[Java] Convert ArrayList to array

How to convert ArrayList to array and notes


//ArrayList generation
ArrayList<String> arrayList = new ArrayList<String>();

String[] array = (String[])arrayList.toArray();

System.out.println("ArrayList = " + arrayList.toString());
System.out.println("Array= " + Arrays.toString(array));

Result (NG)

Exception in thread "main" java.lang.ClassCastException: [Ljava.lang.Object; cannot be cast to [Ljava.lang.String;


Try to get the class to return


//ArrayList generation


class [Ljava.lang.Object;

An Object type array is returned. .. ..



//ArrayList generation
//0 for the number of elements in the array or arrayList.size()Put in
String[] array = arrayList.toArray(new String[arrayList.size()]);
//String[] array = arrayList.toArray(new String[0]);* Processing will be slower (described later)

System.out.println("ArrayList = " + arrayList.toString());
System.out.println("Array= " + Arrays.toString(array));

Result (OK)

ArrayList = [A, B, C]
Array= [A, B, C]

Try deep digging


public <T> T[] toArray(T[] a) {
   if (a.length < size)
     return (T[]) Arrays.copyOf(elementData, size, a.getClass());
     System.arraycopy(elementData, 0, a, 0, size);
     if (a.length > size)
         a[size] = null;
     return a;

System.arraycopy(elementData, 0, a, 0, size); I was copying String [] as it was. If it is ~~ new String [0], it will enter Arrays.copyOf (3rd line) and copy the elements one by one, which will slow down the process. ~~ Updated on 2020/04/29

I tried to verify the processing time [Java] ArrayList → Should I specify the size for array conversion

By the way

Only the type specified in the conversion source ArrayList can be specified in the toArray argument. If you have a different class, you will get a compile error.

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