Does java have a skip list implementation

Question

I find ConcurrentSkipListSet in Java Collection Framework, which is backed up with a skip list. But is there a skip list in Java? A set does not work in my use case. I need a in

Answer 1

I am not claiming that this is my own implementation. I just cannot remember where I found it. If you know let me know and I will update. This has been working quite well for me:

public class SkipList> implements Iterable {

Node _head = new Node<>(null, 33);
private final Random rand = new Random();
private int _levels = 1;
private AtomicInteger size = new AtomicInteger(0);

/// 

/// Inserts a value into the skip list.
/// 
public void insert(T value) {
    // Determine the level of the new node. Generate a random number R. The
    // number of
    // 1-bits before we encounter the first 0-bit is the level of the node.
    // Since R is
    // 32-bit, the level can be at most 32.
    int level = 0;
    size.incrementAndGet();
    for (int R = rand.nextInt(); (R & 1) == 1; R >>= 1) {
        level++;
        if (level == _levels) {
            _levels++;
            break;
        }
    }

    // Insert this node into the skip list
    Node newNode = new Node<>(value, level + 1);
    Node cur = _head;
    for (int i = _levels - 1; i >= 0; i--) {
        for (; cur.next[i] != null; cur = cur.next[i]) {
            if (cur.next[i].getValue().compareTo(value) > 0)
                break;
        }

        if (i <= level) {
            newNode.next[i] = cur.next[i];
            cur.next[i] = newNode;
        }
    }
}

/// 

/// Returns whether a particular value already exists in the skip list
/// 
public boolean contains(T value) {
    Node cur = _head;
    for (int i = _levels - 1; i >= 0; i--) {
        for (; cur.next[i] != null; cur = cur.next[i]) {
            if (cur.next[i].getValue().compareTo(value) > 0)
                break;
            if (cur.next[i].getValue().compareTo(value) == 0)
                return true;
        }
    }
    return false;
}

/// 

/// Attempts to remove one occurence of a particular value from the skip
/// list. Returns
/// whether the value was found in the skip list.
/// 
public boolean remove(T value) {

    Node cur = _head;

    boolean found = false;
    for (int i = _levels - 1; i >= 0; i--) {
        for (; cur.next[i] != null; cur = cur.next[i]) {
            if (cur.next[i].getValue().compareTo(value) == 0) {
                found = true;
                cur.next[i] = cur.next[i].next[i];
                break;
            }

            if (cur.next[i].getValue().compareTo(value) > 0)
                break;
        }
    }
    if (found)
        size.decrementAndGet();
    return found;
}

@SuppressWarnings({ "rawtypes", "unchecked" })
@Override
public Iterator iterator() {
    return new SkipListIterator(this, 0);
}

public int size() {
    return size.get();
}

public Double[] toArray() {
    Double[] a = new Double[size.get()];
    int i = 0;
    for (T t : this) {
        a[i] = (Double) t;
        i++;
    }
    return a;
  }

 }

 class Node> {
public Node[] next;
public N value;

@SuppressWarnings("unchecked")
public Node(N value, int level) {
    this.value = value;
    next = new Node[level];
}

public N getValue() {
    return value;
}

public Node[] getNext() {
    return next;
}

public Node getNext(int level) {
    return next[level];
}

public void setNext(Node[] next) {
    this.next = next;
}
}

class SkipListIterator> implements Iterator {
   SkipList list;
   Node current;
   int level;

public SkipListIterator(SkipList list, int level) {
    this.list = list;
    this.current = list._head;
    this.level = level;
}

public boolean hasNext() {
    return current.getNext(level) != null;
}

public E next() {
    current = current.getNext(level);
    return current.getValue();
}

public void remove() throws UnsupportedOperationException {
    throw new UnsupportedOperationException();
}
}