Java tutorial
/* * 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.util.AbstractCollection; import java.util.AbstractSet; import java.util.Collection; import java.util.Iterator; import java.util.Map; import java.util.NoSuchElementException; import java.util.Set; /** * A memory-efficient hash map. * * @param <K> the key type * @param <V> the value type */ public class HashMap<K, V> implements Map<K, V>, Serializable { /** * 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 class EntryIterator implements Iterator<Entry<K, V>> { private int index = 0; private int last = -1; { advanceToItem(); } public boolean hasNext() { return index < keys.length; } public Entry<K, V> next() { if (!hasNext()) { throw new NoSuchElementException(); } last = index; Entry<K, V> toReturn = new HashEntry(index++); advanceToItem(); return toReturn; } public void remove() { if (last < 0) { throw new IllegalStateException(); } internalRemove(last); if (keys[last] != null) { index = last; } last = -1; } private void advanceToItem() { for (; index < keys.length; ++index) { if (keys[index] != null) { return; } } } } private class EntrySet extends AbstractSet<Entry<K, V>> { @Override public boolean add(Entry<K, V> entry) { boolean result = !HashMap.this.containsKey(entry.getKey()); HashMap.this.put(entry.getKey(), entry.getValue()); return result; } @Override public boolean addAll(Collection<? extends Entry<K, V>> c) { HashMap.this.ensureSizeFor(size() + c.size()); return super.addAll(c); } @Override public void clear() { HashMap.this.clear(); } @Override @SuppressWarnings("unchecked") public boolean contains(Object o) { if (!(o instanceof Entry)) { return false; } Entry<K, V> entry = (Entry<K, V>) o; V value = HashMap.this.get(entry.getKey()); return HashMap.this.valueEquals(value, entry.getValue()); } @Override public int hashCode() { return HashMap.this.hashCode(); } @Override public Iterator<java.util.Map.Entry<K, V>> iterator() { return new EntryIterator(); } @Override @SuppressWarnings("unchecked") public boolean remove(Object o) { if (!(o instanceof Entry)) { return false; } Entry<K, V> entry = (Entry<K, V>) o; int index = findKey(entry.getKey()); if (index >= 0 && valueEquals(values[index], entry.getValue())) { internalRemove(index); return true; } return false; } @Override public boolean removeAll(Collection<?> c) { boolean didRemove = false; for (Object o : c) { didRemove |= remove(o); } return didRemove; } @Override public int size() { return HashMap.this.size; } } private class HashEntry implements Entry<K, V> { private final int index; public HashEntry(int index) { this.index = index; } @Override @SuppressWarnings("unchecked") public boolean equals(Object o) { if (!(o instanceof Entry)) { return false; } Entry<K, V> entry = (Entry<K, V>) o; return keyEquals(getKey(), entry.getKey()) && valueEquals(getValue(), entry.getValue()); } @SuppressWarnings("unchecked") public K getKey() { return (K) unmaskNullKey(keys[index]); } @SuppressWarnings("unchecked") public V getValue() { return (V) values[index]; } @Override public int hashCode() { return keyHashCode(getKey()) ^ valueHashCode(getValue()); } @SuppressWarnings("unchecked") public V setValue(V value) { V previous = (V) values[index]; values[index] = value; return previous; } @Override public String toString() { return getKey() + "=" + getValue(); } } private class KeyIterator implements Iterator<K> { private int index = 0; private int last = -1; { advanceToItem(); } public boolean hasNext() { return index < keys.length; } @SuppressWarnings("unchecked") public K next() { if (!hasNext()) { throw new NoSuchElementException(); } last = index; Object toReturn = unmaskNullKey(keys[index++]); advanceToItem(); return (K) toReturn; } public void remove() { if (last < 0) { throw new IllegalStateException(); } internalRemove(last); if (keys[last] != null) { index = last; } last = -1; } private void advanceToItem() { for (; index < keys.length; ++index) { if (keys[index] != null) { return; } } } } private class KeySet extends AbstractSet<K> { @Override public void clear() { HashMap.this.clear(); } @Override public boolean contains(Object o) { return HashMap.this.containsKey(o); } @Override public int hashCode() { int result = 0; for (int i = 0; i < keys.length; ++i) { Object key = keys[i]; if (key != null) { result += keyHashCode(unmaskNullKey(key)); } } return result; } @Override public Iterator<K> iterator() { return new KeyIterator(); } @Override public boolean remove(Object o) { int index = findKey(o); if (index >= 0) { internalRemove(index); return true; } return false; } @Override public boolean removeAll(Collection<?> c) { boolean didRemove = false; for (Object o : c) { didRemove |= remove(o); } return didRemove; } @Override public int size() { return HashMap.this.size; } } private class ValueIterator implements Iterator<V> { private int index = 0; private int last = -1; { advanceToItem(); } public boolean hasNext() { return index < keys.length; } @SuppressWarnings("unchecked") public V next() { if (!hasNext()) { throw new NoSuchElementException(); } last = index; Object toReturn = values[index++]; advanceToItem(); return (V) toReturn; } public void remove() { if (last < 0) { throw new IllegalStateException(); } internalRemove(last); if (keys[last] != null) { index = last; } last = -1; } private void advanceToItem() { for (; index < keys.length; ++index) { if (keys[index] != null) { return; } } } } private class Values extends AbstractCollection<V> { @Override public void clear() { HashMap.this.clear(); } @Override public boolean contains(Object o) { return HashMap.this.containsValue(o); } @Override public int hashCode() { int result = 0; for (int i = 0; i < keys.length; ++i) { if (keys[i] != null) { result += valueHashCode(values[i]); } } return result; } @Override public Iterator<V> iterator() { return new ValueIterator(); } @Override public boolean remove(Object o) { if (o == null) { for (int i = 0; i < keys.length; ++i) { if (keys[i] != null && values[i] == null) { internalRemove(i); return true; } } } else { for (int i = 0; i < keys.length; ++i) { if (valueEquals(values[i], o)) { internalRemove(i); return true; } } } return false; } @Override public boolean removeAll(Collection<?> c) { boolean didRemove = false; for (Object o : c) { didRemove |= remove(o); } return didRemove; } @Override public int size() { return HashMap.this.size; } } private static final Object NULL_KEY = new Serializable() { Object readResolve() { return NULL_KEY; } }; static Object maskNullKey(Object k) { return (k == null) ? NULL_KEY : k; } static Object unmaskNullKey(Object k) { return (k == NULL_KEY) ? null : k; } /** * Backing store for all the keys; transient due to custom serialization. * Default access to avoid synthetic accessors from inner classes. */ transient Object[] keys; /** * Number of pairs 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 values; transient due to custom serialization. * Default access to avoid synthetic accessors from inner classes. */ transient Object[] values; public HashMap() { initTable(INITIAL_TABLE_SIZE); } public HashMap(Map<? extends K, ? extends V> m) { int newCapacity = INITIAL_TABLE_SIZE; int expectedSize = m.size(); while (newCapacity * 3 < expectedSize * 4) { newCapacity <<= 1; } initTable(newCapacity); internalPutAll(m); } public void clear() { initTable(INITIAL_TABLE_SIZE); size = 0; } public boolean containsKey(Object key) { return findKey(key) >= 0; } public boolean containsValue(Object value) { if (value == null) { for (int i = 0; i < keys.length; ++i) { if (keys[i] != null && values[i] == null) { return true; } } } else { for (Object existing : values) { if (valueEquals(existing, value)) { return true; } } } return false; } public Set<Entry<K, V>> entrySet() { return new EntrySet(); } @Override @SuppressWarnings("unchecked") public boolean equals(Object o) { if (!(o instanceof Map)) { return false; } Map<K, V> other = (Map<K, V>) o; return entrySet().equals(other.entrySet()); } @SuppressWarnings("unchecked") public V get(Object key) { int index = findKey(key); return (index < 0) ? null : (V) values[index]; } @Override public int hashCode() { int result = 0; for (int i = 0; i < keys.length; ++i) { Object key = keys[i]; if (key != null) { result += keyHashCode(unmaskNullKey(key)) ^ valueHashCode(values[i]); } } return result; } public boolean isEmpty() { return size == 0; } public Set<K> keySet() { return new KeySet(); } @SuppressWarnings("unchecked") public V put(K key, V value) { ensureSizeFor(size + 1); int index = findKeyOrEmpty(key); if (keys[index] == null) { ++size; keys[index] = maskNullKey(key); values[index] = value; return null; } else { Object previousValue = values[index]; values[index] = value; return (V) previousValue; } } public void putAll(Map<? extends K, ? extends V> m) { ensureSizeFor(size + m.size()); internalPutAll(m); } @SuppressWarnings("unchecked") public V remove(Object key) { int index = findKey(key); if (index < 0) { return null; } Object previousValue = values[index]; internalRemove(index); return (V) previousValue; } public int size() { return size; } @Override public String toString() { if (size == 0) { return "{}"; } StringBuilder buf = new StringBuilder(32 * size()); buf.append('{'); boolean needComma = false; for (int i = 0; i < keys.length; ++i) { Object key = keys[i]; if (key != null) { if (needComma) { buf.append(',').append(' '); } key = unmaskNullKey(key); Object value = values[i]; buf.append(key == this ? "(this Map)" : key).append('=') .append(value == this ? "(this Map)" : value); needComma = true; } } buf.append('}'); return buf.toString(); } public Collection<V> values() { return new Values(); } /** * Adapted from {@link org.apache.commons.collections.map.AbstractHashedMap}. */ @SuppressWarnings("unchecked") protected void doReadObject(ObjectInputStream in) throws IOException, ClassNotFoundException { int capacity = in.readInt(); initTable(capacity); int items = in.readInt(); for (int i = 0; i < items; i++) { Object key = in.readObject(); Object value = in.readObject(); put((K) key, (V) value); } } /** * Adapted from {@link org.apache.commons.collections.map.AbstractHashedMap}. */ protected void doWriteObject(ObjectOutputStream out) throws IOException { out.writeInt(keys.length); out.writeInt(size); for (int i = 0; i < keys.length; ++i) { Object key = keys[i]; if (key != null) { out.writeObject(unmaskNullKey(key)); out.writeObject(values[i]); } } } /** * Returns whether two keys are equal for the purposes of this set. */ protected boolean keyEquals(Object a, Object b) { return (a == null) ? (b == null) : a.equals(b); } /** * Returns the hashCode for a key. */ protected int keyHashCode(Object k) { return (k == null) ? 0 : k.hashCode(); } /** * Returns whether two values are equal for the purposes of this set. */ protected boolean valueEquals(Object a, Object b) { return (a == null) ? (b == null) : a.equals(b); } /** * Returns the hashCode for a value. */ protected int valueHashCode(Object v) { return (v == null) ? 0 : v.hashCode(); } /** * Ensures the map is large enough to contain the specified number of entries. * Default access to avoid synthetic accessors from inner classes. */ void ensureSizeFor(int expectedSize) { if (keys.length * 3 >= expectedSize * 4) { return; } int newCapacity = keys.length << 1; while (newCapacity * 3 < expectedSize * 4) { newCapacity <<= 1; } Object[] oldKeys = keys; Object[] oldValues = values; initTable(newCapacity); for (int i = 0; i < oldKeys.length; ++i) { Object k = oldKeys[i]; if (k != null) { int newIndex = getKeyIndex(unmaskNullKey(k)); while (keys[newIndex] != null) { if (++newIndex == keys.length) { newIndex = 0; } } keys[newIndex] = k; values[newIndex] = oldValues[i]; } } } /** * Returns the index in the key table at which a particular key resides, or -1 * if the key is not in the table. Default access to avoid synthetic accessors * from inner classes. */ int findKey(Object k) { int index = getKeyIndex(k); while (true) { Object existing = keys[index]; if (existing == null) { return -1; } if (keyEquals(k, unmaskNullKey(existing))) { return index; } if (++index == keys.length) { index = 0; } } } /** * Returns the index in the key table at which a particular key resides, or * the index of an empty slot in the table where this key should be inserted * if it is not already in the table. Default access to avoid synthetic * accessors from inner classes. */ int findKeyOrEmpty(Object k) { int index = getKeyIndex(k); while (true) { Object existing = keys[index]; if (existing == null) { return index; } if (keyEquals(k, unmaskNullKey(existing))) { return index; } if (++index == keys.length) { index = 0; } } } /** * Removes the entry 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) { keys[index] = null; values[index] = null; --size; plugHole(index); } private int getKeyIndex(Object k) { int h = keyHashCode(k); // 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 & (keys.length - 1); } private void initTable(int capacity) { keys = new Object[capacity]; values = new Object[capacity]; } private void internalPutAll(Map<? extends K, ? extends V> m) { for (Entry<? extends K, ? extends V> entry : m.entrySet()) { K key = entry.getKey(); V value = entry.getValue(); int index = findKeyOrEmpty(key); if (keys[index] == null) { ++size; keys[index] = maskNullKey(key); values[index] = value; } else { values[index] = value; } } } /** * 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 == keys.length) { index = 0; } while (keys[index] != null) { int targetIndex = getKeyIndex(unmaskNullKey(keys[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! keys[hole] = keys[index]; values[hole] = values[index]; keys[index] = null; values[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! keys[hole] = keys[index]; values[hole] = values[index]; keys[index] = null; values[index] = null; hole = index; } } if (++index == keys.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); } }