Converting array to list in Java

Question

How do I convert an array to a list in Java?

I used the Arrays.asList() but the behavior (and signature) somehow changed from Java SE 1.4.2 (docs now in archive) to 8 and most snippets I found on the web use the 1.4.2 behaviour.

For example:

int[] numbers = new int[] { 1, 2, 3 };
Arrays.asList(numbers)

• on 1.4.2 returns a list containing the elements 1, 2, 3
• on 1.5.0+ returns a list containing the array 'numbers'

In many cases it should be easy to detect, but sometimes it can slip unnoticed:

Assert.assertTrue(Arrays.asList(numbers).indexOf(4) == -1);

Answer 1

In your example, it is because you can't have a List of a primitive type. In other words, List<int> is not possible.

You can, however, have a List<Integer> using the Integer class that wraps the int primitive. Convert your array to a List with the Arrays.asList utility method.

Integer[] numbers = new Integer[] { 1, 2, 3 };
List<Integer> list = Arrays.asList(numbers);

See this code run live at IdeOne.com.

Answer 2

In Java 8, you can use streams:

int[] numbers = new int[] { 1, 2, 3 };
Arrays.stream(numbers)
      .boxed()
      .collect(Collectors.toList());

Answer 3

We cannot have List<int> as int is a primitive type so we can only have List<Integer>.

Java 16

Java 16 introduces a new method on Stream API called toList(). This handy method returns an unmodifiable List containing the stream elements. So, trying to add a new element to the list will simply lead to UnsupportedOperationException.

int[] ints = new int[] {1,2,3,4,5};
Arrays.stream(ints).boxed().toList();

Java 8 (int array)

int[] ints = new int[] {1,2,3,4,5};
List<Integer> list11 =Arrays.stream(ints).boxed().collect(Collectors.toList()); 

Java 8 and below (Integer array)

Integer[] integers = new Integer[] {1,2,3,4,5};
List<Integer> list21 =  Arrays.asList(integers); // returns a fixed-size list backed by the specified array.
List<Integer> list22 = new ArrayList<>(Arrays.asList(integers)); // good
List<Integer> list23 = Arrays.stream(integers).collect(Collectors.toList()); //Java 8 only

Need ArrayList and not List?

In case we want a specific implementation of List e.g. ArrayList then we can use toCollection as:

ArrayList<Integer> list24 = Arrays.stream(integers)
                          .collect(Collectors.toCollection(ArrayList::new));

---

Why list21 cannot be structurally modified?

When we use Arrays.asList the size of the returned list is fixed because the list returned is not java.util.ArrayList, but a private static class defined inside java.util.Arrays. So if we add or remove elements from the returned list, an UnsupportedOperationException will be thrown. So we should go with list22 when we want to modify the list. If we have Java8 then we can also go with list23.

To be clear list21 can be modified in sense that we can call list21.set(index,element) but this list may not be structurally modified i.e. cannot add or remove elements from the list. You can also check this answer of mine for more explanation.

---

If we want an immutable list then we can wrap it as:

List<Integer> list22 = Collections.unmodifiableList(Arrays.asList(integers));

Another point to note is that the method Collections.unmodifiableList returns an unmodifiable view of the specified list. An unmodifiable view collection is a collection that is unmodifiable and is also a view onto a backing collection. Note that changes to the backing collection might still be possible, and if they occur, they are visible through the unmodifiable view.

We can have a truly immutable list in Java 9 and 10.

Truly Immutable list

Java 9:

String[] objects = {"Apple", "Ball", "Cat"};
List<String> objectList = List.of(objects);

Java 10 (Truly Immutable list):

We can use List.of introduced in Java 9. Also other ways:

1. List.copyOf(Arrays.asList(integers))
2. Arrays.stream(integers).collect(Collectors.toUnmodifiableList());

Answer 4

Speaking about conversion way, it depends on why do you need your List. If you need it just to read data. OK, here you go:

Integer[] values = { 1, 3, 7 };
List<Integer> list = Arrays.asList(values);

But then if you do something like this:

list.add(1);

you get java.lang.UnsupportedOperationException. So for some cases you even need this:

Integer[] values = { 1, 3, 7 };
List<Integer> list = new ArrayList<Integer>(Arrays.asList(values));

First approach actually does not convert array but 'represents' it like a List. But array is under the hood with all its properties like fixed number of elements. Please note you need to specify type when constructing ArrayList.

Answer 5

The problem is that varargs got introduced in Java 5 and unfortunately, Arrays.asList() got overloaded with a vararg version too. So Arrays.asList(numbers) is understood by the Java 5 compiler as a vararg parameter of int arrays.

This problem is explained in more details in Effective Java 2nd Ed., Chapter 7, Item 42.

Answer 6

I recently had to convert an array to a List. Later on the program filtered the list attempting to remove the data. When you use the Arrays.asList(array) function, you create a fixed size collection: you can neither add nor delete. This entry explains the problem better than I can: Why do I get an UnsupportedOperationException when trying to remove an element from a List?.

In the end, I had to do a "manual" conversion:

    List<ListItem> items = new ArrayList<ListItem>();
    for (ListItem item: itemsArray) {
        items.add(item);
    }

I suppose I could have added conversion from an array to a list using an List.addAll(items) operation.

Answer 7

Even shorter:

List<Integer> list = Arrays.asList(1, 2, 3, 4);

Answer 8

Using Arrays

This is the simplest way to convert an array to List. However, if you try to add a new element or remove an existing element from the list, an UnsupportedOperationException will be thrown.

Integer[] existingArray = {1, 2, 3};
List<Integer> list1 = Arrays.asList(existingArray);
List<Integer> list2 = Arrays.asList(1, 2, 3);

// WARNING:
list2.add(1);     // Unsupported operation!
list2.remove(1);  // Unsupported operation!

Using ArrayList or Other List Implementations

You can use a for loop to add all the elements of the array into a List implementation, e.g. ArrayList:

List<Integer> list = new ArrayList<>();
for (int i : new int[]{1, 2, 3}) {
  list.add(i);
}

Using Stream API in Java 8

You can turn the array into a stream, then collect the stream using different collectors: The default collector in Java 8 use ArrayList behind the screen, but you can also impose your preferred implementation.

List<Integer> list1, list2, list3;
list1 = Stream.of(1, 2, 3).collect(Collectors.toList());
list2 = Stream.of(1, 2, 3).collect(Collectors.toCollection(ArrayList::new));
list3 = Stream.of(1, 2, 3).collect(Collectors.toCollection(LinkedList::new));

See also:

• Why do we use autoboxing and unboxing in Java?
• When to use LinkedList over ArrayList?

Answer 9

In Java 9 you have the even more elegant solution of using immutable lists via the new convenience factory method List.of:

List<String> immutableList = List.of("one","two","three");

(shamelessly copied from here )

Answer 10

Another workaround if you use Apache commons-lang:

int[] numbers = new int[] { 1, 2, 3 };
Arrays.asList(ArrayUtils.toObject(numbers));

Where ArrayUtils.toObject converts int[] to Integer[]

Answer 11

One-liner:

List<Integer> list = Arrays.asList(new Integer[] {1, 2, 3, 4});

Answer 12

If you are targeting Java 8 (or later), you can try this:

int[] numbers = new int[] {1, 2, 3, 4};
List<Integer> integers = Arrays.stream(numbers)
                        .boxed().collect(Collectors.<Integer>toList());

NOTE:

Pay attention to the Collectors.<Integer>toList(), this generic method helps you to avoid the error "Type mismatch: cannot convert from List<Object> to List<Integer>".

Answer 13

you have to cast in to array

Arrays.asList((Object[]) array)

Answer 14

1. Using Guava:

   Integer[] array = { 1, 2, 3};
   List<Integer> list = Lists.newArrayList(sourceArray);
   

2. Using Apache Commons Collections:

   Integer[] array = { 1, 2, 3};
   List<Integer> list = new ArrayList<>(6);
   CollectionUtils.addAll(list, array);
   

Answer 15

I've had the same problem and wrote a generic function that takes an array and returns an ArrayList of the same type with the same contents:

public static <T> ArrayList<T> ArrayToArrayList(T[] array) {
    ArrayList<T> list = new ArrayList<T>();
    for(T elmt : array) list.add(elmt);
    return list;
}

Answer 16

Given Array:

    int[] givenArray = {2,2,3,3,4,5};

Converting integer array to Integer List

One way: boxed() -> returns the IntStream

    List<Integer> givenIntArray1 = Arrays.stream(givenArray)
                                  .boxed()
                                  .collect(Collectors.toList());

Second Way: map each element of the stream to Integer and then collect

NOTE: Using mapToObj you can covert each int element into string stream, char stream etc by casing i to (char)i

    List<Integer> givenIntArray2 = Arrays.stream(givenArray)
                                         .mapToObj(i->i)
                                         .collect(Collectors.toList());

Converting One array Type to Another Type Example:

List<Character> givenIntArray2 = Arrays.stream(givenArray)
                                             .mapToObj(i->(char)i)
                                             .collect(Collectors.toList());

Answer 17

So it depends on which Java version you are trying-

Java 7

 Arrays.asList(1, 2, 3);

OR

       final String arr[] = new String[] { "G", "E", "E", "K" };
       final List<String> initialList = new ArrayList<String>() {{
           add("C");
           add("O");
           add("D");
           add("I");
           add("N");
       }};

       // Elements of the array are appended at the end
       Collections.addAll(initialList, arr);

OR

Integer[] arr = new Integer[] { 1, 2, 3 };
Arrays.asList(arr);

In Java 8

int[] num = new int[] {1, 2, 3};
List<Integer> list = Arrays.stream(num)
                        .boxed().collect(Collectors.<Integer>toList())

Reference - http://www.codingeek.com/java/how-to-convert-array-to-list-in-java/

Answer 18

Use this to convert an Array arr to List. Arrays.stream(arr).collect(Collectors.toList());

An example of defining a generic method to convert an array to a list:

public <T> List<T> fromArrayToList(T[] a) {   
    return Arrays.stream(a).collect(Collectors.toList());
}

Answer 19

use two line of code to convert array to list if you use it in integer value you must use autoboxing type for primitive data type

  Integer [] arr={1,2};
  List<Integer> listInt=Arrays.asList(arr);

Answer 20

As of Java 8, the following should do

int[] temp = {1, 2, 3, 4, 5};
List<Integer> tempList = Arrays.stream(temp).boxed().collect(Collectors.toList());

Answer 21

"... How do I convert an array to a list in Java? ..."

There are several ways to accomplish this task.
And, depending on the data-type, some solutions may require more code.

The most simple approach is to traverse the array, and append each value to a List.

int[] values = { 1, 2, 3 };
List<Integer> list = new ArrayList<>();
for (int value : values) list.add(value);

Although, if you're not using a primitive data-type, you can utilize the List#of method, and get rid of the loop.
To create a mutable List, I supplied the value as a new ArrayList parameter.

String[] strings = { "a", "b", "c" };
List<String> list = new ArrayList<>(List.of(strings));

Integer[] numbers = { 1, 2, 3 };
List<Integer> list = new ArrayList<>(List.of(numbers));

Additionally, you can stream the array, to derive a List object.

For a primitive data-type array, it will take slightly more effort.
The Collections Framework was built specifically for objects, and not primitive data-types.
It's recommended to use the wrapper-classes, Integer, Float, etc., for any resourced array objects.

You can stream the array using the Arrays#stream method.
This will return an IntStream, which you can then call the boxed method on, which returns a stream of wrapped, or "boxed" values.
Finally, use the Stream#toList method to return the boxed stream as a List.

Again, to obtain a mutable List, I supplied the value as a new ArrayList parameter.

int[] numbers = { 1, 2, 3 };
List<Integer> list = new ArrayList<>(Arrays.stream(numbers).boxed().toList());

On a final note, if you're simply looking to add an array of values to a List, just use the List#of method.
Just remember this produces an immutable List.

Immutable:

List<Integer> list = List.of(1, 2, 3);

Mutable:

List<Integer> list = new ArrayList<>(List.of(1, 2, 3));

Answer 22

Can you improve this answer please as this is what I use but im not 100% clear. It works fine but intelliJ added new WeatherStation[0]. Why the 0 ?

    public WeatherStation[] removeElementAtIndex(WeatherStation[] array, int index)
    {
        List<WeatherStation> list = new ArrayList<WeatherStation>(Arrays.asList(array));
        list.remove(index);
        return list.toArray(new WeatherStation[0]);
    }

Answer 23

If you are trying to optimize for memory, etc., (and don't want to pull in external libraries) it's simpler than you think to implement your own immutable "array view list" – you just need to extend java.util.AbstractList.

class IntArrayViewList extends AbstractList<Integer> {
    int[] backingArray;
    int size;

    IntArrayViewList(int[] backingArray, int size) {
        this.backingArray = backingArray;
        this.size = size;
    }

    public Iterator<Integer> iterator() {
        return new Iterator<Integer>() {
            int i = 0;

            @Override
            public boolean hasNext() {
                return i < size;
            }

            @Override
            public Integer next() {
                return get(i++);
            }
        };
    }

    public int size() {
        return size;
    }

    public Integer get(int i) {
        return backingArray[i];
    }
}

Answer 24

int is a primitive. Primitives can’t accept null and have default value. Hence, to accept null you need to use wrapper class Integer.

Option 1:

int[] nos = { 1, 2, 3, 4, 5 };
Integer[] nosWrapped = Arrays.stream(nos).boxed()   
                                        .toArray(Integer[]::new);
nosWrapped[5] = null // can store null

Option 2: You can use any data structure that uses the wrapper class Integer

int[] nos = { 1, 2, 3, 4, 5 };
List<Integer> = Arrays.asList(nos)

Answer 25

I started looking at this by trying to reduce the amount of code preparing the input of some test cases. I see a lot of effort around trying to include advanced and new features along with Arrays.asList(), but below the code chosen due simplicity:

    //Integer input[]
    List<Integer> numbers = Arrays.asList(new Integer[]{1, 2 ,3, 4, 5, 4, 3, 2, 1, 3, 4});
    
    //String input[]
    List<String> names = Arrays.asList(new String[]{"Jhon", "Lucas", "Daniel", "Jim", "Sam"});
    
    //String input[]
    List<Character> letters = Arrays.asList(new Character[]{'A', 'B', 'K', 'J', 'F'});
    

Please notice that Anonymous array example will work just with Arrays of Non Primitive Types as the API uses Generics, that's the reason you can see several 2 line examples around, more info here: Why don't Java Generics support primitive types?

For newer JDKs there is another simpler option, the below examples are equivalent to the ones show above:

    //Integer
    List<Integer> numbers = Arrays.asList(1, 2 ,3, 4, 5, 4, 3, 2, 1, 3, 4);
    
    //String
    List<String> names = Arrays.asList("Jhon", "Lucas", "Daniel", "Jim", "Sam"); 
    
    //Character
    List<Character> letters = Arrays.asList('A', 'B', 'K', 'J', 'F');