2-D (concurrent) HashMap: 2-property key type? hashmap of hashmaps? [update]

Question

So I need a 2-dimensional ConcurrentHashMap.

It has to be as blazing fast as possible, as I'm going to be adding to and updating its values extremely frequently. It's in a multithreaded application, hence the choice to use ConcurrentHashMap instead of just HashMap.

Both the "x" and "y" indices are integers with a known range (0 through 40,000,000).

What I need to know is: What's the most efficient way to implement this so it'll be as speedy as possible? The most obvious route is to do a literal 2-D hashmap:

ConcurrentHashMap<Integer, ConcurrentHashMap<Integer, ValueObj>> foo;

Or I could make a private class "IntPair" with two properties x and y, and use that as a key... though if I do that, what's the most efficient way to do equals() and hashcode()? and will I wind up allocating too many new IntPairs? Could I keep a set of IntPairs for each x/y I've assigned, and then use a purely reflexive equals() such that I'm just checking for the exact same object instance?

---

Update:

Now that I've taken a closer look at Integer.valueOf(int), the specific caching model it uses wouldn't make sense here, since I'm dealing with a very sparse matrix with unpredictable entries. I really need to be caching all those IntPairs which are used, not a prespecified subset.

Intuitively, it seems to me that looking up an IntPair in a big map to see if I've already created it would, in fact, be more-or-less the same as just looking it up in the big "2-D" ConcurrentHashMap anyway, wouldn't it? So it seems the solution here is really to just use new IntPair(x,y) each time I look up a key. Yes?

Answer 1

ConcurrentHashMap is quite large, so you probably don't want a collection of them.

Short lived objects are actually very fast to allocate. Are you going to have to create the Integers anyway?

You could intern the coordinate objects, but the cost for just a lookup would probably be comparable to creating them anyway. The real win with Integer is that the same instances are shared when you keep around lots of them for some time.

If performance is really a huge issue, you could write (or use) a map-type object that maps longs to references. I wouldn't be surprised to see custom maps out there which also have functionality associated with coordinate systems (like finding nearest or within a range).

Answer 2

It depends on how sparse your (x,y) points are, in the 40,000,000 x 40,000,000 matrix. My guess is that the matrix is going to be quite sparse anyway, so creating a lot of ConcurrentHashMaps is going to be expensive.

Your (immutable) IntPair suggestion seems more attractive in comparison. As you've suggested, you can even cache some of these pairs to improve performance (see Integer.valueOf(int) to see how this can be implemented using a static nested class and a static factory method). Since the hashcode will always be required, you can pre-compute it in the constructor and save it as a final field. To compute equals, you could use the identity equality for objects in the cache, otherwise you'll need to compare x and y individually.

EDIT: Here's the source code (OpenJDK) for Integer.valueOf(int).

Answer 3

In response to Zach, Yes, the matrix will be very sparse.

I looked at the page you linked, and without a doubt the functionality of Integer.valueOf(int) would be ideal. If I developed a similar static method within my IntPair class, can I assume that I could define equals() to only check for strict reflexive equality?

That said, I don't see in that page where it explains how to implement that functionality using a static nested class and static factory method.... am I just missing it somehow? How do I do that?

Thanks!

Answer 4

I've made a Int2DMap implementation based on the standard Java HashMap. I find it is faster than using an IntPair as key. However it will need to be synchronized.

import java.io.*;
import java.util.*;

public class Int2DMap implements Map, Serializable {
    private static final int DEFAULT_INITIAL_CAPACITY = 16;
    private static final int MAXIMUM_CAPACITY = 1 << 30;
    private static final float DEFAULT_LOAD_FACTOR = 0.75f;
    protected Entry[] table;
    protected int size;
    protected int threshold;
    protected float loadFactor;
    protected transient volatile int modCount;

    public Int2DMap(int initialCapacity, float loadFactor) {
     if (initialCapacity < 0)
      throw new IllegalArgumentException("Illegal initial capacity: " + initialCapacity);
     if (initialCapacity > MAXIMUM_CAPACITY)
      initialCapacity = MAXIMUM_CAPACITY;
     if (loadFactor <= 0 || Float.isNaN(loadFactor))
      throw new IllegalArgumentException("Illegal load factor: " + loadFactor);
     // Find a power of 2 >= initialCapacity
     int capacity = 1;
     while (capacity < initialCapacity) {
      capacity <<= 1;
     }
     this.loadFactor = loadFactor;
     this.threshold = (int) (capacity * loadFactor);
     this.table = new Entry[capacity];
    }

    public Int2DMap(int initialCapacity) {
     this(initialCapacity, DEFAULT_LOAD_FACTOR);
    }

    public Int2DMap() {
     this(DEFAULT_INITIAL_CAPACITY, DEFAULT_LOAD_FACTOR);
    }

    public boolean containsKey(Object key) {
     int[] xy = (int[]) key;
     return containsKey(xy[0], xy[1]);
    }

    public Object get(Object key) {
     int[] xy = (int[]) key;
     return get(xy[0], xy[1]);
    }

    public Object put(Object key, Object value) {
     int[] xy = (int[]) key;
     return put(xy[0], xy[1], value);
    }

    public Object remove(Object key) {
     int[] xy = (int[]) key;
     return remove(xy[0], xy[1]);
    }

    public int size() {
     return size;
    }

    public boolean isEmpty() {
     return size == 0;
    }

    protected static final int indexFor(int x, int y, int length) {
     return (x * 31 + y) & (length - 1);
    }

    public Object get(int x, int y) {
     for (Entry e = table[indexFor(x, y, table.length)]; e != null; e = e.next) {
      if (e.x == x && e.y == y) {
       return e.value;
      }
     }
     return null;
    }

    public boolean containsKey(int x, int y) {
     return getEntry(x, y) != null;
    }

    protected Entry getEntry(int x, int y) {
     for (Entry e = table[indexFor(x, y, table.length)]; e != null; e = e.next) {
      if (e.x == x && e.y == y) {
       return e;
      }
     }
     return null;
    }

    public Object put(int x, int y, Object value) {
     int i = indexFor(x, y, table.length);
     for (Entry e = table[i]; e != null; e = e.next) {
      if (e.x == x && e.y == y) {
       Object oldValue = e.value;
       e.value = value;
       e.recordAccess(this);
       return oldValue;
      }
     }
     modCount++;
     addEntry(x, y, value, i);
     return null;
    }

    protected void resize(int newCapacity) {
     Entry[] oldTable = table;
     int oldCapacity = oldTable.length;
     if (oldCapacity == MAXIMUM_CAPACITY) {
      threshold = Integer.MAX_VALUE;
      return;
     }
     Entry[] newTable = new Entry[newCapacity];
     transfer(newTable);
     table = newTable;
     threshold = (int) (newCapacity * loadFactor);
    }

    protected void transfer(Entry[] newTable) {
     Entry[] src = table;
     int newCapacity = newTable.length;
     for (int j = 0; j < src.length; j++) {
      Entry e = src[j];
      if (e != null) {
       src[j] = null;
       do {
        Entry next = e.next;
        int i = indexFor(e.x, e.y, newCapacity);
        e.next = newTable[i];
        newTable[i] = e;
        e = next;
       } while (e != null);
      }
     }
    }

    public void putAll(Map m) {
     int numKeysToBeAdded = m.size();
     if (numKeysToBeAdded == 0) {
      return;
     }
     if (numKeysToBeAdded > threshold) {
      int targetCapacity = (int) (numKeysToBeAdded / loadFactor + 1);
      if (targetCapacity > MAXIMUM_CAPACITY)
       targetCapacity = MAXIMUM_CAPACITY;
      int newCapacity = table.length;
      while (newCapacity < targetCapacity)
       newCapacity <<= 1;
      if (newCapacity > table.length)
       resize(newCapacity);
     }
     for (Iterator i = m.entrySet().iterator(); i.hasNext();) {
      Map.Entry e = (Map.Entry) i.next();
      put(e.getKey(), e.getValue());
     }
    }

    public Object remove(int x, int y) {
     Entry e = removeEntryForKey(x, y);
     return (e == null ? null : e.value);
    }

    protected Entry removeEntryForKey(int x, int y) {
     int i = indexFor(x, y, table.length);
     Entry prev = table[i];
     Entry e = prev;
     while (e != null) {
      Entry next = e.next;
      Object k;
      if (e.x == x && e.y == y) {
       modCount++;
       size--;
       if (prev == e)
        table[i] = next;
       else
        prev.next = next;
       e.recordRemoval(this);
       return e;
      }
      prev = e;
      e = next;
     }
     return e;
    }

    protected Entry removeMapping(Object o) {
     if (!(o instanceof Entry))
      return null;
     Entry entry = (Entry) o;
     int x = entry.x;
     int y = entry.y;
     int i = indexFor(x, y, table.length);
     Entry prev = table[i];
     Entry e = prev;
     while (e != null) {
      Entry next = e.next;
      if (e.x == x && e.y == y) {
       modCount++;
       size--;
       if (prev == e)
        table[i] = next;
       else
        prev.next = next;
       e.recordRemoval(this);
       return e;
      }
      prev = e;
      e = next;
     }
     return e;
    }

    public void clear() {
     modCount++;
     Entry[] tab = table;
     for (int i = 0; i < tab.length; i++)
      tab[i] = null;
     size = 0;
    }

    public boolean containsValue(Object value) {
     Entry[] tab = table;
     for (int i = 0; i < tab.length; i++)
      for (Entry e = tab[i]; e != null; e = e.next)
       if (value.equals(e.value))
        return true;
     return false;
    }

    static class Entry implements Map.Entry {
     final int x;
     final int y;
     Object value;
     Entry next;

     Entry(int x, int y, Object value, Entry next) {
      this.x = x;
      this.y = y;
      this.value = value;
      this.next = next;
     }

     public final Object getKey() {
      return new int[] { x, y };
     }

     public final Object getValue() {
      return value;
     }

     public final Object setValue(Object newValue) {
      Object oldValue = value;
      value = newValue;
      return oldValue;
     }

     public final boolean equals(Object o) {
      if (!(o instanceof Map.Entry))
       return false;
      Map.Entry e = (Map.Entry) o;
      int[] xy = (int[])e.getKey();
      if (x == xy[0] && y == xy[1]) {
       Object v1 = getValue();
       Object v2 = e.getValue();
       if (v1 == v2 || (v1 != null && v1.equals(v2)))
        return true;
      }
      return false;
     }

     public final int hashCode() {
      return ((31 + x) * 31 + y);
     }

     public final String toString() {
      return "[" + x + ", " + y + "]=" + value;
     }

     /**
      * This method is invoked whenever the value in an entry is overwritten by
      * an invocation of put(k,v) for a key k that's already in the HashMap.
      */
     void recordAccess(Int2DMap m) {
     }

     /**
      * This method is invoked whenever the entry is removed from the table.
      */
     void recordRemoval(Int2DMap m) {
     }
    }

    void addEntry(int x, int y, Object value, int bucketIndex) {
     Entry e = table[bucketIndex];
     table[bucketIndex] = new Entry(x, y, value, e);
     if (size++ >= threshold)
      resize(2 * table.length);
    }


    private abstract class HashIterator implements Iterator {
     Entry next; // next entry to return
     int expectedModCount; // For fast-fail
     int index; // current slot
     Entry current; // current entry

     HashIterator() {
      expectedModCount = modCount;
      if (size > 0) { // advance to first entry
       Entry[] t = table;
       while (index < t.length && (next = t[index++]) == null)
        ;
      }
     }

     public final boolean hasNext() {
      return next != null;
     }

     final Entry nextEntry() {
      if (modCount != expectedModCount)
       throw new ConcurrentModificationException();
      Entry e = current = next;
      if (e == null)
       throw new NoSuchElementException();
      if ((next = e.next) == null) {
       Entry[] t = table;
       while (index < t.length && (next = t[index++]) == null)
        ;
      }
      return e;
     }

     public void remove() {
      if (current == null)
       throw new IllegalStateException();
      if (modCount != expectedModCount)
       throw new ConcurrentModificationException();
      int x = current.x;
      int y = current.y;
      current = null;
      Int2DMap.this.removeEntryForKey(x, y);
      expectedModCount = modCount;
     }
    }

    private final class ValueIterator extends HashIterator {
     public Object next() {
      return nextEntry().value;
     }
    }

    private final class KeyIterator extends HashIterator {
     public Object next() {
      return nextEntry().getKey();
     }
    }

    private final class EntryIterator extends HashIterator {
     public Map.Entry next() {
      return nextEntry();
     }
    }

    // Subclass overrides these to alter behavior of views' iterator() method
    Iterator newKeyIterator() {
     return new KeyIterator();
    }

    Iterator newValueIterator() {
     return new ValueIterator();
    }

    Iterator newEntryIterator() {
     return new EntryIterator();
    }

    public Set keySet() {
     return new KeySet();
    }

    private final class KeySet extends AbstractSet {
     public Iterator iterator() {
      return newKeyIterator();
     }

     public int size() {
      return size;
     }

     public boolean contains(Object o) {
      return containsKey(o);
     }

     public boolean remove(Object o) {
      int[] xy = (int[]) o;
      return Int2DMap.this.removeEntryForKey(xy[0], xy[1]) != null;
     }

     public void clear() {
      Int2DMap.this.clear();
     }
    }

    public Collection values() {
     return new Values();
    }

    private final class Values extends AbstractCollection {
     public Iterator iterator() {
      return newValueIterator();
     }

     public int size() {
      return size;
     }

     public boolean contains(Object o) {
      return containsValue(o);
     }

     public void clear() {
      Int2DMap.this.clear();
     }
    }

    public Set entrySet() {
     return new EntrySet();
    }

    private final class EntrySet extends AbstractSet {

     public Iterator iterator() {
      return newEntryIterator();
     }

     public boolean contains(Object o) {
      if (!(o instanceof Map.Entry))
       return false;
      Entry e = (Entry) o;
      Entry candidate = getEntry(e.x, e.y);
      return candidate != null && candidate.equals(e);
     }

     public boolean remove(Object o) {
      return removeMapping(o) != null;
     }

     public int size() {
      return size;
     }

     public void clear() {
      Int2DMap.this.clear();
     }
    }

    public static void main(String[] args) {    
     try {

      Int2DMap map = new Int2DMap();

      map.put(20, 6000, "Test");
      System.out.println(map.size() == 1);

      System.out.println(map.get(20, 6000) != null);

      System.out.println("Test".equals(map.get(20, 6000)));

      for (Iterator iter = map.values().iterator(); iter.hasNext();) {
       System.out.println("Test".equals(iter.next()));
      }

      for (Iterator iter = map.keySet().iterator(); iter.hasNext();) {
       int[] key = (int[])iter.next();
       System.out.println(key[0] == 20 && key[1] == 6000);
      }

      for (Iterator iter = map.entrySet().iterator(); iter.hasNext();) {
       Map.Entry e = (Map.Entry)iter.next();
       System.out.println(e.toString().equals("[20, 6000]=Test"));
      }

      map.remove(20, 6000);
      System.out.println(map.size() == 0 && map.get(20, 6000) == null);


      long start = System.nanoTime();
      int max = 40000000;
      for (int i = 0; i < 500000; i++) {
       int x = (int)(Math.random() * max);
       int y = (int)(Math.random() * max);
       map.put(x, y, "");

       int x2 = (int)(Math.random() * max);
       int y2 = (int)(Math.random() * max);
       Object o = map.get(x2, y2);

      }
      System.out.println(map.size());
      System.out.println((System.nanoTime() - start) / 1000000);


      Map map2 = new HashMap();
      start = System.nanoTime();

      for (int i = 0; i < 500000; i++) {
       String key = "" + (int)(Math.random() * max) + "," + (int)(Math.random() * max);
       map2.put(key, "");

       String key2 = "" + (int)(Math.random() * max) + "," + (int)(Math.random() * max);
       Object o = map2.get(key2);

      }
      System.out.println(map2.size());
      System.out.println((System.nanoTime() - start) / 1000000);


     } catch (Throwable t) {
      t.printStackTrace();
     }
    }
}