# Detailed explanation of HashMap automatic capacity expansion mechanism source code

Posted by MrLister on Mon, 14 Feb 2022 05:01:28 +0100

# 1, Introduction

• HashMap's source code, which we have interpreted before, is an array plus a linked list. If the linked list is too long, it will split into a red black tree. The automatic capacity expansion mechanism is not detailed. Let's take a look at it in detail today # 2, Capacity expansion mechanism

• The capacity of hashmap is a multiple of 2, such as 2, 4, 8, 16, 32, 64
• Each expansion is doubled, 2 to 4, 4 to 8, 8 to 16, 16 to 32, etc
• Expansion factor: 0.75 by default, or a decimal point can be specified
• Capacity expansion time point: when the number of elements in the container reaches: capacity * capacity expansion factor, start capacity expansion

# 3, Source code analysis

(1) Let's look at the constructor first

```static final int DEFAULT_INITIAL_CAPACITY = 1 << 4; // aka 16
static final float DEFAULT_LOAD_FACTOR = 0.75f;

public HashMap() {
}
```

The default constructor specifies the expansion factor: 0.75, and the default capacity is 16

```public HashMap(int initialCapacity) {
}
```

Specify the initial capacity. The default expansion factor is 0.75

```public HashMap(int initialCapacity, float loadFactor) {
if (initialCapacity < 0)
throw new IllegalArgumentException("Illegal initial capacity: " +
initialCapacity);
if (initialCapacity > MAXIMUM_CAPACITY)
initialCapacity = MAXIMUM_CAPACITY;
throw new IllegalArgumentException("Illegal load factor: " +
this.threshold = tableSizeFor(initialCapacity);
}
```

Specify the initial capacity and expansion factor at the same time

```/**
* The next size value at which to resize (capacity * load factor).
*
* @serial
*/
int threshold;
```
• Note this variable: the next value to be expanded, expansion capacity, capacity * expansion factor
• Look at this sentence: this threshold = tableSizeFor(initialCapacity);
```/**
* Returns a power of two size for the given target capacity.
*/
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;
}
```
• This method is to take the multiple of 2 after rounding the given value, such as 3 - > 4, 15 - > 16, 27 - > 32
• So far, the preparatory work has been done. Let's look at the put method

(2) 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;
// ① At first, the table is null. Call the resize () method
if ((tab = table) == null || (n = tab.length) == 0)
n = (tab = resize()).length;
if ((p = tab[i = (n - 1) & hash]) == null)
tab[i] = newNode(hash, key, value, null);
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);
else {
for (int binCount = 0; ; ++binCount) {
if ((e = p.next) == null) {
p.next = newNode(hash, key, value, null);
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
V oldValue = e.value;
if (!onlyIfAbsent || oldValue == null)
e.value = value;
afterNodeAccess(e);
return oldValue;
}
}
++modCount;
// ② At the end, judge whether the capacity is greater than the expanded capacity. If it is greater than the expanded capacity, call the resize method
if (++size > threshold)
resize();
afterNodeInsertion(evict);
return null;
}
```
• ① At first, the table is null. Call the resize () method
• ② At the end, judge whether the capacity is greater than the expanded capacity. If it is greater than the expanded capacity, call the resize () method
• Look at the resize () method
```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_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;
}
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)
else
loTail.next = e;
loTail = e;
}
else {
if (hiTail == null)
else
hiTail.next = e;
hiTail = e;
}
} while ((e = next) != null);
if (loTail != null) {
loTail.next = null;
}
if (hiTail != null) {
hiTail.next = null;
}
}
}
}
}
return newTab;
}
```
• First, analyze the first case: Map = new hashmap();
• Take the last branch, with a capacity of 16 and an expansion capacity of 12
```else {
newCap = DEFAULT_INITIAL_CAPACITY;
}
```
• Analyze the second case: Map map = new HashMap(20);
• Take the second branch. As analyzed earlier, threshold = tableSizeFor(20) is 32
• New capacity newcap = oldThr is 32
```// capacity
else if (oldThr > 0) // initial capacity was placed in threshold
newCap = oldThr;
```
• New expansion capacity newThr = newCap * loadFactor is 24
```// Expansion capacity
if (newThr == 0) {
float ft = (float)newCap * loadFactor;
newThr = (newCap < MAXIMUM_CAPACITY && ft < (float)MAXIMUM_CAPACITY ?
(int)ft : Integer.MAX_VALUE);
}
threshold = newThr;
```
• Analyze the third case: 24 elements have been inserted into the above map, and a new one needs to be expanded
• Take the first branch, oldCap=32, oldThr=24
• Capacity expansion: newcap = oldcap < < 1 = 64
• Expansion capacity newthr = oldthr < < 1, 48
```if (oldCap > 0) {
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
}
```
• Finally, copy the elements to the new table
• Direct copy of a single element
• If it is a tree, call the copy method of the tree
• If it is a linked list, copy the circular linked list Welcome to WeChat official account: Feng Chi, more technical learning and sharing.