> https://mp.weixin.qq.com/s/AHWzboztt53ZfFZmsSnMSw
HashMap `ConcurrentHashMap ` HashMap
## 1. ConcurrentHashMap 1.7
### 1.
Java 7 ConcurrentHashMap ConcurrnetHashMap Segment Segment HashMap HashMap Segment **** Segment 16 ConcurrentHashMap 16
### 2.
ConcurrentHashMap ConcurrentHashMap
```java
/**
* Creates a new, empty map with a default initial capacity (16),
* load factor (0.75) and concurrencyLevel (16).
*/
public ConcurrentHashMap() {
this(DEFAULT_INITIAL_CAPACITY, DEFAULT_LOAD_FACTOR, DEFAULT_CONCURRENCY_LEVEL);
}
```
```java
/**
*
*/
static final int DEFAULT_INITIAL_CAPACITY = 16;
/**
*
*/
static final float DEFAULT_LOAD_FACTOR = 0.75f;
/**
*
*/
static final int DEFAULT_CONCURRENCY_LEVEL = 16;
```
```java
@SuppressWarnings("unchecked")
public ConcurrentHashMap(int initialCapacity,float loadFactor, int concurrencyLevel) {
//
if (!(loadFactor > 0) || initialCapacity < 0 || concurrencyLevel segmentShift) & segmentMask)