Core concepts for HashMap
Concept of hashing.
Collision resolution in HashMap.
Use of equals() and hashCode() and there importance in HashMap.
Benefit of immutable object.
Race condition on HashMap in Java.
Resizing of Java HashMap.
Null key handled in HashMap.
It is Hash table based implementation of the Map interface. This implementation provides all of the optional map operations, and permits null values and the null key.
The HashMap class is roughly equivalent to Hashtable, except that it is unsynchronized and permits nulls.
This class makes no guarantees as to the order of the map; in particular, it does not guarantee that the order will remain constant over time.
This implementation provides constant-time performance for the basic operations (get and put), assuming the hash function disperses the elements properly among the buckets.
Iteration over collection views requires time proportional to the "capacity" of the HashMap instance (the number of buckets) plus its size (the number of key-value mappings). Thus, it's very important not to set the initial capacity too high (or the load factor too low) if iteration performance is important.
initial capacity and load factor:
These two parameters affects the performance of HashMap.
The capacity is the number of buckets in the hash table, and the initial capacity is simply the capacity at the time the hash table is created.
The load factor is a measure of how full the hash table is allowed to get before its capacity is automatically increased.
When the number of entries in the hash table exceeds the product of the load factor and the current capacity, the hash table is rehashed (that is, internal data structures are rebuilt) so that the hash table has approximately twice the number of buckets.
load factor trade-off:
As a general rule, the default load factor (.75) offers a good tradeoff between time and space costs.
Higher values decrease the space overhead but increase the lookup cost (reflected in most of the operations of the HashMap class, includingget and put).
The expected number of entries in the map and its load factor should be taken into account when setting its initial capacity, so as to minimize the number of rehash operations.
If the initial capacity is greater than the maximum number of entries divided by the load factor, no rehash operations will ever occur.
Synchronization:
HashMap implementation is not synchronized.
If multiple threads access a hash map concurrently, and at least one of the threads modifies the map structurally, it must be synchronized externally.
A structural modification is any operation that adds or deletes one or more mappings; merely changing the value associated with a key that an instance already contains is not a structural modification.
This is typically accomplished by synchronizing on some object that naturally encapsulates the map.
If no such object exists, the map should be "wrapped" using the Collections.synchronizedMap method.
To prevent accidental unsynchronized access to the map:
Map m = Collections.synchronizedMap(new HashMap(...));
The iterators returned by all of this class's "collection view methods" are fail-fast:
If the map is structurally modified at any time after the iterator is created, in any way except through the iterator's own remove method, the iterator will throw a ConcurrentModificationException.
Note that the fail-fast behavior of an iterator cannot be guaranteed as it is, generally speaking, impossible to make any hard guarantees in the presence of unsynchronized concurrent modification.
Fail-fast iterators throw ConcurrentModificationException on a best-effort basis.
Therefore, it would be wrong to write a program that depended on this exception for its correctness: the fail-fast behavior of iterators should be used only to detect bugs.
package java.util;
import java.io.*;
public class HashMap<K,V> extends AbstractMap<K,V> implements Map<K,V>, Cloneable, Serializable {
/**Default initial capacity - MUST be a power of two. */
static final int DEFAULT_INITIAL_CAPACITY = 16;
static final int MAXIMUM_CAPACITY = 1 << 30;
/**load factor used when none specified in constructor. */
static final float DEFAULT_LOAD_FACTOR = 0.75f;
/**table, resized as necessary.
* Length MUST always be a power of two.*/
transient Entry[] table;
/**number of key-value mappings contained in this map.*/
transient int size;
/**next size value at which to resize (capacity * load factor).*/
int threshold;
/**load factor for the hash table.*/
final float loadFactor;
/**number of times this HashMap has been structurally modified
* Structural Modifications are those that change the number
* of mappings in the HashMap or otherwise modify its internal
* structure (e.g., rehash).
* This field is used to make iterators on Collection-views of
* the HashMap fail-fast. (ConcurrentModificationException).
*/
transient volatile int modCount;
/** Constructs an empty HashMap with the specified
* initial capacity and load factor.*/
public HashMap(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;
threshold = (int)(capacity * loadFactor);
table = new Entry[capacity];
init();
}
/** Constructs an empty HashMap with the specified
* initial capacity and the default load factor (0.75).
* @throws IllegalArgumentException
* - if the initial capacity is negative.
*/
public HashMap(int initialCapacity) {
this(initialCapacity, DEFAULT_LOAD_FACTOR);
}
/**Constructs an empty HashMap
* default initial capacity (16) and the default load factor (0.75).
*/
public HashMap() {
this.loadFactor = DEFAULT_LOAD_FACTOR;
threshold =
(int)(DEFAULT_INITIAL_CAPACITY * DEFAULT_LOAD_FACTOR);
table = new Entry[DEFAULT_INITIAL_CAPACITY];
init();
}
/**
* Constructs a new HashMap with the same mappings as the specified Map.
* The HashMap is created with default load factor (0.75) and an initial capacity
* sufficient to hold the mappings in the specified Map.
*/
public HashMap(Map<? extends K, ? extends V> m) {
this(Math.max((int) (m.size() / DEFAULT_LOAD_FACTOR) + 1,
DEFAULT_INITIAL_CAPACITY), DEFAULT_LOAD_FACTOR);
putAllForCreate(m);
}
public V put(K key, V value) {
if (key == null)
return putForNullKey(value);
int hash = hash(key.hashCode());
int i = indexFor(hash, table.length);
for (Entry<K,V> e = table[i]; e != null; e = e.next) {
Object k;
if (e.hash == hash && ((k = e.key) == key || key.equals(k))) {
V oldValue = e.value;
e.value = value;
e.recordAccess(this);
return oldValue;
}
}
modCount++;
addEntry(hash, key, value, i);
return null;
}
void addEntry(int hash, K key, V value, int bucketIndex) {
Entry<K,V> e = table[bucketIndex];
table[bucketIndex] = new Entry<K,V>(hash, key, value, e);
if (size++ >= threshold)
resize(2 * table.length);
}
public V get(Object key) {
if (key == null)
return getForNullKey();
int hash = hash(key.hashCode());
for (Entry<K,V> e = table[indexFor(hash, table.length)];
e != null;
e = e.next) {
Object k;
if (e.hash == hash && ((k = e.key) == key || key.equals(k)))
return e.value;
}
return null;
}
static int hash(int h) {
// This function ensures that hashCodes that differ only by
// constant multiples at each bit position have a bounded
// number of collisions (approximately 8 at default load factor).
h ^= (h >>> 20) ^ (h >>> 12);
return h ^ (h >>> 7) ^ (h >>> 4);
}
/**
* Returns a shallow copy of this HashMap instance: the keys and
* values themselves are not cloned.
* @return a shallow copy of this map
*/
public Object clone() {
HashMap<K,V> result = null;
try {
result = (HashMap<K,V>)super.clone();
} catch (CloneNotSupportedException e) {
// assert false;
}
result.table = new Entry[table.length];
result.entrySet = null;
result.modCount = 0;
result.size = 0;
result.init();
result.putAllForCreate(this);
return result;
}
/**
*Returns true if this map contains a mapping for the specified key.
*/
public boolean containsKey(Object key) {
return getEntry(key) != null;
}
/**
*Returns the entry associated with the specified key in the HashMap.
*Returns null if the HashMap contains no mapping for the key.
*/
final Entry<K,V> getEntry(Object key) {
int hash = (key == null) ? 0 : hash(key.hashCode());
for (Entry<K,V> e = table[indexFor(hash, table.length)];
e != null;
e = e.next) {
Object k;
if(e.hash==hash&&((k=e.key)==key||(key!= null&& key.equals(k))))
return e;
}
return null;
}
/**Returns true if this map maps one or more keys to the
* specified value. */
public boolean containsValue(Object value) {
if (value == null)
return containsNullValue();
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;
}
/**
*Removes the mapping for the specified key from this map if present.
*@param key key whose mapping is to be removed from the map
*@return the previous value associated with key,
* or null if there was no mapping for key.
* A null return can also indicate that the map previously
* associated null with key.
*/
public V remove(Object key) {
Entry<K,V> e = removeEntryForKey(key);
return (e == null ? null : e.value);
}
/**
* Removes and returns the entry associated with the
* specified key in the HashMap.
* Returns null if the HashMap contains no mapping for this key.
*/
final Entry<K,V> removeEntryForKey(Object key) {
int hash = (key == null) ? 0 : hash(key.hashCode());
int i = indexFor(hash, table.length);
Entry<K,V> prev = table[i];
Entry<K,V> e = prev;
while (e != null) {
Entry<K,V> next = e.next;
Object k;
if (e.hash == hash && ((k=e.key)==key||
(key!=null&& key.equals(k)))) {
modCount++;
size--;
if (prev == e)
table[i] = next;
else
prev.next = next;
e.recordRemoval(this);
return e;
}
prev = e;
e = next;
}
return e;
}
}
HashMap working
HashMap works on principle of hashing.
When we pass an both key and value to put() method to store on HashMap , it uses key object hashcode() method to calculate hashcode and they by applying hashing on that hashcode it identifies bucket location for storing value object.
While retrieving it uses key object equals method to find out correct key value pair and return value object associated with that key.
HashMap uses linked list in case of collision and object will be stored in next node of linked list.
What will happen if two different HashMap key objects have same hashcode?
They will be stored in same bucket but no next node of linked list and keys equals () method will be used to identify correct key value pair in HashMap.
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