Concatenate two java.util.LinkedList in constant time

Question

I\'m working on some piece of code, which is quite hot, and I need to add elements of one LinkedList (l1) to another LinkedList (

Answer 1

It seems like required feature is not supported by the standard library. However, you can use reflection to achieve constant time when merging linked lists. I haven't tested the following code properly, just to show how it could be achieved:

public class ListConcatenation {

public static void main(String[] args) throws NoSuchFieldException, IllegalAccessException, ClassNotFoundException {
    LinkedList list1 = new LinkedList<>();
    list1.add("a");
    list1.add("b");
    list1.add("c");

    LinkedList list2 = new LinkedList<>();
    list2.add("d");
    list2.add("e");
    list2.add("f");

    System.out.println(merge(list1, list2));
}

public static List merge(List list1, List list2) throws NoSuchFieldException, IllegalAccessException, ClassNotFoundException {
    Field first = getDeclaredField(LinkedList.class, "first");
    Field last = getDeclaredField(LinkedList.class, "last");
    Field size = getDeclaredField(LinkedList.class, "size");

    Class nodeClass = Class.forName("java.util.LinkedList$Node");
    Field next = nodeClass.getDeclaredField("next");
    next.setAccessible(true);

    Object list1Last = last.get(list1);
    Object list2First = first.get(list2);
    Object list2Last = last.get(list2);

    // link the last element of list1 with the first element of list2
    next.set(list1Last, list2First);
    // set last element of list1 equal to the last element of list2
    last.set(list1, list2Last);
    // update list1 size
    size.set(list1, list1.size() + list2.size());

    return list1;
}

private static Field getDeclaredField(Class clazz, String name) throws NoSuchFieldException {
    Field field = clazz.getDeclaredField(name);
    field.setAccessible(true);
    return field;
}

}