【HashMap 和 ConcurrentHashMap 的区别
原文:https://www . geesforgeks . org/difference-hashmap-concurrenthashmap/
HashMap 是传统集合下的类，ConcurrentHashMap 是并发集合下的类，除此之外，它们之间还有各种不同之处，分别是:
- HashMap 本质上是非同步的，即 HashMap 不是线程安全的，而 ConcurrentHashMap 本质上是线程安全的。
- HashMap 的性能相对较高，因为它本质上是非同步的，任何数量的线程都可以同时执行。但是 ConcurrentHashMap 的性能有时很低，因为有时需要线程来等待 ConcurrentHashMap。
-
While one thread is Iterating the HashMap object, if other thread try to add/modify the contents of Object then we will get Run-time exception saying ConcurrentModificationException.Whereas In ConcurrentHashMap we wont get any exception while performing any modification at the time of Iteration.
使用 HashMap
```java // Java program to illustrate // HashMap drawbacks import java.util.HashMap;
class HashMapDemo extends Thread { static HashMap l=new HashMap();
public void run() {
try { Thread.sleep(1000); // Child thread trying to add // new element in the object l.put(103,"D"); } catch(InterruptedException e) { System.out.println("Child Thread going to add element"); } }
public static void main(String[] args) throws InterruptedException { l.put(100,"A"); l.put(101,"B"); l.put(102,"C"); HashMapDemo t=new HashMapDemo(); t.start();
for (Object o : l.entrySet()) { Object s=o; System.out.println(s); Thread.sleep(1000); } System.out.println(l); } } ```
输出:
```java 100=A Exception in thread "main" java.util.ConcurrentModificationException
```
使用 ConcurrentHashMap
```java // Java program to illustrate // HashMap drawbacks import java.util.HashMap; import java.util.concurrent.*;
class HashMapDemo extends Thread { static ConcurrentHashMap l = new ConcurrentHashMap();
public void run() {
// Child add new element in the object l.put(103,"D");
try { Thread.sleep(2000); } catch(InterruptedException e) { System.out.println("Child Thread going to add element"); } }
public static void main(String[] args) throws InterruptedException { l.put(100,"A"); l.put(101,"B"); l.put(102,"C"); HashMapDemo t=new HashMapDemo(); t.start();
for (Object o : l.entrySet()) { Object s=o; System.out.println(s); Thread.sleep(1000); } System.out.println(l); } } ```
输出:
```java 100=A 101=B 102=C 103=D
```
-
In HashMap, null values are allowed for key and values, whereas in ConcurrentHashMap null value is not allowed for key and value, otherwise we will get Run-time exception saying NullPointerException.
使用 HashMap
java //Java Program to illustrate ConcurrentHashMap behaviour import java.util.*; class ConcurrentHashMapDemo { public static void main(String[] args) { HashMap m=new HashMap(); m.put(100,"Hello"); m.put(101,"Geeks"); m.put(102,"Geeks"); m.put(null,"World"); System.out.println(m); } }输出:
```java
```
使用 ConcurrentHashMap
java //Java Program to illustrate HashMap behaviour import java.util.concurrent.*; class ConcurrentHashMapDemo { public static void main(String[] args) { ConcurrentHashMap m=new ConcurrentHashMap(); m.put(100,"Hello"); m.put(101,"Geeks"); m.put(102,"Geeks"); m.put(null,"World"); System.out.println(m); } }输出:
```java Exception in thread "main" java.lang.NullPointerException
```
-
HashMap 是在 JDK 1.2 中引入的，而 ConcurrentHashMap 是由 SUN 微系统在 JDK 1.5 中引入的。
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