面试中,HashMap可以说是必问的,既然这样,我们应该怎么准备怎么回答呢,看看这篇文章,估计你会懂点东西。
先看看这两张图,是其内部的存储结构
说起HashMap,我们可以先从底层实现说起,HashMap是通过hash算法,基于数组、链表和红黑树实现的,hash算法是一种思想,只要符合该思想的算法都是hash算法,其核心就是给定一个key,通过hash可以对应一个h(key),举个例子就是当我们要存储一个key为字符串的一组数据,那么我们就可以通过hash将key转化为数字,做为数组的索引,然后将value存入该索引所对应的空间内,如果我们的数组长度是16,那么当有20个key来进行hash的时候,肯定存在冲突,也就是h(key1) = h (key2),此时解决的办法就是,让该索引指向一个链表,这样就可以让不同的key对应同一个h(key),获取的时候通过该key确定到某一个索引,然后遍历链表,但是当链表数据量很大的时候,遍历是很耗时的,于是红黑树就出现了,当链表元素大于等于8个的时候,就采用红黑树去存储,时间复杂度直接从O(n)降低到了O(logn)。
说到这,HashMap的核心思想也就差不多了,这些应该大部分人都知道,现在来点亮点,你可以说HashMap线程不安全,然后给他来这个例子进行演示,创建三个线程,同时去访问put方法
import java.util.HashMap;
import java.util.Map;
import java.util.concurrent.ConcurrentHashMap;
public class TestHashMap implements Runnable {
static Map<String,Object> map = new HashMap();
//static Map<String,Object> map = new ConcurrentHashMap<>();
@Override
public void run() {
for (int i = 0; i < 100; i ++) {
map.put(i + "", "hello");
}
}
public static void main(String[] args){
Thread thread1 = new Thread(new TestHashMap());
Thread thread2 = new Thread(new TestHashMap());
Thread thread3 = new Thread(new TestHashMap());
thread1.start();
thread2.start();
thread3.start();
Thread currentThread = Thread.currentThread();
try {
currentThread.sleep(2000);
} catch (Exception e) {
System.out.println(e.getMessage());
}
System.out.println(map.size());
}
}
按正常来说应该输出100的,但是这里并没有,而是输出了204
你可以给put方法上锁,用synchronized
import java.util.HashMap;
import java.util.Map;
import java.util.concurrent.ConcurrentHashMap;
public class TestHashMap implements Runnable {
static Map<String,Object> map = new HashMap();
//static Map<String,Object> map = new ConcurrentHashMap<>();
@Override
public void run() {
for (int i = 0; i < 100; i ++) {
synchronized (TestHashMap.class) { // 这里!!!
map.put(i + "", "hello");
}
}
}
public static void main(String[] args){
Thread thread1 = new Thread(new TestHashMap());
Thread thread2 = new Thread(new TestHashMap());
Thread thread3 = new Thread(new TestHashMap());
thread1.start();
thread2.start();
thread3.start();
Thread currentThread = Thread.currentThread();
try {
currentThread.sleep(2000);
} catch (Exception e) {
System.out.println(e.getMessage());
}
System.out.println(map.size());
}
}
这样是可以解决线程问题,但是,你这有点暴力了吧,直接把整个put方法都给上锁了,你看看put方法中有多少代码,都给锁上,有点过分
Hash Map的put源码
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;
}
好家伙,你这一锁,都给锁上了,不好,于是来了ConcurrentHashMap,线程安全的,先看看人家是怎么锁的
public V put(K key, V value) {
return putVal(key, value, false);
}
/** Implementation for put and putIfAbsent */
final V putVal(K key, V value, boolean onlyIfAbsent) {
if (key == null || value == null) throw new NullPointerException();
int hash = spread(key.hashCode());
int binCount = 0;
for (Node<K,V>[] tab = table;;) {
Node<K,V> f; int n, i, fh;
if (tab == null || (n = tab.length) == 0)
tab = initTable();
else if ((f = tabAt(tab, i = (n - 1) & hash)) == null) {
if (casTabAt(tab, i, null,
new Node<K,V>(hash, key, value, null)))
break; // no lock when adding to empty bin
}
else if ((fh = f.hash) == MOVED)
tab = helpTransfer(tab, f);
else {
V oldVal = null;
synchronized (f) { // 在这!!!!!!!!!!!!
if (tabAt(tab, i) == f) {
if (fh >= 0) {
binCount = 1;
for (Node<K,V> e = f;; ++binCount) {
K ek;
if (e.hash == hash &&
((ek = e.key) == key ||
(ek != null && key.equals(ek)))) {
oldVal = e.val;
if (!onlyIfAbsent)
e.val = value;
break;
}
Node<K,V> pred = e;
if ((e = e.next) == null) {
pred.next = new Node<K,V>(hash, key,
value, null);
break;
}
}
}
else if (f instanceof TreeBin) {
Node<K,V> p;
binCount = 2;
if ((p = ((TreeBin<K,V>)f).putTreeVal(hash, key,
value)) != null) {
oldVal = p.val;
if (!onlyIfAbsent)
p.val = value;
}
}
}
}
if (binCount != 0) {
if (binCount >= TREEIFY_THRESHOLD)
treeifyBin(tab, i);
if (oldVal != null)
return oldVal;
break;
}
}
}
addCount(1L, binCount);
return null;
}
这样一来,锁的就少了,效率当然也就高了,说实话,用着也方便,下面来展示高端操作
import java.util.HashMap;
import java.util.Map;
import java.util.concurrent.ConcurrentHashMap;
public class TestHashMap implements Runnable {
//static Map<String,Object> map = new HashMap();
static Map<String,Object> map = new ConcurrentHashMap<>();
@Override
public void run() {
for (int i = 0; i < 100; i ++) {
map.put(i + "", "hello");
}
}
public static void main(String[] args){
Thread thread1 = new Thread(new TestHashMap());
Thread thread2 = new Thread(new TestHashMap());
Thread thread3 = new Thread(new TestHashMap());
thread1.start();
thread2.start();
thread3.start();
Thread currentThread = Thread.currentThread();
try {
currentThread.sleep(2000);
} catch (Exception e) {
System.out.println(e.getMessage());
}
System.out.println(map.size());
}
}
最后,如果考虑线程安全,ConcurrentHashMap香呀。