Hashtable
1. Contrast HashMap
1.
| project | HashMap | Hashtable |
| Default capacity | 16 | 11 |
| load factor | 0.75 | 0.75 |
| null value | Allow key/value to be empty | Do not allow key/value to be empty (null pointer exception during put operation) |
| Expansion | length*2 | length*2+1 |
| Safety | thread unsafe | thread safety |
| extends | AbstractMap | Dictionary |
| capacity | >=1 | 2^n |
2. The load factor is
too low , the space utilization is low, the frequent expansion is
too high, the addressing time increases, and the key value is too much;
3. The fast failure
modCount
is in the traversal process, if modCount > expectedCount
throw new ConcurrentModificationException
2. Class definition
1. Source code
public class Hashtable<K,V>
extends Dictionary<K,V>
implements Map<K,V>, Cloneable, java.io.Serializable
3. Properties
1. Source code
/**
* The hash table data.
*/
// array of Entry, Entry
private transient Entry[] table;
/**
* The total number of entries in the hash table.
*/
// capacity
private transient int count;
/**
* The table is rehashed when its size exceeds this threshold. (The
* value of this field is (int)(capacity * loadFactor).)
*
* @serial
*/
// critical value
private int threshold;
/**
* The load factor for the hashtable.
*
* @serial
*/
// load factor
private float loadFactor;
/**
* The number of times this Hashtable has been structurally modified
* Structural modifications are those that change the number of entries in
* the Hashtable or otherwise modify its internal structure (e.g.,
* rehash). This field is used to make iterators on Collection-views of
* the Hashtable fail-fast. (See ConcurrentModificationException).
*/
// modification times
private transient int modCount = 0;
/** use serialVersionUID from JDK 1.0.2 for interoperability */
private static final long serialVersionUID = 1421746759512286392L;
Fourth, the construction method
1.
// Initialize capacity, load factor
public Hashtable(int initialCapacity, float loadFactor) {
// Initialize capacity cannot be less than 0
if (initialCapacity < 0)
throw new IllegalArgumentException("Illegal Capacity: "+
initialCapacity);
// The load factor cannot be less than 0 and it is a float type of data
if (loadFactor <= 0 || Float.isNaN(loadFactor))
throw new IllegalArgumentException("Illegal Load: "+loadFactor);
if (initialCapacity==0)
// Initialize the capacity to any number, HashMap must be the N power of 2
initialCapacity = 1;
this.loadFactor = loadFactor;
table = new Entry[initialCapacity];
threshold = (int)(initialCapacity * loadFactor);
}
2.
// default load factor 0.75
public Hashtable(int initialCapacity) {
this(initialCapacity, 0.75f);
}
/**
* Constructs a new, empty hashtable with a default initial capacity (11)
* and load factor (0.75).
*/
// default initialize capacity 11
public Hashtable() {
this(11, 0.75f);
}
3. Take an existing collection as a parameter
/**
* Constructs a new hashtable with the same mappings as the given
* Map. The hashtable is created with an initial capacity sufficient to
* hold the mappings in the given Map and a default load factor (0.75).
*
* @param t the map whose mappings are to be placed in this map.
* @throws NullPointerException if the specified map is null.
* @since 1.2
*/
public Hashtable(Map<? extends K, ? extends V> t) {
// Call the constructor, and the initialization capacity is twice the number of key-value pairs in the existing set
this(Math.max(2*t.size(), 11), 0.75f);
putAll(t);
}
// Traverse the existing collection, call the put method, synchronized keyword, thread safety
public synchronized void putAll(Map<? extends K, ? extends V> t) {
for (Map.Entry<? extends K, ? extends V> e : t.entrySet())
put(e.getKey(), e.getValue());
}
Five, put ()
1. Source code
public synchronized V put(K key, V value) {
// Make sure the value is not null
// value is not allowed to be null, null pointer exception
if (value == null) {
throw new NullPointerException();
}
// Makes sure the key is not already in the hashtable.
// assign
Entry tab[] = table;
// key cannot be empty, otherwise a null pointer exception will be reported when the hashcode is obtained
// A hash, the hash value of the key itself
int hash = key.hashCode();
// Calculate the hash value twice, and then take the remainder with the length to determine the position stored in the array
int index = (hash & 0x7FFFFFFF) % tab.length;
// Take out the data at the index position, traverse the linked list under the array, and determine whether the key already exists
// If it already exists, replace the existing value and return the old value
for (Entry<K,V> e = tab[index] ; e != null ; e = e.next) {
if ((e.hash == hash) && e.key.equals(key)) {
V old = e.value;
e.value = value;
return old;
}
}
// increment the number of operations by 1
modCount++;
// Whether the number count of Entry in Hashtable is greater than the critical value
if (count >= threshold) {
// Rehash the table if the threshold is exceeded
// expand
rehash();
tab = table;
index = (hash & 0x7FFFFFFF) % tab.length;
}
// Creates the new entry.
// Assign the linked list at tab[index] position to e
Entry<K,V> e = tab[index];
// Add an Entry and put the original linked list in the next of the newly added Entry
// That is, the newly added Entry is placed at the top of the linked list
tab[index] = new Entry<K,V>(hash, key, value, e);
count++;
return null;
}
// Constructor of Entry
protected Entry(int hash, K key, V value, Entry<K,V> next) {
this.hash = hash;
this.key = key;
this.value = value;
this.next = next;
}
// expand
protected void rehash() {
// Capacity before expansion
int oldCapacity = table.length;
Entry[] oldMap = table;
// The expanded capacity length*2 + 1
int newCapacity = oldCapacity * 2 + 1;
Entry[] newMap = new Entry[newCapacity];
// increment the number of operations by 1
modCount++;
threshold = (int)(newCapacity * loadFactor);
table = newMap;
// Two-layer loop, the outer layer traverses the total length of the original array, and puts the old array into the new array in turn
for (int i = oldCapacity ; i-- > 0 ;) {
// Get the linked list at index i in the old data
for (Entry<K,V> old = oldMap[i] ; old != null ; ) {
// Get the element value at the current position of the linked list
Entry<K,V> e = old;
// pointer down
old = old.next;
// Calculate storage location
int index = (e.hash & 0x7FFFFFFF) % newCapacity;
// The current element points to the index subscript position in the new array
e.next = newMap[index];
// The index position of the new array points to e
newMap[index] = e;
}
// The result of the traversal is to put the elements in the linked list upside down into a new array
}
}
Six, remove ()
1. Source code
public synchronized V remove(Object key) {
Entry tab[] = table;
int hash = key.hashCode();
// locate the storage location
int index = (hash & 0x7FFFFFFF) % tab.length;
for (Entry<K,V> e = tab[index], prev = null ; e != null ; prev = e, e = e.next) {
if ((e.hash == hash) && e.key.equals(key)) {
modCount++;
// If the number of elements in the linked list is > 1, point the previous Entry.next to the next of the current element
if (prev != null) {
prev.next = e.next;
} else {
// traverse the conditions
// Entry<K,V> e = tab[index], prev = null
// In the next loop, assign prev = e; if prev == null
// Explain that the first Entry in the linked list is the target
tab[index] = e.next;
}
count--; // count represents the number of stored Entry
V oldValue = e.value;
e.value = null;
return oldValue;
}
}
Seven, get ()
1. Source code
public synchronized V get(Object key) {
Entry tab[] = table;
// key cannot be null, a null pointer is reported here
int hash = key.hashCode();
int index = (hash & 0x7FFFFFFF) % tab.length;
for (Entry<K,V> e = tab[index] ; e != null ; e = e.next) {
if ((e.hash == hash) && e.key.equals(key)) {
return e.value;
}
}
return null;
}