IntObjectHashMap和HashMap的区别？
源码来源地址：https://github.com/netty/netty/tree/4.0
不恰当的数据结构导致内存在占用过大。这个问题，就完全可以使用 Netty 的IntObjectHashMap 数据结构来解决，只需要换个类，就可以节省非常多的资源。
相同点：
IntObjectHashMap和HashMap都是以键值对的形式，储存对象。
不同点：
IntObjectHashMap的键只能是int类型，值是Object类型。
HashMap的键和值都是Object类型。
目录
一、IntObjectMap.java
package com;
import java.util.Collection;
/**
* Interface for a primitive map that uses {@code int}s as keys.
*
* @param <V> the value type stored in the map.
*/
public interface IntObjectMap<V> {
/**
* An Entry in the map.
*
* @param <V> the value type stored in the map.
*/
interface Entry<V> {
/**
* Gets the key for this entry.
*/
int key();
/**
* Gets the value for this entry.
*/
V value();
/**
* Sets the value for this entry.
*/
void setValue(V value);
}
/**
* Gets the value in the map with the specified key.
*
* @param key the key whose associated value is to be returned.
* @return the value or {@code null} if the key was not found in the map.
*/
V get(int key);
/**
* Puts the given entry into the map.
*
* @param key the key of the entry.
* @param value the value of the entry.
* @return the previous value for this key or {@code null} if there was no
* previous mapping.
*/
V put(int key, V value);
/**
* Puts all of the entries from the given map into this map.
*/
void putAll(IntObjectMap<V> sourceMap);
/**
* Removes the entry with the specified key.
*
* @param key the key for the entry to be removed from this map.
* @return the previous value for the key, or {@code null} if there was no
* mapping.
*/
V remove(int key);
/**
* Returns the number of entries contained in this map.
*/
int size();
/**
* Indicates whether or not this map is empty (i.e {@link #size()} ==
* {@code 0]).
*/
boolean isEmpty();
/**
* Clears all entries from this map.
*/
void clear();
/**
* Indicates whether or not this map contains a value for the specified key.
*/
boolean containsKey(int key);
/**
* Indicates whether or not the map contains the specified value.
*/
boolean containsValue(V value);
/**
* Gets an iterable collection of the entries contained in this map.
*/
Iterable<Entry<V>> entries();
/**
* Gets the keys contained in this map.
*/
int[] keys();
/**
* Gets the values contained in this map.
*/
V[] values(Class<V> clazz);
/**
* Gets the values contatins in this map as a {@link Collection}.
*/
Collection<V> values();
}二、IntObjectHashMap.java
package com;
/*
* Copyright 2014 The Netty Project
*
* The Netty Project licenses this file to you 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.lang.reflect.Array;
import java.util.AbstractCollection;
import java.util.Arrays;
import java.util.Collection;
import java.util.Iterator;
import java.util.NoSuchElementException;
/**
* A hash map implementation of {@link IntObjectMap} that uses open addressing
* for keys. To minimize the memory footprint, this class uses open addressing
* rather than chaining. Collisions are resolved using linear probing. Deletions
* implement compaction, so cost of remove can approach O(N) for full maps,
* which makes a small loadFactor recommended.
*
* @param <V> The value type stored in the map.
*/
public class IntObjectHashMap<V> implements IntObjectMap<V>, Iterable<IntObjectMap.Entry<V>> {
/** Default initial capacity. Used if not specified in the constructor */
private static final int DEFAULT_CAPACITY = 11;
/** Default load factor. Used if not specified in the constructor */
private static final float DEFAULT_LOAD_FACTOR = 0.5f;
/**
* Placeholder for null values, so we can use the actual null to mean available.
* (Better than using a placeholder for available: less references for GC
* processing.)
*/
private static final Object NULL_VALUE = new Object();
/** The maximum number of elements allowed without allocating more space. */
private int maxSize;
/** The load factor for the map. Used to calculate {@link #maxSize}. */
private final float loadFactor;
private int[] keys;
private V[] values;
private Collection<V> valueCollection;
private int size;
public IntObjectHashMap() {
this(DEFAULT_CAPACITY, DEFAULT_LOAD_FACTOR);
}
public IntObjectHashMap(int initialCapacity) {
this(initialCapacity, DEFAULT_LOAD_FACTOR);
}
public IntObjectHashMap(int initialCapacity, float loadFactor) {
if (initialCapacity < 1) {
throw new IllegalArgumentException("initialCapacity must be >= 1");
}
if (loadFactor <= 0.0f || loadFactor > 1.0f) {
// Cannot exceed 1 because we can never store more than capacity elements;
// using a bigger loadFactor would trigger rehashing before the desired load is
// reached.
throw new IllegalArgumentException("loadFactor must be > 0 and <= 1");
}
this.loadFactor = loadFactor;
// Adjust the initial capacity if necessary.
int capacity = adjustCapacity(initialCapacity);
// Allocate the arrays.
keys = new int[capacity];
@SuppressWarnings({ "unchecked", "SuspiciousArrayCast" })
V[] temp = (V[]) new Object[capacity];
values = temp;
// Initialize the maximum size value.
maxSize = calcMaxSize(capacity);
}
private static <T> T toExternal(T value) {
return value == NULL_VALUE ? null : value;
}
@SuppressWarnings("unchecked")
private static <T> T toInternal(T value) {
return value == null ? (T) NULL_VALUE : value;
}
@Override
public V get(int key) {
int index = indexOf(key);
return index == -1 ? null : toExternal(values[index]);
}
@Override
public V put(int key, V value) {
int startIndex = hashIndex(key);
int index = startIndex;
for (;;) {
if (values[index] == null) {
// Found empty slot, use it.
keys[index] = key;
values[index] = toInternal(value);
growSize();
return null;
}
if (keys[index] == key) {
// Found existing entry with this key, just replace the value.
V previousValue = values[index];
values[index] = toInternal(value);
return toExternal(previousValue);
}
// Conflict, keep probing ...
if ((index = probeNext(index)) == startIndex) {
// Can only happen if the map was full at MAX_ARRAY_SIZE and couldn't grow.
throw new IllegalStateException("Unable to insert");
}
}
}
private int probeNext(int index) {
return index == values.length - 1 ? 0 : index + 1;
}
@Override
public void putAll(IntObjectMap<V> sourceMap) {
if (sourceMap instanceof IntObjectHashMap) {
// Optimization - iterate through the arrays.
IntObjectHashMap<V> source = (IntObjectHashMap<V>) sourceMap;
for (int i = 0; i < source.values.length; ++i) {
V sourceValue = source.values[i];
if (sourceValue != null) {
put(source.keys[i], sourceValue);
}
}
return;
}
// Otherwise, just add each entry.
for (Entry<V> entry : sourceMap.entries()) {
put(entry.key(), entry.value());
}
}
@Override
public V remove(int key) {
int index = indexOf(key);
if (index == -1) {
return null;
}
V prev = values[index];
removeAt(index);
return toExternal(prev);
}
@Override
public int size() {
return size;
}
@Override
public boolean isEmpty() {
return size == 0;
}
@Override
public void clear() {
Arrays.fill(keys, 0);
Arrays.fill(values, null);
size = 0;
}
@Override
public boolean containsKey(int key) {
return indexOf(key) >= 0;
}
@Override
public boolean containsValue(V value) {
V v1 = toInternal(value);
for (V v2 : values) {
// The map supports null values; this will be matched as
// NULL_VALUE.equals(NULL_VALUE).
if (v2 != null && v2.equals(v1)) {
return true;
}
}
return false;
}
@Override
public Iterable<Entry<V>> entries() {
return this;
}
@Override
public Iterator<Entry<V>> iterator() {
return new IteratorImpl();
}
@Override
public int[] keys() {
int[] outKeys = new int[size()];
int targetIx = 0;
for (int i = 0; i < values.length; ++i) {
if (values[i] != null) {
outKeys[targetIx++] = keys[i];
}
}
return outKeys;
}
@Override
public V[] values(Class<V> clazz) {
@SuppressWarnings("unchecked")
V[] outValues = (V[]) Array.newInstance(clazz, size());
int targetIx = 0;
for (V value : values) {
if (value != null) {
outValues[targetIx++] = value;
}
}
return outValues;
}
@Override
public Collection<V> values() {
Collection<V> valueCollection = this.valueCollection;
if (valueCollection == null) {
this.valueCollection = valueCollection = new AbstractCollection<V>() {
@Override
public Iterator<V> iterator() {
return new Iterator<V>() {
final Iterator<Entry<V>> iter = IntObjectHashMap.this.iterator();
@Override
public boolean hasNext() {
return iter.hasNext();
}
@Override
public V next() {
return iter.next().value();
}
@Override
public void remove() {
throw new UnsupportedOperationException();
}
};
}
@Override
public int size() {
return size;
}
};
}
return valueCollection;
}
@Override
public int hashCode() {
// Hashcode is based on all non-zero, valid keys. We have to scan the whole keys
// array, which may have different lengths for two maps of same size(), so the
// capacity cannot be used as input for hashing but the size can.
int hash = size;
for (int key : keys) {
// 0 can be a valid key or unused slot, but won't impact the hashcode in either
// case.
// This way we can use a cheap loop without conditionals, or hard-to-unroll
// operations,
// or the devastatingly bad memory locality of visiting value objects.
// Also, it's important to use a hash function that does not depend on the
// ordering
// of terms, only their values; since the map is an unordered collection and
// entries can end up in different positions in different maps that have the
// same
// elements, but with different history of puts/removes, due to conflicts.
hash ^= key;
}
return hash;
}
@Override
public boolean equals(Object obj) {
if (this == obj) {
return true;
}
if (!(obj instanceof IntObjectMap)) {
return false;
}
@SuppressWarnings("rawtypes")
IntObjectMap other = (IntObjectMap) obj;
if (size != other.size()) {
return false;
}
for (int i = 0; i < values.length; ++i) {
V value = values[i];
if (value != null) {
int key = keys[i];
Object otherValue = other.get(key);
if (value == NULL_VALUE) {
if (otherValue != null) {
return false;
}
} else if (!value.equals(otherValue)) {
return false;
}
}
}
return true;
}
/**
* Locates the index for the given key. This method probes using double hashing.
*
* @param key the key for an entry in the map.
* @return the index where the key was found, or {@code -1} if no entry is found
* for that key.
*/
private int indexOf(int key) {
int startIndex = hashIndex(key);
int index = startIndex;
for (;;) {
if (values[index] == null) {
// It's available, so no chance that this value exists anywhere in the map.
return -1;
}
if (key == keys[index]) {
return index;
}
// Conflict, keep probing ...
if ((index = probeNext(index)) == startIndex) {
return -1;
}
}
}
/**
* Returns the hashed index for the given key.
*/
private int hashIndex(int key) {
// Allowing for negative keys by adding the length after the first mod
// operation.
return (key % keys.length + keys.length) % keys.length;
}
/**
* Grows the map size after an insertion. If necessary, performs a rehash of the
* map.
*/
private void growSize() {
size++;
if (size > maxSize) {
// Need to grow the arrays. We take care to detect integer overflow,
// also limit array size to ArrayList.MAX_ARRAY_SIZE.
rehash(adjustCapacity((int) Math.min(keys.length * 2.0, Integer.MAX_VALUE - 8)));
} else if (size == keys.length) {
// Open addressing requires that we have at least 1 slot available. Need to
// refresh
// the arrays to clear any removed elements.
rehash(keys.length);
}
}
/**
* Adjusts the given capacity value to ensure that it's odd. Even capacities can
* break probing.
*/
private static int adjustCapacity(int capacity) {
return capacity | 1;
}
/**
* Removes entry at the given index position. Also performs opportunistic,
* incremental rehashing if necessary to not break conflict chains.
*
* @param index the index position of the element to remove.
*/
private void removeAt(int index) {
--size;
// Clearing the key is not strictly necessary (for GC like in a regular
// collection),
// but recommended for security. The memory location is still fresh in the cache
// anyway.
keys[index] = 0;
values[index] = null;
// In the interval from index to the next available entry, the arrays may have
// entries
// that are displaced from their base position due to prior conflicts. Iterate
// these
// entries and move them back if possible, optimizing future lookups.
// Knuth Section 6.4 Algorithm R, also used by the JDK's IdentityHashMap.
int nextFree = index;
for (int i = probeNext(index); values[i] != null; i = probeNext(i)) {
int bucket = hashIndex(keys[i]);
if (i < bucket && (bucket <= nextFree || nextFree <= i) || bucket <= nextFree && nextFree <= i) {
// Move the displaced entry "back" to the first available position.
keys[nextFree] = keys[i];
values[nextFree] = values[i];
// Put the first entry after the displaced entry
keys[i] = 0;
values[i] = null;
nextFree = i;
}
}
}
/**
* Calculates the maximum size allowed before rehashing.
*/
private int calcMaxSize(int capacity) {
// Clip the upper bound so that there will always be at least one available
// slot.
int upperBound = capacity - 1;
return Math.min(upperBound, (int) (capacity * loadFactor));
}
/**
* Rehashes the map for the given capacity.
*
* @param newCapacity the new capacity for the map.
*/
private void rehash(int newCapacity) {
int[] oldKeys = keys;
V[] oldVals = values;
keys = new int[newCapacity];
@SuppressWarnings({ "unchecked", "SuspiciousArrayCast" })
V[] temp = (V[]) new Object[newCapacity];
values = temp;
maxSize = calcMaxSize(newCapacity);
// Insert to the new arrays.
for (int i = 0; i < oldVals.length; ++i) {
V oldVal = oldVals[i];
if (oldVal != null) {
// Inlined put(), but much simpler: we don't need to worry about
// duplicated keys, growing/rehashing, or failing to insert.
int oldKey = oldKeys[i];
int index = hashIndex(oldKey);
for (;;) {
if (values[index] == null) {
keys[index] = oldKey;
values[index] = toInternal(oldVal);
break;
}
// Conflict, keep probing. Can wrap around, but never reaches startIndex again.
index = probeNext(index);
}
}
}
}
/**
* Iterator for traversing the entries in this map.
*/
private final class IteratorImpl implements Iterator<Entry<V>>, Entry<V> {
private int prevIndex = -1;
private int nextIndex = -1;
private int entryIndex = -1;
private void scanNext() {
for (;;) {
if (++nextIndex == values.length || values[nextIndex] != null) {
break;
}
}
}
@Override
public boolean hasNext() {
if (nextIndex == -1) {
scanNext();
}
return nextIndex < keys.length;
}
@Override
public Entry<V> next() {
if (!hasNext()) {
throw new NoSuchElementException();
}
prevIndex = nextIndex;
scanNext();
// Always return the same Entry object, just change its index each time.
entryIndex = prevIndex;
return this;
}
@Override
public void remove() {
if (prevIndex < 0) {
throw new IllegalStateException("next must be called before each remove.");
}
removeAt(prevIndex);
prevIndex = -1;
}
// Entry implementation. Since this implementation uses a single Entry, we
// coalesce that
// into the Iterator object (potentially making loop optimization much easier).
@Override
public int key() {
return keys[entryIndex];
}
@Override
public V value() {
return toExternal(values[entryIndex]);
}
@Override
public void setValue(V value) {
values[entryIndex] = toInternal(value);
}
}
@Override
public String toString() {
if (size == 0) {
return "{}";
}
StringBuilder sb = new StringBuilder(4 * size);
for (int i = 0; i < values.length; ++i) {
V value = values[i];
if (value != null) {
sb.append(sb.length() == 0 ? "{" : ", ");
sb.append(keyToString(keys[i])).append('=').append(value == this ? "(this Map)" : value);
}
}
return sb.append('}').toString();
}
/**
* Helper method called by {@link #toString()} in order to convert a single map
* key into a string.
*/
protected String keyToString(int key) {
return Integer.toString(key);
}
}
