Java tutorial
package pt.utl.ist.online.learning.utils; /* * Copyright 2009 Google Inc. * * Licensed under the Apache License, Version 2.0 (the "License"); you may not * use this file except in compliance with the License. You may obtain a copy of * the License at * * http://www.apache.org/licenses/LICENSE-2.0 * * Unless required by applicable law or agreed to in writing, software * distributed under the License is distributed on an "AS IS" BASIS, WITHOUT * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the * License for the specific language governing permissions and limitations under * the License. */ import java.io.IOException; import java.io.ObjectInputStream; import java.io.ObjectOutputStream; import java.io.Serializable; import java.lang.reflect.Array; import java.util.AbstractSet; import java.util.Collection; import java.util.Iterator; import java.util.NoSuchElementException; /** * A memory-efficient hash set. * * @param <E> the element type */ public class MemoryEfficientHashSet<E> extends AbstractSet<E> implements Serializable { private class SetIterator implements Iterator<E> { private int index = 0; private int last = -1; public SetIterator() { advanceToItem(); } public boolean hasNext() { return index < table.length; } @SuppressWarnings("unchecked") public E next() { if (!hasNext()) { throw new NoSuchElementException(); } last = index; E toReturn = (E) unmaskNull(table[index++]); advanceToItem(); return toReturn; } public void remove() { if (last < 0) { throw new IllegalStateException(); } internalRemove(last); if (table[last] != null) { index = last; } last = -1; } private void advanceToItem() { for (; index < table.length; ++index) { if (table[index] != null) { return; } } } } /** * In the interest of memory-savings, we start with the smallest feasible * power-of-two table size that can hold three items without rehashing. If we * started with a size of 2, we'd have to expand as soon as the second item * was added. */ private static final int INITIAL_TABLE_SIZE = 4; private static final Object NULL_ITEM = new Serializable() { Object readResolve() { return NULL_ITEM; } }; static Object maskNull(Object o) { return (o == null) ? NULL_ITEM : o; } static Object unmaskNull(Object o) { return (o == NULL_ITEM) ? null : o; } /** * Number of objects in this set; transient due to custom serialization. * Default access to avoid synthetic accessors from inner classes. */ transient int size = 0; /** * Backing store for all the objects; transient due to custom serialization. * Default access to avoid synthetic accessors from inner classes. */ transient Object[] table; public MemoryEfficientHashSet() { table = new Object[INITIAL_TABLE_SIZE]; } public MemoryEfficientHashSet(Collection<? extends E> c) { int newCapacity = INITIAL_TABLE_SIZE; int expectedSize = c.size(); while (newCapacity * 3 < expectedSize * 4) { newCapacity <<= 1; } table = new Object[newCapacity]; super.addAll(c); } @Override public boolean add(E e) { ensureSizeFor(size + 1); int index = findOrEmpty(e); if (table[index] == null) { ++size; table[index] = maskNull(e); return true; } return false; } @Override public boolean addAll(Collection<? extends E> c) { ensureSizeFor(size + c.size()); return super.addAll(c); } @Override public void clear() { table = new Object[INITIAL_TABLE_SIZE]; size = 0; } @Override public boolean contains(Object o) { return find(o) >= 0; } @Override public Iterator<E> iterator() { return new SetIterator(); } @Override public boolean remove(Object o) { int index = find(o); if (index < 0) { return false; } internalRemove(index); return true; } @Override public int size() { return size; } @Override public Object[] toArray() { return toArray(new Object[size]); } @SuppressWarnings("unchecked") @Override public <T> T[] toArray(T[] a) { if (a.length < size) { a = (T[]) Array.newInstance(a.getClass().getComponentType(), size); } int index = 0; for (int i = 0; i < table.length; ++i) { Object e = table[i]; if (e != null) { a[index++] = (T) unmaskNull(e); } } while (index < a.length) { a[index++] = null; } return a; } /** * Adapted from {@link org.apache.commons.collections.map.AbstractHashedMap}. */ @SuppressWarnings("unchecked") protected void doReadObject(ObjectInputStream in) throws IOException, ClassNotFoundException { table = new Object[in.readInt()]; int items = in.readInt(); for (int i = 0; i < items; i++) { add((E) in.readObject()); } } /** * Adapted from {@link org.apache.commons.collections.map.AbstractHashedMap}. */ protected void doWriteObject(ObjectOutputStream out) throws IOException { out.writeInt(table.length); out.writeInt(size); for (int i = 0; i < table.length; ++i) { Object e = table[i]; if (e != null) { out.writeObject(unmaskNull(e)); } } } /** * Returns whether two items are equal for the purposes of this set. */ protected boolean itemEquals(Object a, Object b) { return (a == null) ? (b == null) : a.equals(b); } /** * Return the hashCode for an item. */ protected int itemHashCode(Object o) { return (o == null) ? 0 : o.hashCode(); } /** * Works just like {@link #addAll(Collection)}, but for arrays. Used to avoid * having to synthesize a collection in {@link Sets}. */ void addAll(E[] elements) { ensureSizeFor(size + elements.length); for (E e : elements) { int index = findOrEmpty(e); if (table[index] == null) { ++size; table[index] = maskNull(e); } } } /** * Removes the item at the specified index, and performs internal management * to make sure we don't wind up with a hole in the table. Default access to * avoid synthetic accessors from inner classes. */ void internalRemove(int index) { table[index] = null; --size; plugHole(index); } /** * Ensures the set is large enough to contain the specified number of entries. */ private void ensureSizeFor(int expectedSize) { if (table.length * 3 >= expectedSize * 4) { return; } int newCapacity = table.length << 1; while (newCapacity * 3 < expectedSize * 4) { newCapacity <<= 1; } Object[] oldTable = table; table = new Object[newCapacity]; for (Object o : oldTable) { if (o != null) { int newIndex = getIndex(unmaskNull(o)); while (table[newIndex] != null) { if (++newIndex == table.length) { newIndex = 0; } } table[newIndex] = o; } } } /** * Returns the index in the table at which a particular item resides, or -1 if * the item is not in the table. */ private int find(Object o) { int index = getIndex(o); while (true) { Object existing = table[index]; if (existing == null) { return -1; } if (itemEquals(o, unmaskNull(existing))) { return index; } if (++index == table.length) { index = 0; } } } /** * Returns the index in the table at which a particular item resides, or the * index of an empty slot in the table where this item should be inserted if * it is not already in the table. */ private int findOrEmpty(Object o) { int index = getIndex(o); while (true) { Object existing = table[index]; if (existing == null) { return index; } if (itemEquals(o, unmaskNull(existing))) { return index; } if (++index == table.length) { index = 0; } } } private int getIndex(Object o) { int h = itemHashCode(o); // Copied from Apache's AbstractHashedMap; prevents power-of-two collisions. h += ~(h << 9); h ^= (h >>> 14); h += (h << 4); h ^= (h >>> 10); // Power of two trick. return h & (table.length - 1); } /** * Tricky, we left a hole in the map, which we have to fill. The only way to * do this is to search forwards through the map shuffling back values that * match this index until we hit a null. */ private void plugHole(int hole) { int index = hole + 1; if (index == table.length) { index = 0; } while (table[index] != null) { int targetIndex = getIndex(unmaskNull(table[index])); if (hole < index) { /* * "Normal" case, the index is past the hole and the "bad range" is from * hole (exclusive) to index (inclusive). */ if (!(hole < targetIndex && targetIndex <= index)) { // Plug it! table[hole] = table[index]; table[index] = null; hole = index; } } else { /* * "Wrapped" case, the index is before the hole (we've wrapped) and the * "good range" is from index (exclusive) to hole (inclusive). */ if (index < targetIndex && targetIndex <= hole) { // Plug it! table[hole] = table[index]; table[index] = null; hole = index; } } if (++index == table.length) { index = 0; } } } private void readObject(ObjectInputStream in) throws IOException, ClassNotFoundException { in.defaultReadObject(); doReadObject(in); } private void writeObject(ObjectOutputStream out) throws IOException { out.defaultWriteObject(); doWriteObject(out); } }