:45

Daniel · Accepted Answer · 2020-02-19 17:29:28Z

2

The answer depends on whether you need to copy the contents out of the String[]s, or if it is acceptable to use those exact arrays.

If you can keep the exact arrays, it would be easiest to just do the following:

return convertor.toArray(Object[][]::new);

However, if you need to copy the values out of the array, you could use streams to easily solve it:

return converter.stream()
                .map(s -> Arrays.copyOf(s, s.length, Object[].class))
                .toArray(Object[][]::new);

answered Feb 19, 2020 at 17:29

Daniel

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1 Comment

WJS Over a year ago

Both are nice solutions and I would prefer the first. But depending on the nature of the question and questioner I tend to focus on more vanilla approaches.

WJS · Accepted Answer · 2020-02-19 17:25:17Z

1

Try this.

    public static void main(String[] args) {
        List<String[]> convertor = 
              List.of(new String[]{"a","b","c"}, new String[]{"e","f","g"});
        Object[][] objs = listToObject2dConvert(convertor);
        for (Object[] o : objs) {
            System.out.println(Arrays.toString(o));
        }
    }
    // just assign the array that's in the List to the 2D array column entry.
    private static Object[][] listToObject2dConvert(List<String[]> convertor){
        Object[][] array= new Object[convertor.size()][];
        int i = 0;
        for (String[] s : convertor) {
            array[i++] = s;
        }
        return array;
    }

answered Feb 19, 2020 at 17:25

WJS

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1 Comment

ska Over a year ago

Thanks a lot WJS!!

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