JDK7（7 U80）比较靠近8的版本。
HashMap<String,Integer> map = new HashMap<>();
创建一个HashMap对象，首先调用构造器HashMap();
public HashMap() {
this(DEFAULT_INITIAL_CAPACITY, DEFAULT_LOAD_FACTOR);
}
在构造器中调用HashMap(int initialCapacity, float loadFactor）构造方法进行初始化；
其中initialCapacity是初始容量，loadFactor 是影响因子。
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;
threshold = initialCapacity;
init();
}
// 初始化容量
static final int DEFAULT_INITIAL_CAPACITY = 1 << 4; // aka 16
// 初始影响因子
static final float DEFAULT_LOAD_FACTOR = 0.75f;
完成上述操作后创建对象的操作就完成了，可以看到并没有创建Entry数组(JDK7中维护的是一个Entry数组，等到8的时候就改名为Node)。
接着向map中put()看看如何执行
map.put("AA",12);
public V put(K key, V value) {
if (table == EMPTY_TABLE) {
inflateTable(threshold);
}
if (key == null)
return putForNullKey(value);
int hash = hash(key);
int i = indexFor(hash, table.length);
for (Entry<K,V> e = table[i]; e != null; e = e.next) {
Object k;
if (e.hash == hash && ((k = e.key) == key || key.equals(k))) {
V oldValue = e.value;
e.value = value;
e.recordAccess(this);
return oldValue;
}
}
modCount++;
addEntry(hash, key, value, i);
return null;
}
// 1，当table为空时,即当创建map后第一次插入时，调用inflateTable(threshold)创建entry数组
if (table == EMPTY_TABLE) {
inflateTable(threshold);
}
// threshold 在上文创建map对象时，将initialCapacity赋值给了threshold
private void inflateTable(int toSize) {
// Find a power of 2 >= toSize
// 返回创建数组的大小
int capacity = roundUpToPowerOf2(toSize);
//
threshold = (int) Math.min(capacity * loadFactor, MAXIMUM_CAPACITY + 1);
table = new Entry[capacity];
initHashSeedAsNeeded(capacity);
}
private static int roundUpToPowerOf2(int number) {
// assert number >= 0 : "number must be non-negative";
return number >= MAXIMUM_CAPACITY
? MAXIMUM_CAPACITY
: (number > 1) ? Integer.highestOneBit((number - 1) << 1) : 1;
}
```
```java
// 当所传的key值是空时，调用putForNullKey(value)方法，将key为空的entry对象放在数组的最头部。如果头部有数据，则往下一个找，直到找到一个为空的位置，或者找到一个key为null的更换value值。
if (key == null)
return putForNullKey(value);
```
```java
1,根据key1.hashCode()计算出key1的哈希值1，在根据hash()算出哈希值2 hash，在根据哈希值2和算法算出key1要插入数组的下标i。
int hash = hash(key);
int i = indexFor(hash, table.length);
2，若key1要插入的数组位置i没有元素，直接插入
若有数据，则比较两个key的哈希2的值，若相等，比较key1==key 或者比较 key1.equals(k) 若符合条件就替换value值
若一直没找到一样的就添加(key1,value)
for (Entry<K,V> e = table[i]; e != null; e = e.next) {
Object k;
if (e.hash == hash && ((k = e.key) == key || key.equals(k))) {
V oldValue = e.value;
e.value = value;
e.recordAccess(this);
return oldValue;
}
}
modCount++;
addEntry(hash, key, value, i);
return null;
```
总结：
添加/修改过程：
将（key1,value1）添加到当前的map中
1.1首先，需要调用key1所在类的hashCode()方法，计算key1对应的哈希值1，此哈希值1经过某种（hash()）之后，得到哈希值2.哈希值2再经过某种算法（indexFor()）之后就确定了（key1,value1）在数组种的索引位置i --------> 情况A
1.2 如果此索引位置i的数组上有元素（key2,value2）,则需要比较key和key2的哈希值 --->哈希冲突
2.1 如果key1的哈希值2与key2的哈希值2不相同，则（key1,value1）添加成功。------->情况B
2.2 如果key1的哈希值与key2的哈希值2相同，则需要继续比较key1与key2的equals（）。要调用key1所在类的equals(),将key2作为参数传递进去
3.1. 调用equals(),返回false:则（key1,value1）添加成功。------->情况B
3.2 调用equals(),返回true:则认为key1和key2是相同的。默认情况下，value1替换原有的values2。
情况A 放到数组的i位置
情况B
jdk8 与 jdk 7 的不同之处。
jdk8源码
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);
}
/**
* Constructs an empty <tt>HashMap</tt> with the specified initial
* capacity and the default load factor (0.75).
*
* @param initialCapacity the initial capacity.
* @throws IllegalArgumentException if the initial capacity is negative.
*/
public HashMap(int initialCapacity) {
this(initialCapacity, DEFAULT_LOAD_FACTOR);
}
/**
* Constructs an empty <tt>HashMap</tt> with the default initial capacity
* (16) and the default load factor (0.75).
*/
public HashMap() {
this.loadFactor = DEFAULT_LOAD_FACTOR; // all other fields defaulted
}
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;
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();
afterNodeInsertion(evict);
return null;
}
jdk17 源码
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);
}
/**
* Constructs an empty {@code HashMap} with the specified initial
* capacity and the default load factor (0.75).
*
* @param initialCapacity the initial capacity.
* @throws IllegalArgumentException if the initial capacity is negative.
*/
public HashMap(int initialCapacity) {
this(initialCapacity, DEFAULT_LOAD_FACTOR);
}
/**
* Constructs an empty {@code HashMap} with the default initial capacity
* (16) and the default load factor (0.75).
*/
public HashMap() {
this.loadFactor = DEFAULT_LOAD_FACTOR; // all other fields defaulted
}
/**
* Constructs a new {@code HashMap} with the same mappings as the
* specified {@code Map}. The {@code HashMap} is created with
* default load factor (0.75) and an initial capacity sufficient to
* hold the mappings in the specified {@code Map}.
*
* @param m the map whose mappings are to be placed in this map
* @throws NullPointerException if the specified map is null
*/
public HashMap(Map<? extends K, ? extends V> m) {
this.loadFactor = DEFAULT_LOAD_FACTOR;
putMapEntries(m, false);
}
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;
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();
afterNodeInsertion(evict);
return null;
}
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