Recently, I began to review some basic knowledge , Find out a lot about HashMap There are some problems in the summary of , In particular, there are still some questions about the timing and threshold of capacity expansion , So decisive or look at the source .

This paper only points out some problems about capacity expansion , Other detailed analysis can refer to other articles .

When is the capacity expansion

• When put They found table When not initialized , Initial capacity expansion
• When put After joining the node , Find out size（ Number of key-value pairs ）>threshold when , super-popular

In the following source code respectively corresponding to the notes

 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; // Initialize expansion
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;
if (++size > threshold)
resize(); // Over threshold capacity expansion
afterNodeInsertion(evict);
return null;
}

How is the threshold determined

In fact, the key is to determine the threshold , Most articles on the Internet only say that the threshold is load factor*current capacity, This conclusion is correct , But the whole process is different depending on the constructor used .
Next we only analyze the relationship between threshold and capacity .
Here is resize Part of the code

 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;
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;
/*** It is omitted here table The changing process of the watch ***/
}

Parameter free constructor

public HashMap() {
this.loadFactor = DEFAULT_LOAD_FACTOR; // all other fields defaulted
}

You can see that the parameterless constructor here only initializes the load factor to the default value 0.75.
== Be careful , here hashmap Of table,threshold It’s not initialized yet , That is, the capacity at this time is 0, The threshold is also 0.==
And for the first time put when , Check table==null, For the first time resize

At this time in resize Will go into the following branches

So capacity cap As the default value 16, threshold threshold by 16*0.75=12.
And in every subsequent expansion , Both capacity and threshold are doubled .
That is to say, both of them are still 0.75 The proportion of .

Constructor with parameters

 public HashMap(int initialCapacity) {
this(initialCapacity, DEFAULT_LOAD_FACTOR);
}

 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);
}

In addition to the above code parameters in addition to the legitimate judgment , Mainly set load factor and threshold , And the threshold is tableSizeFor() The function determines .

 /**
* 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;
}

You can see the comments , threshold threshold Will be set closest to the given parameter 2ⁿ Value .
Be careful , At this point, only the threshold value is set ,table It’s not initialized yet , Capacity cap Also for 0.

It’s also in put when , For the first time , And the expansion branches are as follows

You can see , Capacity cap Set to the threshold just calculated 2ⁿ, And the threshold is reset to cap*loadFactor. Although the two still hold loadFactor The proportion of , But first set the threshold , And then assigned to the capacity , Then the threshold value is recalculated according to the capacity and load factor . In the process , Threshold takes on the role of an intermediate variable .

And in the subsequent expansion process , Capacity and threshold still keep the proportion of load factor , And at the same time become twice the original .