延迟构造的ArrayList和默认的emptyList以获得更好的性能

Question

Does this increase the performance if the list stay empty its life cycle?

public class LazyArrayList<E> implements List<E> {

    private List<E> list = Collections.emptyList();

    public add(E e) {
        if (list == Collections.EMPTY_LIST) {
            list = new ArrayList();
        }
        list.add(e);
    }

    // other necessary methods
}

For example:

List<Integer> deleteList = new LazyArrayList<Integer>();
for ( MyObj o : otherList ) {
    if (o.isOutdated()) {
        deleteList.add(o.getId());
    }
}
return deleteList;

Answer 1

No. ArrayList already has this optimization (in recent versions of the JRE). It instantiates its internal array only when needed:

/**
 * Shared empty array instance used for empty instances.
 */
private static final Object[] EMPTY_ELEMENTDATA = {};

public ArrayList() {
    super();
    this.elementData = EMPTY_ELEMENTDATA;
}

Answer 2

Try with ArrayList.trimToSize() that says:

Trims the capacity of this ArrayList instance to be the list's current size. An application can use this operation to minimize the storage of an ArrayList instance.

Look at ArrayList.ensureCapacity() that says:

Increases the capacity of this ArrayList instance, if necessary, to ensure that it can hold at least the number of elements specified by the minimum capacity argument.