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Convert between list and array in Java

2017-07-19

List to Array

There are two methods in List to convert a List object to Array object as follows.

public Object[] toArray();
// Convert a List object to an Array object. All elements in this array will be type Object.

public <T> T[] toArray(T[] a);
// Convert a List<T> object to an Array object. All elements in this array will be type T.

First method

If we use the first method, a casting from Object to T is needed as below. It can be unsuccessful (down casting). Correspondingly, the code below will report Runtime Error.

List<String> list = new ArrayList<>();
String[] array = (String[]) list.toArray();

Second method

We cannot new a type T array with T arr=new T[size];. So the second method returns what we want with following code. It use reflect to create the array. a.getClass().getComponentType() returns the type.

public <T> T[] toArray(T[] a) {
    if (a.length < size)
        a = (T[])java.lang.reflect.Array.
            newInstance(a.getClass().getComponentType(), size);
    System.arraycopy(elementData, 0, a, 0, size);
    if (a.length > size)
        a[size] = null;
    return a;
}

Inplementation

So the final implementation for the conversion from list to array is as below.

List<String> list = new ArrayList<>();
for (int i = 0; i < 10; i++) {
    list.add(String.valueOf(i));
}
String[] array = new String[list.size()];
list.toArray(array);

Array to List

String[] array = {"1","2"};
List<String> list = Arrays.asList(array);

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