A brief introduction to the principle of HashMap
All of the following code comes from the JDk1.8 source hashmap.java
Source code Reading 1
HashMap which part of the source
// Initialize the array size by default
static final int DEFAULT_INITIAL_CAPACITY = 1 << 4; // aka 16
// Maximum capacity
static final int MAXIMUM_CAPACITY = 1 << 30;
// Default load factor
static final float DEFAULT_LOAD_FACTOR = 0.75 f;
// The threshold for the size of the red-black tree's list
static final int TREEIFY_THRESHOLD = 8;
// Remove the minimum threshold for red-black tree structures
static final int UNTREEIFY_THRESHOLD = 6;
To avoid conflicts between resizing and the TREEIFY_THRESHOLD, the threshold cannot be higher than 4 * TREEIFY_THRESHOLD
static final int MIN_TREEIFY_CAPACITY = 64; .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);
}
public HashMap(int initialCapacity) {
this(initialCapacity, DEFAULT_LOAD_FACTOR);
}
public HashMap(a) {
this.loadFactor = DEFAULT_LOAD_FACTOR; // all other fields defaulted
}
public HashMap(Map<? extends K, ? extends V> m) {
this.loadFactor = DEFAULT_LOAD_FACTOR;
putMapEntries(m, false);
}
Copy the codeFrom the constructor of a HashMap, the initialization array size and load factor can be specified
Source code Reading 2
What exactly is the underlying HashMap made of
// Node array
transientNode<K,V>[] table; .// Single linked list
static class Node<K.V> implements Map.Entry<K.V> {
final int hash;
final K key;
V value;
Node<K,V> next;
Node(int hash, K key, V value, Node<K,V> next) {
this.hash = hash;
this.key = key;
this.value = value;
this.next = next; }... }.../ / tree node
static final class TreeNode<K.V> extends LinkedHashMap.Entry<K.V> {
TreeNode<K,V> parent; // red-black tree links
TreeNode<K,V> left;
TreeNode<K,V> right;
TreeNode<K,V> prev; // needed to unlink next upon deletion
boolean red;
TreeNode(int hash, K key, V val, Node<K,V> next) {
super(hash, key, val, next); }... }Copy the codeAs can be seen from the code, the bottom layer is composed of array + linked list + tree
Read source code 3
public V put(K key, V value) {
return putVal(hash(key), key, value, false.true);
}
/**
* Implements Map.put and related methods
*
* @param hash hash for key
* @param key the key
* @param value the value to put
* @param onlyIfAbsent if true, don't change existing value
* @param evict if false, the table is in creation mode.
* @return previous value, or null if none
*/
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)
n = (tab = resize()).length;
// Determine the length of the node array
if ((p = tab[i = (n - 1) & hash]) == null)
tab[i] = newNode(hash, key, value, null);
// Add a new node if the index is hashed
else {
Node<K,V> e; K k;
if(p.hash == hash && ((k = p.key) == key || (key ! =null && key.equals(k))))
e = p;
else if (p instanceof TreeNode)
e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);
// If it is a tree structure, it needs to be added as a tree
else {
for (int binCount = 0; ; ++binCount) {
if ((e = p.next) == null) {
p.next = newNode(hash, key, value, null);
// If the number of linked lists reaches 8, convert to a tree structure
if (binCount >= TREEIFY_THRESHOLD - 1) // -1 for 1st
treeifyBin(tab, hash);
break;
}
if(e.hash == hash && ((k = e.key) == key || (key ! =null && key.equals(k))))
break; p = e; }}if(e ! =null) { // existing mapping for key
// Key exists, update value
V oldValue = e.value;
if(! onlyIfAbsent || oldValue ==null)
e.value = value;
afterNodeAccess(e);
return oldValue;
}
}
++modCount;
if (++size > threshold)
resize();
/ / capacity
afterNodeInsertion(evict);
return null;
}
Copy the codePut process analysis:
- Determine the length of the node array
- If there is no hash conflict, add the first node; if there is hash conflict, link the node; if the list length exceeds 8, convert it to a tree; Index = HashCode (Key) & (length-1)
- If a key already exists, the old value is replaced
Read source code 4
public V get(Object key) {
Node<K,V> e;
return (e = getNode(hash(key), key)) == null ? null : e.value;
}
/**
* Implements Map.get and related methods
*
* @param hash hash for key
* @param key the key
* @return the node, or null if none
*/
final Node<K,V> getNode(int hash, Object key) {
Node<K,V>[] tab; Node<K,V> first, e; int n; K k;
if((tab = table) ! =null && (n = tab.length) > 0 &&
(first = tab[(n - 1) & hash]) ! =null) {
if (first.hash == hash && // always check first node((k = first.key) == key || (key ! =null && key.equals(k))))
return first;
if((e = first.next) ! =null) {
if (first instanceof TreeNode)
return ((TreeNode<K,V>)first).getTreeNode(hash, key);
do {
if(e.hash == hash && ((k = e.key) == key || (key ! =null && key.equals(k))))
return e;
} while((e = e.next) ! =null); }}return null;
}
Copy the codeGet process analysis:
- Compute the hash value from key
- The first node is checked each time to determine whether it is a list node or a tree node
- List search if it’s a linked list, tree search if it’s a tree
Read source code 5
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) {
// Determine if the maximum value is exceeded. If not, return the original array
if (oldCap >= MAXIMUM_CAPACITY) {
threshold = Integer.MAX_VALUE;
return oldTab;
}
else if ((newCap = oldCap << 1) < MAXIMUM_CAPACITY &&
oldCap >= DEFAULT_INITIAL_CAPACITY)
newThr = oldThr << 1; // double threshold
}
else if (oldThr > 0) // initial capacity was placed in threshold
newCap = oldThr;
else { // zero initial threshold signifies using defaults
newCap = DEFAULT_INITIAL_CAPACITY;
newThr = (int)(DEFAULT_LOAD_FACTOR * DEFAULT_INITIAL_CAPACITY);
// New threshold, load factor * initial value
}
if (newThr == 0) {
float ft = (float)newCap * loadFactor;
newThr = (newCap < MAXIMUM_CAPACITY && ft < (float)MAXIMUM_CAPACITY ?
(int)ft : Integer.MAX_VALUE);
// If the capacity does not exceed the maximum, the capacity expansion value will be updated
}
threshold = newThr;
@SuppressWarnings({"rawtypes","unchecked"})
Node<K,V>[] newTab = (Node<K,V>[])new Node[newCap];
Create a new array object
table = newTab;
// Update the new threshold
if(oldTab ! =null) {
// Migrate the data from the old array to the new array
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;
elsehiTail.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;
}
Copy the codeCapacity expansion process analysis:
- Check whether the value exceeds the maximum value. If not, return the original array. If yes, expand the array
- Calculate the size of the new expansion, newCap = oldCap << 1, double the size
- Creating a new array
- Migrate the data from the old array to the new array according to the rules
- Return a new array
The illustration process
Put the process
Initialize a new array
The get process
Get (k2), calculate the hash value of K2, search according to the index, and judge the key value
Expansion process
According to theHashMap.Size >= Capacity * LoadFactorFormula, the capacity expansion condition is met, 6>=8*0.75
Multithreading problem
conclusion
- A HashMap is a collection used to store key-value pairs
- The bottom layer consists of arrays, linked lists and red-black trees
- Initial capacity is 8 and load factor is 0.75
- Threads are not safe and linked list loops may occur
Ps: limited ability, if there are mistakes, please actively point out, thank you! Pay attention to CSDN