Returning an empty array

Question

I'm trying to think of the best way to return an empty array, rather than null.

Is there any difference between foo() and bar()?

private static File[] foo() {
    return Collections.emptyList().toArray(new File[0]);
}

private static File[] bar() {
    return new File[0];
}

@Carlos, I admit it's convoluted, but the two methods are essentially equivalent, right? — Moonbeam, Jul 21 '11 at 19:03
equivalent if you ignore readability... you can complicate almost everything. What is the best way to return zero? `return 15 - 3*5 + 0;`? — user85421, Jul 21 '11 at 19:08
just a note: `foo()` will return the array created as the parameter passed to `toArray`! — user85421, Jul 21 '11 at 19:17
See also [Performance benefits of a static empty array instance](https://stackoverflow.com/questions/34301251/performance-benefits-of-a-static-empty-array-instance) with proper benchmarks. — Vadzim, May 13 '19 at 20:26

score 87 · Accepted Answer · answered Jul 21 '11 at 18:16

87

A different way to return an empty array is to use a constant as all empty arrays of a given type are the same.

private static final File[] NO_FILES = {};
private static File[] bar(){
    return NO_FILES;
}

answered Jul 21 '11 at 18:16

Peter Lawrey

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Yeah, that's the other way I've seen it done. I think the fact that `foo()` worked disoriented me a bit. And I wasn't sure if referring to a constant would have any benefit. I don't know...just my inexperience, I suppose. – Moonbeam Jul 21 '11 at 18:18
+1: Since an empty array is immutable, this is the best solution. – Martijn Courteaux Jul 21 '11 at 18:20
@Moonbeam, From a performance point of view it means you don't need to create an object each time. – Peter Lawrey Jul 21 '11 at 18:22
@Peter, That being said, would `foo()` be more preferable than `bar()`, where the solution you provided would be optimal? – Moonbeam Jul 21 '11 at 18:23
2

@Moonbeam, It still think foo() is doing more than it needs to, which can lead to confusion about what it is doing. It is often the case that people can assume a) extra was put there for a reason and it takes along time to find something which is not there, b) it is doing more work than it needs to, even if it doesn't) – Peter Lawrey Jul 21 '11 at 18:30
@Peter, Thanks for the elaboration! :) – Moonbeam Jul 21 '11 at 18:31
another way: `return byte[]{};` – Captain Fantastic Apr 20 '18 at 19:31

score 44 · Answer 2 · answered Nov 28 '14 at 13:01

44

Both foo() and bar() may generate warnings in some IDEs. For example, IntelliJ IDEA will generate a Allocation of zero-length array warning.

An alternative approach is to use Apache Commons Lang 3 ArrayUtils.toArray() function with empty arguments:

public File[] bazz() {
    return ArrayUtils.toArray();
}

This approach is both performance and IDE friendly, yet requires a 3rd party dependency. However, if you already have commons-lang3 in your classpath, you could even use statically-defined empty arrays for primitive types:

public String[] bazz() {
    return ArrayUtils.EMPTY_STRING_ARRAY;
}

answered Nov 28 '14 at 13:01

weekens

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This is what I was looking for. Thank you. – Bandham Manikanta Jun 01 '20 at 14:00
Your IDE is just not smart enough to detect memory allocation in this case. `ArrayUtils.toArray()` creates a new File[0] every time it is called, which can be easily checked with comparing hash codes. – Fizz Areh Aug 09 '22 at 08:32

Martijn Courteaux · Answer 3 · 2011-07-21T18:18:35.497

18

Definitely the second one. In the first one, you use a constant empty List<?> and then convert it to a File[], which requires to create an empty File[0] array. And that is what you do in the second one in one single step.

edited Jul 21 '11 at 18:18

answered Jul 21 '11 at 18:13

Martijn Courteaux

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+1: The first one is confusing. But it doesn't create a list. The emptyList() returns a constant. – Peter Lawrey Jul 21 '11 at 18:15
@Peter: Thanks, I didn't know. I edited my answer. I'll give you +1. – Martijn Courteaux Jul 21 '11 at 18:19

MAHENDRA SINGH CHAUHAN · Answer 4 · 2018-05-08T10:16:10.047

9

You can return empty array by following two ways:

If you want to return array of int then

Using {}:
```
int arr[] = {};
return arr;
```
Using new int[0]:
```
int arr[] = new int[0];
return arr;
```

Same way you can return array for other datatypes as well.

edited May 08 '18 at 10:16

answered May 08 '18 at 10:07

MAHENDRA SINGH CHAUHAN

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Or a combination of both: `return new int[] {}` – Steve Chambers Oct 08 '21 at 08:01

score 5 · Answer 5 · answered Aug 27 '19 at 10:57

5

return new File[0];

This is better and efficient approach.

answered Aug 27 '19 at 10:57

Gourav Goutam

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This doesn't answer the question at all. – Ignatius Aug 27 '19 at 11:17
A good answer would explain why it is more efficient, so readers can validate that it is correct and - more importantly - applicable for their situation. (And a bad comment alleges deficits that may or may not be true, as you say yourself in "I guess not" - that's just uncivil; such comments tend to get flagged and ultimatedly removed by a moderator.) – toolforger Sep 01 '20 at 08:58

score 3 · Answer 6 · answered Aug 27 '19 at 10:29

3

In a single line you could do:

private static File[] bar(){
    return new File[]{};
}

answered Aug 27 '19 at 10:29

Marco

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`new File[0]` would to the same thing as `new File[]{}`, with less symbols, and I'd expect it to compile to the same bytecode. – toolforger Sep 01 '20 at 08:59

score 3 · Answer 7 · answered Jul 21 '11 at 18:15

3

There is no difference except the fact that foo performs 3 visible method calls to return empty array that is anyway created while bar() just creates this array and returns it.

answered Jul 21 '11 at 18:15

AlexR

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score 3 · Answer 8 · answered Jul 21 '11 at 18:15

3

I'm pretty sure you should go with bar(); because with foo(); it creates a List (for nothing) since you create a new File[0] in the end anyway, so why not go with directly returning it!

answered Jul 21 '11 at 18:15

dominicbri7

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Returning an empty array

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