This question already has answers here: How to convert an ArrayList containing Integers to primitive int array? (19 answers) Closed 3 years ago.
I'm new to Java. How can I convert a List<Integer>
to int[]
in Java?
I'm confused because List.toArray()
actually returns an Object[]
, which can be cast to neither Integer[]
nor int[]
.
Right now I'm using a loop to do so:
int[] toIntArray(List<Integer> list) {
int[] ret = new int[list.size()];
for(int i = 0; i < ret.length; i++)
ret[i] = list.get(i);
return ret;
}
Is there's a better way to do this?
This is similar to the question How can I convert int[] to Integer[] in Java?.
Integer[] arr = (Integer[])list.toArray(new Integer[list.size]);
With streams added in Java 8 we can write code like:
int[] example1 = list.stream().mapToInt(i->i).toArray();
// OR
int[] example2 = list.stream().mapToInt(Integer::intValue).toArray();
Thought process:
The simple Stream#toArray returns an Object[] array, so it is not what we want. Also, Stream#toArray(IntFunction generator) doesn't do what we want, because the generic type A can't represent the primitive type int
So it would be nice to have some stream which could handle the primitive type int instead of the wrapper Integer, because its toArray method will most likely also return an int[] array (returning something else like Object[] or even boxed Integer[] would be unnatural here). And fortunately Java 8 has such a stream which is IntStream
So now the only thing we need to figure out is how to convert our Stream
Unfortunately, I don't believe there really is a better way of doing this due to the nature of Java's handling of primitive types, boxing, arrays and generics. In particular:
List
You can't use int as a type argument for generics, so it would have to be an int-specific method (or one which used reflection to do nasty trickery).
I believe there are libraries which have autogenerated versions of this kind of method for all the primitive types (i.e. there's a template which is copied for each type). It's ugly, but that's the way it is I'm afraid :(
Even though the Arrays
class came out before generics arrived in Java, it would still have to include all the horrible overloads if it were introduced today (assuming you want to use primitive arrays).
binarySearch
, copyOf
, copyOfRange
...? I wonder why they couldn't add another set of horrible overloads.
In addition to Commons Lang, you can do this with Guava's method Ints.toArray(Collection<Integer> collection)
:
List<Integer> list = ...
int[] ints = Ints.toArray(list);
This saves you having to do the intermediate array conversion that the Commons Lang equivalent requires yourself.
Object[] boxedArray = collection.toArray();
The easiest way to do this is to make use of Apache Commons Lang. It has a handy ArrayUtils class that can do what you want. Use the toPrimitive
method with the overload for an array of Integer
s.
List<Integer> myList;
... assign and fill the list
int[] intArray = ArrayUtils.toPrimitive(myList.toArray(new Integer[myList.size()]));
This way you don't reinvent the wheel. Commons Lang has a great many useful things that Java left out. Above, I chose to create an Integer list of the right size. You can also use a 0-length static Integer array and let Java allocate an array of the right size:
static final Integer[] NO_INTS = new Integer[0];
....
int[] intArray2 = ArrayUtils.toPrimitive(myList.toArray(NO_INTS));
toPrimitive
link is broken.
myList.toArray()
will create an Integer[]
and populate it, while ArrayUtils.toPrimitive()
will allocate an int[]
and unbox the input.
Java 8 has given us an easy way to do this via streams...
Using the collections stream()
function and then mapping to ints, you'll get an IntStream. With the IntStream
we can call toArray() which gives us int []
int [] ints = list.stream().mapToInt(Integer::intValue).toArray();
Use:
int[] toIntArray(List<Integer> list) {
int[] ret = new int[list.size()];
int i = 0;
for (Integer e : list)
ret[i++] = e;
return ret;
}
This slight change to your code is to avoid expensive list indexing (since a List is not necessarily an ArrayList, but it could be a linked list, for which random access is expensive).
list.get(i)
performs bounds checks and stuff for each access. I don't know whether the new solution is actually better, but Mike says so at least, so perhaps it is. Edit: I forgot that Java allows indexing into a linked list (I'm used to C++'s std::list, which does not allow it). So what arjantop said about non-ArrayList is true as well; indexing isn't necessarily fast even without the bounds checks.
ArrayList
are free in a modern JIT. However, I'd prefer if Java more followed STL and implemented only methods which really make sense (LinkedList::get(int)
does not as it may be arbitrary slow).
Here is a Java 8 single line code for this:
public int[] toIntArray(List<Integer> intList){
return intList.stream().mapToInt(Integer::intValue).toArray();
}
If you are simply mapping an Integer
to an int
then you should consider using parallelism, since your mapping logic does not rely on any variables outside its scope.
int[] arr = list.parallelStream().mapToInt(Integer::intValue).toArray();
Just be aware of this
Note that parallelism is not automatically faster than performing operations serially, although it can be if you have enough data and processor cores. While aggregate operations enable you to more easily implement parallelism, it is still your responsibility to determine if your application is suitable for parallelism.
There are two ways to map Integers to their primitive form:
Via a ToIntFunction. mapToInt(Integer::intValue) Via explicit unboxing with lambda expression. mapToInt(i -> i.intValue()) Via implicit (auto-) unboxing with lambda expression. mapToInt(i -> i)
Given a list with a null
value
List<Integer> list = Arrays.asList(1, 2, null, 4, 5);
Here are three options to handle null
:
Filter out the null values before mapping. int[] arr = list.parallelStream().filter(Objects::nonNull).mapToInt(Integer::intValue).toArray(); Map the null values to a default value. int[] arr = list.parallelStream().map(i -> i == null ? -1 : i).mapToInt(Integer::intValue).toArray(); Handle null inside the lambda expression. int[] arr = list.parallelStream().mapToInt(i -> i == null ? -1 : i.intValue()).toArray();
mapToInt(Integer::intValue)
and mapToInt(i -> i.intValue())
are strictly identical (two ways of expressing the exact same method call), and all three are effectively identical (same bytecode).
This simple loop is always correct! no bugs
int[] integers = new int[myList.size()];
for (int i = 0; i < integers.length; i++) {
integers[i] = myList.get(i);
}
List#get(int)
is not guaranteed to be a constant-time operation, as you may have assumed, so it shouldn't be used for iteration. Instead, use an iterator, which is designed for this use case. Either use the Java 5+ foreach loop or call List#iterator()
and work with the iterator. Also, the list's size could change during this loop, leading to either an IndexOutOfBoundsException
or an incomplete array. Many iterator implementations have well-documented strategies to handle this situation.
I've noticed several uses of for loops, but you don't even need anything inside the loop. I mention this only because the original question was trying to find less verbose code.
int[] toArray(List<Integer> list) {
int[] ret = new int[ list.size() ];
int i = 0;
for( Iterator<Integer> it = list.iterator();
it.hasNext();
ret[i++] = it.next() );
return ret;
}
If Java allowed multiple declarations in a for loop the way C++ does, we could go a step further and do for(int i = 0, Iterator it...
In the end though (this part is just my opinion), if you are going to have a helping function or method to do something for you, just set it up and forget about it. It can be a one-liner or ten; if you'll never look at it again you won't know the difference.
There is really no way of "one-lining" what you are trying to do, because toArray returns an Object[] and you cannot cast from Object[] to int[] or Integer[] to int[].
int[] ret = new int[list.size()];
Iterator<Integer> iter = list.iterator();
for (int i=0; iter.hasNext(); i++) {
ret[i] = iter.next();
}
return ret;
Also try Dollar (check this revision):
import static com.humaorie.dollar.Dollar.*
...
List<Integer> source = ...;
int[] ints = $(source).convert().toIntArray();
With Eclipse Collections, you can do the following if you have a list of type java.util.List<Integer>
:
List<Integer> integers = Lists.mutable.with(1, 2, 3, 4, 5);
int[] ints = LazyIterate.adapt(integers).collectInt(i -> i).toArray();
Assert.assertArrayEquals(new int[]{1, 2, 3, 4, 5}, ints);
If you already have an Eclipse Collections type like MutableList
, you can do the following:
MutableList<Integer> integers = Lists.mutable.with(1, 2, 3, 4, 5);
int[] ints = integers.asLazy().collectInt(i -> i).toArray();
Assert.assertArrayEquals(new int[]{1, 2, 3, 4, 5}, ints);
Note: I am a committer for Eclipse Collections
I would recommend you to use the List<?>
skeletal implementation from the Java collections API. It appears to be quite helpful in this particular case:
package mypackage;
import java.util.AbstractList;
import java.util.Arrays;
import java.util.Collections;
import java.util.List;
public class Test {
// Helper method to convert int arrays into Lists
static List<Integer> intArrayAsList(final int[] a) {
if(a == null)
throw new NullPointerException();
return new AbstractList<Integer>() {
@Override
public Integer get(int i) {
return a[i]; // Autoboxing
}
@Override
public Integer set(int i, Integer val) {
final int old = a[i];
a[i] = val; // Auto-unboxing
return old; // Autoboxing
}
@Override
public int size() {
return a.length;
}
};
}
public static void main(final String[] args) {
int[] a = {1, 2, 3, 4, 5};
Collections.reverse(intArrayAsList(a));
System.out.println(Arrays.toString(a));
}
}
Beware of boxing/unboxing drawbacks.
Using a lambda you could do this (compiles in JDK lambda):
public static void main(String ars[]) {
TransformService transformService = (inputs) -> {
int[] ints = new int[inputs.size()];
int i = 0;
for (Integer element : inputs) {
ints[ i++ ] = element;
}
return ints;
};
List<Integer> inputs = new ArrayList<Integer>(5) { {add(10); add(10);} };
int[] results = transformService.transform(inputs);
}
public interface TransformService {
int[] transform(List<Integer> inputs);
}
Success story sharing
mapTo...
functions don't allow fornull
lambda.... like sort does.... which makes it default to a default behavior... in this case,i -> i
would be a perfect default behavior.