Proper way to assign a new array to an existing array

Question

Taking into consideration that Java operates off of pass-by-value. Which of the following would be deemed the most conventional/correct way to assign a new array value to an already existing array:

1:

 int[] x = {1,2};

    public static int[] changeValue(int[] arr) {
        for (int i = 0; i < arr.length; i++) {
            arr[i]*=2;
        }
        return arr;
    }

    changeValue(x);

2:

 int[] x = {1,2};

    public static int[] changeValue(int[] arr) {
        for (int i = 0; i < arr.length; i++) {
            arr[i]*=2;
        }
        return arr;
    }

    x = changeValue(x);

3:

 int[] x = {1,2};

    public static void changeValue(int[] arr) {
        for (int i = 0; i < arr.length; i++) {
            arr[i]*=2;
        }
    }

    changeValue(x);

Thanks.

The third one works. You're passing a copy of the reference but what you're modifying is the contents of the actual object. — Nikos Tzianas
– Nikos Tzianas, Commented Mar 26, 2019 at 16:53
They all work, but I would have thought that just conventionally the second one would of made the most sense? — Danny
– Danny, Commented Mar 26, 2019 at 16:55
@LppEdd The stream option does seem to be rather good, however I was more interested in figuring out which of the 3 options I listed was the most conventional. — Danny
– Danny, Commented Mar 27, 2019 at 16:17
@Danny imho the third one. A void return type explicitly says that the input array will be modified. Returning int[] might be misleading. A user might think a new array is returned, while it is not. But still, prefer immutability. — LppEdd
– LppEdd, Commented Mar 27, 2019 at 16:19

Murat Karagöz · Accepted Answer · 2019-03-26 16:56:26Z

2

Either is fine. As for code guidelines you can stick to the native java methods e.g. Collections#reverse does not return anything. It mutates the passed object. So it really depends on exactly what the method does.

So in your case I would go with the third version.

answered Mar 26, 2019 at 16:56

Murat Karagöz

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Comments

Ruslan · Accepted Answer · 2019-03-26 17:05:57Z

2

Actually I would prefer return a new array instead of modified existing one:

public static int[] changeValue(int[] arr) {
    int[] newArr = new int[arr.length];
    for (int i = 0; i < arr.length; i++) {
        newArr[i] = arr[i] * 2;
    }
    return newArr;
}

See more about immutability importance

And Why is immutability so important in software development

answered Mar 26, 2019 at 17:05

Ruslan

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1 Comment

Danny Over a year ago

Or perhaps keeping the same method, but adding arr = Arrays.copyOf(arr, arr.length); instead of declaring a new variable.

LppEdd · Accepted Answer · 2019-03-26 17:28:07Z

0

Using a Stream is the cleanest way

public static int[] changeValue(final int[] arr) {
   return IntStream.of(arr)
                   .map(i -> i *= 2)
                   .toArray();
}

This will also let you pass in a customized/variable transformation function

public static int[] changeValue(final int[] arr, final IntUnaryOperator function) {
    return IntStream.of(arr)
                    .map(function)
                    .toArray();
}

final int[] result1 = changeValue(ints, i -> i *= 2);
final int[] result2 = changeValue(ints, i -> i /= 4);

Immutability, clean code, and parallelizable (.parallel()) for big arrays.

answered Mar 26, 2019 at 17:28

LppEdd

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