HashTable internal storage structure
The internal storage structure of HashTable stores data in the form of array + one-way linked list, i.e. the defined entry <?,? > [] table variable
Source code analysis:
Variable definition:
//Use Entry Array storage data (Entry One-way linked list) private transient Entry<?,?>[] table; //Already stored in table Of Entry Number private transient int count; /**** * Entry When the array expansion threshold (count) count > = threshold, the expansion Entry array will be expanded * It is recommended not to set more than 1 or set too large, which will lead to too long linked list and slow query * For example, if hashtble initialCapacity = 5 LoadFactor = 0.75, then calculate threshold =3 * when count >=3 When table starts to expand (how to expand depends on the expansion code) * ***/ private int threshold; //Load factor for calculation threshold private float loadFactor; //Record modification times private transient int modCount = 0; //table Maximum length of array private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;
Entry point linked list structure
/** * Hashtable bucket collision list entry *One-way linked list */ private static class Entry<K,V> implements Map.Entry<K,V> { //hash value final int hash; //key final K key; //value V value; //Successor Entry<K,V> next; protected Entry(int hash, K key, V value, Entry<K,V> next) { this.hash = hash; this.key = key; this.value = value; this.next = next; } @SuppressWarnings("unchecked") protected Object clone() { return new Entry<>(hash, key, value, (next==null ? null : (Entry<K,V>) next.clone())); } // Map.Entry Ops public K getKey() { return key; } public V getValue() { return value; } public V setValue(V value) { if (value == null) throw new NullPointerException(); V oldValue = this.value; this.value = value; return oldValue; } public boolean equals(Object o) { if (!(o instanceof Map.Entry)) return false; Map.Entry<?,?> e = (Map.Entry<?,?>)o; return (key==null ? e.getKey()==null : key.equals(e.getKey())) && (value==null ? e.getValue()==null : value.equals(e.getValue())); } public int hashCode() { return hash ^ Objects.hashCode(value); } public String toString() { return key.toString()+"="+value.toString(); } }
Constructor
/*** * Initialize HashTable specifies initial capacity and load factor * HashTable Initialize core code * **/ public Hashtable(int initialCapacity, float loadFactor) { if (initialCapacity < 0) throw new IllegalArgumentException("Illegal Capacity: "+ initialCapacity); if (loadFactor <= 0 || Float.isNaN(loadFactor)) throw new IllegalArgumentException("Illegal Load: "+loadFactor); if (initialCapacity==0) initialCapacity = 1; this.loadFactor = loadFactor; //Initialization Entry array table = new Entry<?,?>[initialCapacity]; //Calculate expansion threshold threshold = (int)Math.min(initialCapacity * loadFactor, MAX_ARRAY_SIZE + 1); } /*** * Initialize HashTable specifies the initial capacity * The default load factor size is 0.75 * ***/ public Hashtable(int initialCapacity) { this(initialCapacity, 0.75f); } /*** * The default initialization Entry array size is 11 * loadFactor =0.75 ***/ public Hashtable() { this(11, 0.75f); } /*** * * ***/ public Hashtable(Map<? extends K, ? extends V> t) { this(Math.max(2*t.size(), 11), 0.75f); putAll(t); }
put method
/*** **Add element key value ** Note: put method is locked, which is why hashTable is blocked due to thread safety ****/ public synchronized V put(K key, V value) { // Make sure the value is not null if (value == null) { throw new NullPointerException(); } // Makes sure the key is not already in the hashtable. Entry<?,?> tab[] = table; //hashCode What is taken is key Of HashCode int hash = key.hashCode(); //according to hashCode & long The maximum hash gets the index of the array to be inserted by taking the modulus of the array length int index = (hash & 0x7FFFFFFF) % tab.length; @SuppressWarnings("unchecked") //Check whether there are elements under the subscript of the array. If there is convenience value(new)cover key Same value Entry<K,V> entry = (Entry<K,V>)tab[index]; for(; entry != null ; entry = entry.next) { if ((entry.hash == hash) && entry.key.equals(key)) { V old = entry.value; entry.value = value; return old; } } //Preservation addEntry(hash, key, value, index); return null; } /*** *Preservation ***/ private void addEntry(int hash, K key, V value, int index) { //Record put frequency +1 modCount++; Entry<?,?> tab[] = table; //Stored Entry Number >= Threshold capacity expansion if (count >= threshold) { // Rehash the table if the threshold is exceeded rehash(); tab = table; hash = key.hashCode(); index = (hash & 0x7FFFFFFF) % tab.length; } // Creates the new entry. @SuppressWarnings("unchecked") //Query the original Entry Linked list Entry<K,V> e = (Entry<K,V>) tab[index]; //take Entry<K,V> Save and Entry<K,V>.next Point to the original Entry Linked list tab[index] = new Entry<>(hash, key, value, e); //Array length+1 count++; }
Capacity expansion mechanism
/**** **Capacity expansion ** Trigger count > = threshold ****/ protected void rehash() { //Array before expansion(table)length int oldCapacity = table.length; //Before expansion table array Entry<?,?>[] oldMap = table; // overflow-conscious code //The expanded array length is oldCapacity*2+1 int newCapacity = (oldCapacity << 1) + 1; //Check if the length exceeds the upper limit if (newCapacity - MAX_ARRAY_SIZE > 0) { if (oldCapacity == MAX_ARRAY_SIZE) // Keep running with MAX_ARRAY_SIZE buckets return; newCapacity = MAX_ARRAY_SIZE; } // Create a new array with a length of oldCapacity*2+1 array Entry<?,?>[] newMap = new Entry<?,?>[newCapacity]; modCount++; //Computing threshold threshold = (int)Math.min(newCapacity * loadFactor, MAX_ARRAY_SIZE + 1); table = newMap; //takeIn the old array Entry Recalculate transfer to new array for (int i = oldCapacity ; i-- > 0 ;) { for (Entry<K,V> old = (Entry<K,V>)oldMap[i] ; old != null ; ) { Entry<K,V> e = old; old = old.next; int index = (e.hash & 0x7FFFFFFF) % newCapacity; e.next = (Entry<K,V>)newMap[index]; newMap[index] = e; } } }
Characteristic:
- key and value are not allowed to be empty. key is not allowed to be empty because hashcode takes the HashCode()
- The default construction method capacity of HashTable is initialized to 11 with a load factor of 0.75. If you use a construction method with a load factor to create a HashTable, please don't say that the load factor setting is too large. For example, if the initialization capacity is set to 1 and the load factor is set to 1000, the query will be slow
- HashTable is an array + single linked list entry < K, V > structure to store data
- The maximum length of array table is Integer.Max - 8;
- Capacity expansion condition count (number of hashtbale storage chain tables) > = threshold (threshold calculation method table.length * loadFactor)
- Thread safety