The HashMap of Java.

Class:

public class HashMap<K,V> extends AbstractMap<K,V>    implements Map<K,V>, Cloneable, Serializable

• Inherited from Abstractmap implements the Map,cloneable,serializable interface
• Can be serialized
• Can be clone

//默认初始容量为16static final int DEFAULT_INITIAL_CAPACITY = 1 << 4; // aka 16//最大容量static final int MAXIMUM_CAPACITY = 1 << 30//默认扩容因子static final float DEFAULT_LOAD_FACTOR = 0.75f//红黑树转链表的阀值static final int TREEIFY_THRESHOLD = 8;//链表转红黑树阀值static final int UNTREEIFY_THRESHOLD = 6;//存储方式由链表转成红黑树的容量的最小阈值static final int MIN_TREEIFY_CAPACITY = 64;

Initialization

Map<String, Object> map = new HashMap<>();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);    this.loadFactor = loadFactor;    this.threshold = tableSizeFor(initialCapacity);}

• New Hashmap<> () will enter HashMap (int initialcapacity, float loadfactor) method
• Initialcapacity to 0xB
• Loadfactor defaults to 0.75
• The first step is to determine if the value of initialcapacity is less than 0 or greater than 1 << 30, the second step is to determine if Loadfactor is greater than 0 and whether it is a floating-point number, the third step sets the loadfactor of the instance to 0.75, and the fourth step is 16

Tablesizefor

//返回大于或等于cap且为2的幂的数值static final int tableSizeFor(int cap) {    int n = cap - 1;    n |= n >>> 1;    n |= n >>> 2;    n |= n >>> 4;    n |= n >>> 8;    n |= n >>> 16;    return (n < 0) ? 1 : (n >= MAXIMUM_CAPACITY) ? MAXIMUM_CAPACITY : n + 1;}

• Cap-1: If the cap does not subtract the power of the 1,cap=2, the result is twice times the cap and does not match the expected

Hash method

static final int hash(Object key) {    int h;    return (key == null) ? 0 : (h = key.hashCode()) ^ (h >>> 16);}

• Hashcode high 16bit with low 16bit participation XOR operation
• Null in NO. 0 place

Node Object

Put method

public V put(K key, V value) {    return putVal(hash(key), key, value, false, true);}

Final V putval (int hash, K key, V value, Boolean onlyifabsent, Boolean evict) {node<k,v>[] tab ; Node<k,v> p;    int n, I; if (tab = table) = = NULL | |    (n = tab.length) = = 0)//capacity expansion, the first expansion length of n = (tab = resize ()). length; (n-1) & hash length must be between the 0-n//(n-1) & hash equals hash% n, and the operation ratio modulo fast//n-1 2 is 01111...,n 2 of the second square if (p = tab[    i = (n-1) & hash]) = = null) tab[i] = NewNode (hash, key, value, NULL); else {node<k,v> E;        K K; hash and equals or = phase, replace if (P.hash = = Hash && (k = p.key) = = Key | |            (Key! = null && key.equals (k))))        e = p; else if (P instanceof TreeNode)//node is a tree node, add e = ((treenode<k,v>) p) to the red-black tree. Puttreeval (This, tab, hash, key        , value); else {//list for (int bincount = 0;; ++bincount) {//Add at end of list if (E = p.next) = = null) {P.next = NewNode (Hash, key, value, NULL);                        When bincount>=7 is converted to red black tree if (bincount >= treeify_threshold-1)//-1 for 1st                    Treeifybin (tab, hash);                Break }//already exists if (E.hash = = Hash && (k = e.key) = = Key | |                    (Key! = null && key.equals (k))))                Break            Next object p = e;            }}//already exists key if (E! = null) {V oldValue = E.value;            if (!onlyifabsent | | oldValue = = NULL) E.value = value;            Afternodeaccess (e);        return oldValue;    }} ++modcount;    Expansion if (++size > Threshold) resize ();    Afternodeinsertion (evict); return null;}

• modulo x 2^n = = x & (2^n-1)

Expansion

Final node<k,v>[] Resize () {node<k,v>[] oldtab = table; int Oldcap = (Oldtab = = null)?    0:oldtab.length;    int oldthr = threshold;    int Newcap, newthr = 0;            if (Oldcap > 0) {if (Oldcap >= maximum_capacity) {threshold = Integer.max_value;        return oldtab; } else if ((Newcap = oldcap << 1) < maximum_capacity && Oldcap >= Default_initi al_capacity) Newthr = oldthr << 1;    Double threshold} else if (Oldthr > 0)//initial capacity is placed in threshold Newcap = OLDTHR;        else {//Zero initial threshold signifies using defaults newcap = default_initial_capacity;    NEWTHR = (int) (Default_load_factor * default_initial_capacity);        } if (Newthr = = 0) {Float ft = (float) newcap * loadfactor;                  Newthr = (Newcap < maximum_capacity && ft < (float) maximum_capacity? (int) Ft:inteGer.    Max_value);    } threshold = Newthr;    @SuppressWarnings ({"Rawtypes", "Unchecked"}) node<k,v>[] NewTab = (node<k,v>[]) new NODE[NEWCAP];    Table = NewTab;            if (oldtab! = null) {for (int j = 0; j < Oldcap; ++j) {node<k,v> e;                if ((e = oldtab[j])! = null) {OLDTAB[J] = null;                if (E.next = = null) Newtab[e.hash & (newCap-1)] = e;                else if (e instanceof TreeNode) ((treenode<k,v>) e). Split (This, NewTab, J, Oldcap);                    else {//preserve order node<k,v> Lohead = null, lotail = NULL;                    Node<k,v> hihead = null, hitail = NULL;                    Node<k,v> Next;                        do {next = E.next; if ((E.hash & oldcap) = = 0) {if (Lotail = = null) Lohead = E         ;                   else Lotail.next = e;                        Lotail = e;                            } else {if (Hitail = = null) Hihead = e;                            else Hitail.next = e;                        Hitail = e;                    }} while ((e = next) = null);                        if (lotail! = null) {lotail.next = null;                    NEWTAB[J] = Lohead;                        } if (Hitail! = null) {hitail.next = null;                    Newtab[j + oldcap] = Hihead; }}}}} return newTab;}

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