问题
I want them to do a custom iterator, for a wrapper set.
ListIterator from(E elem)
回答1:
Extend the implementation and add some façade methods
回答2:
If you want a custom iterator for a Map, do need to use a LinkedHashMap (which iterates in the same order as entries are added). Just use a HashMap and override the entrySet() method:
public class Map<K, V> extends HashMap<K, V> {
public Set<K, V> entrySet() {
return new HashSet<K, V>(super.entrySet()) {
public Iterator<Map.Entry<K, V>> iterator () {
return // some custom implementation
}
};
}
// similar for keySet() if you wish
}
回答3:
I've got a solution now that doesn't use extension or reflection (or indeed LinkedHashMap)
What do you think?
package i3.util;
import java.util.AbstractSet;
import java.util.Collection;
import java.util.ConcurrentModificationException;
import java.util.HashMap;
import java.util.Iterator;
import java.util.ListIterator;
import java.util.Map;
import java.util.NoSuchElementException;
/**
* This class is a like LinkedHashSet (insertion order) but it allows querying
* the relative position of a element and has a ListIterator that can set and
* insert anywhere.
*
* Warning: the iterator can change the order of the set by moving elements when
* setting or adding. Elements that already exist are not ignored, but moved the
* requested place. This changes iteration order
*
*
* The iterators of this class are fail fast and will throw a
* ConcurrentModificationException if their iterator are used with intervening
* main class (or other iterators) mutative calls
*
* @author i30817 <i30817@gmail.com>
*/
public class LinkedSet<E> extends AbstractSet<E> {
//It holds the linked list
private Map<E, Node> m = new HashMap<E, Node>();
//head of that
protected Node head = new Node();
//this is copied to the map value in increments of iteratorAddStep on set.add
//(which only adds to the end, by insertion indexing)
private int monotonicallyIncreasing = 0;
//iterator add step may change when doing rebuilds of the 'space' between elements
//for the before/after functions on LinkedKeyIterator.add
private int iteratorAddStep = 10;
//for fail fast iterators
private int modCount;
/**
* Start iterating from elem (inclusive)
*
*
* @throws NoSuchElementException if E not part of the set
* @param elem a element of the set
* @return a ListIterator - doesn't support nextIndex() or previousIndex()
*/
public ListIterator<E> from(E elem) {
Node e = m.get(elem);
if (e == null) {
throw new NoSuchElementException("the given element isn't part of the set");
}
return new LinkedKeyIterator(e);
}
@Override
public ListIterator<E> iterator() {
return new LinkedKeyIterator();
}
/**
* Returns true if the value target was added before (exclusive) limitElem
* in insertion order.
*
* If target or limit are not present on the set this method returns false
*
* @param limitElem a E that may be a element of the set or not.
* @return if target was added before limit (can be reset by removing and
* re-adding the target, that changes iteration order).
*/
public boolean containsBefore(E target, E limitElem) {
if (isEmpty()) {
return false;
}
Integer targetN = m.get(target).relativeLocation;
Integer highN = m.get(limitElem).relativeLocation;
return targetN != null && highN != null && targetN < highN;
}
/**
* Returns true if the value target was added after (exclusive) previousElem
* in insertion order.
*
* If target or previous are not present on the set this method returns
* false
*
* @param previousElem a E that may be a element of the set or not.
* @return if target was added before previous (can be reset by removing and
* re-adding the target, that changes iteration order).
*/
public boolean containsAfter(E target, E previousElem) {
if (isEmpty()) {
return false;
}
Integer targetN = m.get(target).relativeLocation;
Integer low = m.get(previousElem).relativeLocation;
return targetN != null && low != null && low < targetN;
}
@Override
public boolean add(E e) {
if (!m.containsKey(e)) {
Node n = new Node(e, monotonicallyIncreasing);
monotonicallyIncreasing += iteratorAddStep;
n.addBefore(head);//insertion order
m.put(e, n);
return true;
}
return false;
}
@Override
public int size() {
return m.size();
}
@Override
public boolean isEmpty() {
return m.isEmpty();
}
@Override
public boolean contains(Object o) {
return m.containsKey(o);
}
@Override
public Object[] toArray() {
Object[] result = new Object[size()];
int i = 0;
for (E e : this) {
result[i++] = e;
}
return result;
}
@Override
@SuppressWarnings("unchecked")
public <T> T[] toArray(T[] a) {
int size = size();
if (a.length < size) {
a = (T[]) java.lang.reflect.Array.newInstance(a.getClass().getComponentType(), size);
}
int i = 0;
Object[] result = a;
for (E e : this) {
result[i++] = e;
}
if (a.length > size) {
//peculiar toArray contract where it doesn't care about the rest
a[size] = null;
}
return a;
}
@Override
public boolean remove(Object o) {
Node n = m.remove(o);
if (n != null) {
n.remove();
return true;
}
return false;
}
@Override
public boolean addAll(Collection<? extends E> c) {
boolean changed = false;
for (E e : c) {
changed |= add(e);
}
return changed;
}
@Override
public boolean containsAll(Collection<?> c) {
boolean all = true;
for (Object e : c) {
all &= m.containsKey(e);
}
return all;
}
@Override
public boolean retainAll(Collection<?> c) {
boolean changed = false;
Iterator<E> it = iterator();
while (it.hasNext()) {
E k = it.next();
if (!c.contains(k)) {
it.remove();
changed = true;
}
}
return changed;
}
@Override
public void clear() {
modCount++;
head.after = head.before = head;
m.clear();
}
@Override
public String toString() {
return m.keySet().toString();
}
//linkedlist node class
protected final class Node {
Node before, after;
int relativeLocation;
//needed for map removal during iteration
E key;
private void remove() {
before.after = after;
after.before = before;
modCount++;
}
private void addBefore(Node existingEntry) {
after = existingEntry;
before = existingEntry.before;
before.after = this;
after.before = this;
modCount++;
}
//head const
public Node() {
after = before = this;
relativeLocation = 0;
}
public Node(E key, int value) {
this.key = key;
this.relativeLocation = value;
}
}
protected class LinkedKeyIterator implements ListIterator<E> {
Node nextEntry;
Node lastReturned;
int expectedModCount = modCount;
public LinkedKeyIterator() {
nextEntry = head.after;
}
public LinkedKeyIterator(Node startAt) {
nextEntry = startAt;
}
public boolean hasPrevious() {
return nextEntry.before != head;
}
public boolean hasNext() {
return nextEntry != head;
}
public E next() {
if (modCount != expectedModCount) {
throw new ConcurrentModificationException();
}
if (nextEntry == head) {
throw new NoSuchElementException();
}
Node e = lastReturned = nextEntry;
nextEntry = e.after;
return e.key;
}
public E previous() {
if (modCount != expectedModCount) {
throw new ConcurrentModificationException();
}
if (nextEntry.before == head) {
throw new NoSuchElementException();
}
Node e = lastReturned = nextEntry.before;
nextEntry = e;
return e.key;
}
public void remove() {
if (lastReturned == null) {
throw new IllegalStateException();
}
if (modCount != expectedModCount) {
throw new ConcurrentModificationException();
}
m.remove(lastReturned.key);
nextEntry = lastReturned.after;
lastReturned.remove();
lastReturned = null;
expectedModCount = modCount;
}
@Override
public void set(E e) {
if (lastReturned == null) {
throw new IllegalStateException();
}
if (modCount != expectedModCount) {
throw new ConcurrentModificationException();
}
if (lastReturned.key.equals(e)) {
return;
}
//remove mapping for key since we are changing it
m.remove(lastReturned.key);
//put in the new one
lastReturned.key = e;
Node previousKeyOwner = m.put(e, lastReturned);
if (previousKeyOwner != null) {
//as it is a list mutation call, guard against stale iterator
if(nextEntry == previousKeyOwner){
nextEntry = nextEntry.after;
}
previousKeyOwner.remove();
}
//from m.remove and m.put, may help with 2 concurrent iterators on this instance
//this method may not change modCount if previousKeyOwner is null
expectedModCount = ++modCount;
}
@Override
public void add(E e) {
if (modCount != expectedModCount) {
throw new ConcurrentModificationException();
}
//calculate a good relative location, updating subsequent ones if needed
int candidateLoc = nextEntry.before.relativeLocation + 1;
//opsss, it's full
if (candidateLoc == nextEntry.relativeLocation) {
iteratorAddStep *= 1.6;
for (Node current = nextEntry; current != head; current = current.after) {
current.relativeLocation = current.relativeLocation + iteratorAddStep;
}
}
Node n = m.get(e);
if (n == null) {
n = new Node(e, candidateLoc);
m.put(e, n);
} else {
n.relativeLocation = candidateLoc;
//as it is a list mutation call, guard against stale iterator
if(nextEntry == n){
nextEntry = nextEntry.after;
}
n.remove();
}
n.addBefore(nextEntry);
expectedModCount = modCount;//add before changes modCount
}
@Override
public int nextIndex() {
throw new UnsupportedOperationException("Not supported yet.");
}
@Override
public int previousIndex() {
throw new UnsupportedOperationException("Not supported yet.");
}
}
}
来源：https://stackoverflow.com/questions/12017962/any-way-to-get-access-to-the-internal-nodes-of-linkedhashmap