What is the best way of converting List object to long[] array in java?

Question

Is there any utility method to convert a list of Numerical types to array of primitive type? In other words I am looking for a better solution than this.

private long[] toArray(List<Long> values) {
    long[] result = new long[values.size()];
    int i = 0;
    for (Long l : values)
        result[i++] = l;
    return result;
}

See http://stackoverflow.com/questions/3770289/converting-array-of-primitives-to-array-of-containers-in-java — Slava Semushin, May 30 '11 at 10:19
as You want still your solution do not forget about `l.longValue();` — Damian Leszczyński - Vash, May 30 '11 at 10:21
@Vash: `longValue()` isn't necessary since there's autoboxing — Jonathan, May 30 '11 at 10:43

robinst · Accepted Answer · 2016-07-21T11:35:56.577

65

Since Java 8, you can do the following:

long[] result = values.stream().mapToLong(l -> l).toArray();

What's happening here?

We convert the List<Long> into a Stream<Long>.
We call mapToLong on it to get a LongStream
- The argument to mapToLong is a ToLongFunction, which has a long as the result type.
- Because Java automatically unboxes a Long to a long, writing l -> l as the lambda expression works. The Long is converted to a long there. We could also be more explicit and use Long::longValue instead.
We call toArray, which returns a long[]

edited Jul 21 '16 at 11:35

answered Jun 18 '15 at 06:02

robinst

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Can you explain what is happening here? – VaidAbhishek Jul 20 '16 at 10:55
@VaidAbhishek Done :). – robinst Jul 21 '16 at 11:36
I tried to return Long[] with your example but still when I call toArray it return long[]. can you please give a full method call for returning Long[] – Apr 06 '17 at 18:23
2

@MoayadAbuJaber You want a `Long[]` instead? That's just `values.toArray(new Long[0])` – robinst Apr 07 '17 at 12:34
1

In Android this call requires min API level 24 so if you're targeting a lower level, you can't use this. – KlevisGjN Jun 08 '18 at 15:04

score 34 · Answer 2 · edited Feb 27 '17 at 16:05

34

Google Guava : Longs.toArray(Collection)

long[] result = Longs.toArray(values);

edited Feb 27 '17 at 16:05

cahen

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answered May 30 '11 at 10:20

lschin

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score 17 · Answer 3 · edited Mar 10 '15 at 06:07

17

Use ArrayUtils.toPrimitive(Long[] array) from Apache Commons.

Long[] l = values.toArray(new Long[values.size()]);
long[] l = ArrayUtils.toPrimitive(l);

edited Mar 10 '15 at 06:07

Obicere

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answered May 30 '11 at 10:21

melhosseiny

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3

You might need to specify that you want a `long` array returned: `values.toArray(new long[0]);` – Gyan aka Gary Buyn May 30 '11 at 10:26
4

@Gary Buyn: You can't use primitive types as generic arguments. – Damian Leszczyński - Vash May 30 '11 at 10:27
2

I don't know why you get 4 up votes but this code would not even compile you cannot cast from Object[] to long[]. – Damian Leszczyński - Vash May 30 '11 at 10:28
@Vash Yes, I realized it as soon as I posted it. – melhosseiny May 30 '11 at 10:36
If using gradle (as in android) use dependency `compile 'org.apache.commons:commons-lang3:3.4'`(change to latest version) to add the Apache Commons library (and its class `ArrayUtils`). – et_l Apr 02 '17 at 08:49

score 1 · Answer 4 · answered May 30 '11 at 10:59

I don't recall about some native method that will do that but what is wrong with creating a self one ;-).

public class YasinUtilities {

    public static long[] toArray(Iterator<Long) values) { //Better choice would be Enumerator but easier is this way. 

      if(value == null) {
        //return null or throw exception 
      }

      long[] result = new long[values.size()];

      Long current = null;
      int i = 0;
      while(values.hasNext()) {

       current = values.next();

       if(current == null) {
         result[i++] = 0L; //or -1;
       } else {
         result[i++] = current.longValue();
       }
      }
   } 
}

What is the best way of converting List object to long[] array in java?

4 Answers4

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