Read LinkedList source code

//List's linked list implementation, first look at the constructor
public LinkedList() {
    }

public LinkedList(Collection<? extends E> c) {
        this();
        addAll(c);
    }

public boolean addAll(Collection<? extends E> c) {
        return addAll(size, c);
    }


public boolean addAll(int index, Collection<? extends E> c) {
        checkPositionIndex(index);

        Object[] a = c.toArray();
        int numNew = a.length;
        if (numNew == 0)
            return false;
        //pred the previous element of the current index, the element after the element pred on the current index of succ is the element that needs to be moved
        Node<E> pred, succ;
        if (index == size) {
            succ = null;
            pred = last;
        } else {
            succ = node(index);
            pred = succ.prev;
        }
        
	//Build a linked list from pred
        for (Object o : a) {
            @SuppressWarnings("unchecked") E e = (E) o;
            Node<E> newNode = new Node<>(pred, e, null);
	    //Indicate that the current queue is in the middle
            if (pred == null)
                first = newNode;
            else
                pred.next = newNode;
            pred = newNode;
        }
        // is adding element at size position
        if (succ == null) {
            last = pred;
	//Link the queue that needs to be moved to pred
        } else {
            pred.next = succ;
            succ.prev = pred;
        }

        size += numNew;
        modCount++;
        return true;
    }

//Add element (add at the end)
public boolean add(E e) {
        linkLast(e);
        return true;
    }

void linkLast (E e) {
        final Node<E> l = last;
        final Node<E> newNode = new Node<>(l, e, null);
        last = newNode;
	//if the tail node is empty
        if (l == null)
            first = newNode;
        else
            l.next = newNode;
        size++;
        modCount++;
    }
//add element to the header
public void addFirst(E e) {
        linkFirst(e);
    }

private void linkFirst(E e) {
        final Node<E> f = first;
        final Node<E> newNode = new Node<>(null, e, f);
        first = newNode;
        if (f == null)
            last = newNode;
        else
            f.prev = newNode;
        size++;
        modCount++;
    }

//Insert the element at the specified position
 public void add(int index, E element) {
        checkPositionIndex(index);
        // right at the end of the queue
        if (index == size)
            linkLast(element);
        else
            linkBefore(element, node(index));
    }

Node<E> node(int index) {
        // assert isElementIndex(index);
        // start the search from the head node
        if (index < (size >> 1)) {
            Node<E> x = first;
            for (int i = 0; i < index; i++)
                x = x.next;
            return x;
        } else {
	//Start searching from the tail node
            Node<E> x = last;
            for (int i = size - 1; i > index; i--)
                x = x.prev;
            return x;
        }
    }

void linkBefore(E e, Node<E> succ) {
        // assert succ != null;
        final Node<E> pred = succ.prev;
        final Node<E> newNode = new Node<>(pred, e, succ);
        succ.prev = newNode;
        if (pred == null)
            first = newNode;
        else
            pred.next = newNode;
        size++;
        modCount++;
    }

 // get the header element
 public E element() {
        return getFirst();
    }

//Get the header element and throw an exception if first is null.
public E getFirst() {
        final Node<E> f = first;
        if (f == null)
            throw new NoSuchElementException();
        return f.item;
    }
//Get the element at the specified position
public E get(int index) {
        checkElementIndex(index);
        return node(index).item;
    }

// If the tail element is not obtained, an exception will be thrown
 public E getLast() {
        final Node<E> l = last;
        if (l == null)
            throw new NoSuchElementException();
        return l.item;
    }

public boolean offer(E e) {
        return add(e);
    }

public boolean offerFirst(E e) {
        addFirst(e);
        return true;
    }

 public boolean offerLast (E e)
        addLast(e);
        return true;
    }

//The difference between getting the first element and element is that null will be returned here and no exception will be thrown
 public E peekFirst() {
        final Node<E> f = first;
        return (f == null) ? null : f.item;
     }

//get the tail element
public E peekLast () {
        final Node<E> l = last;
        return (l == null) ? null : l.item;
    }

 public E peek() {
        final Node<E> f = first;
        return (f == null) ? null : f.item;
    }

// get and remove the header element
 public E poll() {
        final Node<E> f = first;
        return (f == null) ? null : unlinkFirst(f);
    }

private E unlinkFirst(Node<E> f) {
        // assert f == first && f != null;
        final E element = f.item;
        final Node<E> next = f.next;
	//Let the garbage collection period recycle to avoid memory leaks
        f.item = null;
        f.next = null; // help GC
	//The next node is set as the head node
        first = next;
        if (next == null)
            last = null;
        else
            next.prev = null;
        size--;
        modCount++;
        return element;
    }

public E pollFirst() {
        final Node<E> f = first;
        return (f == null) ? null : unlinkFirst(f);
    }

// get and remove the tail node
 public E pollLast() {
        final Node<E> l = last;
        return (l == null) ? null : unlinkLast(l);
    }

private E unlinkLast(Node<E> l) {
        // assert l == last && l != null;
        final E element = l.item;
        final Node<E> prev = l.prev;
        l.item = null;
        l.prev = null; // help GC
        last = prev;
        if (prev == null)
            first = null;
        else
            prev.next = null;
        size--;
        modCount++;
        return element;
    }

public void push(E e) {
        addFirst(e);
    }

public E pop() {
        return removeFirst();
    }

 public E removeFirst() {
        final Node<E> f = first;
        if (f == null)
            throw new NoSuchElementException();
        return unlinkFirst(f);
    }


public E removeLast() {
        final Node<E> l = last;
        if (l == null)
            throw new NoSuchElementException();
        return unlinkLast(l);
    }

 public E remove() {
        return removeFirst();
    }

public E remove(int index) {
        checkElementIndex(index);
        return unlink(node(index));
    }

public boolean remove(Object o) {
        if (o == null) {
            for (Node<E> x = first; x != null; x = x.next) {
                if (x.item == null) {
                    unlink(x);
                    return true;
                }
            }
        } else {
            for (Node<E> x = first; x != null; x = x.next) {
                if (o.equals(x.item)) {
                    unlink(x);
                    return true;
                }
            }
        }
        return false;
    }


E unlink(Node<E> x) {
        // assert x != null;
        final E element = x.item;
        final Node<E> next = x.next;
        final Node<E> prev = x.prev;

        if (prev == null) {
            first = next;
        } else {
            prev.next = next;
            x.prev = null;
        }

        if (next == null) {
            last = prev;
        } else {
            next.prev = prev;
            x.next = null;
        }

        x.item = null;
        size--;
        modCount++;
        return element;
    }

// remove the first occurrence of the element from the list
 public boolean removeFirstOccurrence(Object o) {
        return remove(o);
    }

// remove the last occurrence of the element from the list
 public boolean removeLastOccurrence(Object o) {
        if (o == null) {
            for (Node<E> x = last; x != null; x = x.prev) {
                if (x.item == null) {
                    unlink(x);
                    return true;
                }
            }
        } else {
            for (Node<E> x = last; x != null; x = x.prev) {
                if (o.equals(x.item)) {
                    unlink(x);
                    return true;
                }
            }
        }
        return false;
    }

// clear the entire queue
  public void clear() {
        // Clearing all of the links between nodes is "unnecessary", but:
        // - helps a generational GC if the discarded nodes inhabit
        //   more than one generation
        // - is sure to free memory even if there is a reachable Iterator
        for (Node<E> x = first; x != null; ) {
            Node<E> next = x.next;
            x.item = null;
            x.next = null;
            x.prev = null;
            x = next;
        }
        first = last = null;
        size = 0;
        modCount++;
    }


public boolean contains(Object o) {
        return indexOf(o) != -1;
    }

 public int indexOf(Object o) {
        int index = 0;
        if (o == null) {
            for (Node<E> x = first; x != null; x = x.next) {
                if (x.item == null)
                    return index;
                index++;
            }
        } else {
            for (Node<E> x = first; x != null; x = x.next) {
                if (o.equals(x.item))
                    return index;
                index++;
            }
        }
        return -1;
    }

public int lastIndexOf(Object o) {
        int index = size;
        if (o == null) {
            for (Node<E> x = last; x != null; x = x.prev) {
                index--;
                if (x.item == null)
                    return index;
            }
        } else {
            for (Node<E> x = last; x != null; x = x.prev) {
                index--;
                if (o.equals(x.item))
                    return index;
            }
        }
        return -1;

    }

public E set(int index, E element) {
        checkElementIndex(index);
        Node<E> x = node(index);
        E oldVal = x.item;
        x.item = element;
        return oldVal;
    }

 public Object[] toArray() {
        Object[] result = new Object[size];
        int i = 0;
        for (Node<E> x = first; x != null; x = x.next)
            result[i++] = x.item;
        return result;
    }

 public <T> T[] toArray(T[] a) {
        if (a.length < size)
            a = (T[])java.lang.reflect.Array.newInstance(
                                a.getClass().getComponentType(), size);
        int i = 0;
        Object[] result = a;
        for (Node<E> x = first; x != null; x = x.next)
            result[i++] = x.item;

        if (a.length > size)
            a[size] = null;

        return a;
    }


 //Implementation of ListIterator
 private class ListItr implements ListIterator<E> {
        private Node<E> lastReturned = null;
        private Node<E> next;
        private int nextIndex;
        private int expectedModCount = modCount;

        ListItr(int index) {
            // assert isPositionIndex(index);
            next = (index == size) ? null : node(index);
            nextIndex = index;
        }

        public boolean hasNext() {
            return nextIndex < size;
        }

        public E next() {
            checkForComodification();
            if (!hasNext())
                throw new NoSuchElementException();

            lastReturned = next;
            next = next.next;
            nextIndex++;
            return lastReturned.item;
        }

        public boolean hasPrevious() {
            return nextIndex > 0;
        }

        public E previous() {
            checkForComodification();
            if (!hasPrevious())
                throw new NoSuchElementException();
            //The strange implementation here returns the previous element of the next element, that is, calling previous() after calling next() returns the same element
            lastReturned = next = (next == null) ? last : next.prev;
            nextIndex--;
            return lastReturned.item;
        }

        public int nextIndex() {
            return nextIndex;
        }

        public int previousIndex() {
            return nextIndex - 1;
        }

        public void remove() {
            checkForComodification();
            if (lastReturned == null)
                throw new IllegalStateException();

            Node<E> lastNext = lastReturned.next;
            unlink(lastReturned);
	    //That is to say, after calling previous(), nextIndex is already -- so it is no longer reduced
            if (next == lastReturned)
                next = lastNext;
            else
                nextIndex--;
            lastReturned = null;
            expectedModCount++;
        }

        public void set(E e) {
            if (lastReturned == null)
                throw new IllegalStateException();
            checkForComodification();
            lastReturned.item = e;
        }

        public void add(E e) {
            checkForComodification();
            lastReturned = null;
            if (next == null)
                linkLast(e);
            else
                linkBefore(e, next);
            nextIndex++;
            expectedModCount++;
        }

        final void checkForComodification() {
            if (modCount != expectedModCount)
                throw new ConcurrentModificationException();
        }
    }

//return the iterator for reverse iteration
private class DescendingIterator implements Iterator<E> {
        private final ListItr itr = new ListItr(size());
        public boolean hasNext() {
            return itr.hasPrevious();
        }
        public E next() {
            return itr.previous();
        }
        public void remove() {
            itr.remove();
        }
    }