[Java] Convert array to ArrayList * Caution

How to convert an array to ArrayList and precautions / pitfalls

I will show you how to convert an array to an ArrayList. There are pitfalls that you will definitely fall in love with, so read on to the end!

ListToArray.java

String[] array ={"A","B","C"};
List<String> list =  Arrays.asList(array);
System.out.println("Array= " + Arrays.toString(array));
System.out.println("List = " + list.toString());

result

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

did it! It's easy.

... but

Try adding an element

ListToArray.java

//Omission
List<String> list =  Arrays.asList(array);
list.add("D"); //Add D!

result

Exception in thread "main" java.lang.UnsupportedOperationException
	at java.util.AbstractList.add(AbstractList.java:148)
	at java.util.AbstractList.add(AbstractList.java:108)
	at codeSample.ListToArray.main(ListToArray.java:13)

Get an error

why

Try to get the class to return

ListToArray.java

//Omission
List<String> list =  Arrays.asList(array);
ArrayList<String> arrayList = new ArrayList<String>();
System.out.println("Regular ArrayList= " + arrayList.getClass());
System.out.println("Conversion from array= " + list.getClass());

result

Regular ArrayList= class java.util.ArrayList
Conversion from array= class java.util.Arrays$ArrayList

Hmm, something different!

Try deep digging

java:java.util.Arrays.class

//Omission
public static <T> List<T> asList(T... a) {
        return new ArrayList<>(a);
    }
    private static class ArrayList<E> extends AbstractList<E>
        implements RandomAccess, java.io.Serializable
    {
        private static final long serialVersionUID = -2764017481108945198L;
        private final E[] a; //←←← is final!

        ArrayList(E[] array) {
            a = Objects.requireNonNull(array);    //Reference copy (additional note)
        }

        @Override
        public int size() {
            return a.length;
        }
//Omission
    }

It was returning a simplified version of ArrayList defined in the inner class of the Arrays class!

• The original array is referenced and copied to E [] a. (Addition) UnsupportedOperationException occurred because there is no definition of add method and it goes to AbstractList.add (E e).

java:java.util.AbstractList.class

public boolean add(E e) {
    add(size(), e);
    return true;
}
//Omission
public void add(int index, E element) {
    throw new UnsupportedOperationException();
}

Fix

ListToArray.java

//Omission
ArrayList<String> arrayList = new ArrayList<String>(Arrays.asList(array));
arrayList.add("D");
System.out.println("Array= " + Arrays.toString(array));
System.out.println("List = " + arrayList.toString());

Create a new ArrayList area and convert it. (Addition)

result

Array= [A, B, C]
List = [A, B, C, D]

Arrays.asList () just converts the type, and the referenced memory area remains the same. (Addition)