问题
How do I convert int[] into List<Integer> in Java?
Of course, I\'m interested in any other answer than doing it in a loop, item by item. But if there\'s no other answer, I\'ll pick that one as the best to show the fact that this functionality is not part of Java.
回答1:
There is no shortcut for converting from int[] to List<Integer> as Arrays.asList does not deal with boxing and will just create a List<int[]> which is not what you want. You have to make a utility method.
int[] ints = {1, 2, 3};
List<Integer> intList = new ArrayList<Integer>(ints.length);
for (int i : ints)
{
intList.add(i);
}
回答2:
Streams
In Java 8 you can do this
int[] ints = {1,2,3};
List<Integer> list = Arrays.stream(ints).boxed().collect(Collectors.toList());
回答3:
Also from guava libraries... com.google.common.primitives.Ints:
List<Integer> Ints.asList(int...)
回答4:
Arrays.asList will not work as some of the other answers expect.
This code will not create a list of 10 integers. It will print 1, not 10:
int arr[] = { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 };
List lst = Arrays.asList(arr);
System.out.println(lst.size());
This will create a list of integers:
List<Integer> lst = Arrays.asList(1, 2, 3, 4, 5, 6, 7, 8, 9, 10);
If you already have the array of ints, there is not quick way to convert, you're better off with the loop.
On the other hand, if your array has Objects, not primitives in it, Arrays.asList will work:
String str[] = { "Homer", "Marge", "Bart", "Lisa", "Maggie" };
List<String> lst = Arrays.asList(str);
回答5:
I'll add another answer with a different method; no loop but an anonymous class that will utilize the autoboxing features:
public List<Integer> asList(final int[] is)
{
return new AbstractList<Integer>() {
public Integer get(int i) { return is[i]; }
public int size() { return is.length; }
};
}
回答6:
The smallest piece of code would be:
public List<Integer> myWork(int[] array) {
return Arrays.asList(ArrayUtils.toObject(array));
}
where ArrayUtils comes from commons-lang :)
回答7:
In Java 8 with stream:
int[] ints = {1, 2, 3};
List<Integer> list = new ArrayList<Integer>();
Collections.addAll(list, Arrays.stream(ints).boxed().toArray(Integer[]::new));
or with Collectors
List<Integer> list = Arrays.stream(ints).boxed().collect(Collectors.toList());
回答8:
In Java 8 :
int[] arr = {1,2,3};
IntStream.of(arr).boxed().collect(Collectors.toList());
回答9:
It's also worth checking out this bug report, which was closed with reason "Not a defect" and the following text:
"Autoboxing of entire arrays is not specified behavior, for good reason. It can be prohibitively expensive for large arrays."
回答10:
give a try to this class:
class PrimitiveWrapper<T> extends AbstractList<T> {
private final T[] data;
private PrimitiveWrapper(T[] data) {
this.data = data; // you can clone this array for preventing aliasing
}
public static <T> List<T> ofIntegers(int... data) {
return new PrimitiveWrapper(toBoxedArray(Integer.class, data));
}
public static <T> List<T> ofCharacters(char... data) {
return new PrimitiveWrapper(toBoxedArray(Character.class, data));
}
public static <T> List<T> ofDoubles(double... data) {
return new PrimitiveWrapper(toBoxedArray(Double.class, data));
}
// ditto for byte, float, boolean, long
private static <T> T[] toBoxedArray(Class<T> boxClass, Object components) {
final int length = Array.getLength(components);
Object res = Array.newInstance(boxClass, length);
for (int i = 0; i < length; i++) {
Array.set(res, i, Array.get(components, i));
}
return (T[]) res;
}
@Override
public T get(int index) {
return data[index];
}
@Override
public int size() {
return data.length;
}
}
testcase:
List<Integer> ints = PrimitiveWrapper.ofIntegers(10, 20);
List<Double> doubles = PrimitiveWrapper.ofDoubles(10, 20);
// etc
回答11:
If you are using java 8, we can use the stream API to convert it into a list.
List<Integer> list = Arrays.stream(arr) // IntStream
.boxed() // Stream<Integer>
.collect(Collectors.toList());
You can also use the IntStream to convert as well.
List<Integer> list = IntStream.of(arr) // return Intstream
.boxed() // Stream<Integer>
.collect(Collectors.toList());
There are other external library like guava and apache commons also available convert it.
cheers.
回答12:
The best shot:
**
* Integer modifiable fix length list of an int array or many int's.
*
* @author Daniel De Leon.
*/
public class IntegerListWrap extends AbstractList<Integer> {
int[] data;
public IntegerListWrap(int... data) {
this.data = data;
}
@Override
public Integer get(int index) {
return data[index];
}
@Override
public Integer set(int index, Integer element) {
int r = data[index];
data[index] = element;
return r;
}
@Override
public int size() {
return data.length;
}
}
- Support get and set.
- No memory data duplication.
- No wasting time in loops.
Examples:
int[] intArray = new int[]{1, 2, 3};
List<Integer> integerListWrap = new IntegerListWrap(intArray);
List<Integer> integerListWrap1 = new IntegerListWrap(1, 2, 3);
回答13:
If you're open to using a third party library, this will work in Eclipse Collections:
int[] a = {1, 2, 3};
List<Integer> integers = IntLists.mutable.with(a).collect(i -> i);
Assert.assertEquals(Lists.mutable.with(1, 2, 3), integers);
Note: I am a committer for Eclipse Collections.
回答14:
/* Integer[] to List<Integer> */
Integer[] intArr = { 10, 20, 30, 40, 50, 60, 70, 80, 90, 100 };
List<Integer> arrList = new ArrayList<>();
arrList.addAll(Arrays.asList(intArr));
System.out.println(arrList);
/* Integer[] to Collection<Integer> */
Integer[] intArr = { 10, 20, 30, 40, 50, 60, 70, 80, 90, 100 };
Collection<Integer> c = Arrays.asList(intArr);
回答15:
What about this:
int[] a = {1,2,3};
Integer[] b = ArrayUtils.toObject(a);
List<Integer> c = Arrays.asList(b);
回答16:
Here is a solution:
int[] array = { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 };
Integer[] iArray = Arrays.stream(array).boxed().toArray(Integer[]::new);
System.out.println(Arrays.toString(iArray));
List<Integer> list = new ArrayList<>();
Collections.addAll(list, iArray);
System.out.println(list);
Output:
[1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
[1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
回答17:
Here is another possibility, again with Java 8 Streams:
void intArrayToListOfIntegers(int[] arr, List<Integer> list) {
IntStream.range(0, arr.length).forEach(i -> list.add(arr[i]));
}
回答18:
Here is a generic way to convert array to ArrayList
<T> ArrayList<T> toArrayList(Object o, Class<T> type){
ArrayList<T> objects = new ArrayList<>();
for (int i = 0; i < Array.getLength(o); i++) {
//noinspection unchecked
objects.add((T) Array.get(o, i));
}
return objects;
}
Usage
ArrayList<Integer> list = toArrayList(new int[]{1,2,3}, Integer.class);
来源：https://stackoverflow.com/questions/1073919/how-to-convert-int-into-listinteger-in-java