hashMap常见面试题总览
- 为什么重写Equals还要重写HashCode方法？
- HashMap如何避免内存泄漏问题？
- HashMap1.7底层是如何实现的？
- HashMapKey为null存放在什么位置？
- HashMap如何解决Hash冲突问题？
- HashMap底层采用单链表还是双链表？
- HashMap根据key查询的时间复杂度？
- HashMap如何实现数组扩容问题？
- HashMap底层是有序存放的吗？
- LinkedHashMap 和 TreeMap底层如何实现有序的？
- HashMap底层如何降低Hash冲突概率？
- HashMap中hash函数是怎么实现的？
- 为什么不直接将key作为哈希值而是与高16位做异或运算？
- HashMap如何存放1万条key效率最高？
- HashMap高低位与取模运算有那些好处？
- HashMap1.8如何避免多线程扩容死循环问题？
- 为什么加载因子是0.75而不是1？
- 为什么HashMap1.8需要引入红黑树？
- 为什么链表长度>8需要转红黑树？而不是6？
- 什么情况下，需要从红黑树转换成链表存放？
- HashMap底层如何降低Hash冲突概率？
- 如何在高并发的情况下使用HashMap？
- ConcurrentHashMap底层实现的原理？
为什么重写equals还要重写hashCode方法？
HashMap如何避免内存泄漏问题？
- 回答
- 为什么重写Equals还要重写HashCode方法？
- HashMap如何避免内存泄漏问题？
Hashcode方法：底层采用C语言编写，根据对象内存地址转换成整数类型
public native int hashCode();
基础1: 两个对象的hashCode值相等，对象不一定相等**
String a1="a";
Integer a2=97;
//97 97
System.out.println(a1.hashCode());
System.out.println(a2.hashCode());
引发hash碰撞问题，所以还要重写Equals，String类型的Equals方法已经帮我们重写，所以下面比较是不相等的
String a1="a";
Integer a2=97;
//false
System.out.println(a1.equals(a2));
String类型的Equals
/**
* Compares this string to the specified object. The result is {@code
* true} if and only if the argument is not {@code null} and is a {@code
* String} object that represents the same sequence of characters as this
* object.
*
* @param anObject
* The object to compare this {@code String} against
*
* @return {@code true} if the given object represents a {@code String}
* equivalent to this string, {@code false} otherwise
*
* @see #compareTo(String)
* @see #equalsIgnoreCase(String)
*/
public boolean equals(Object anObject) {
if (this == anObject) {
return true;
}
if (anObject instanceof String) {
String anotherString = (String)anObject;
int n = value.length;
if (n == anotherString.value.length) {
char v1[] = value;
char v2[] = anotherString.value;
int i = 0;
while (n-- != 0) {
if (v1[i] != v2[i])
return false;
i++;
}
return true;
}
}
return false;
}
Integer类型的Equals
/**
* Compares this object to the specified object. The result is
* {@code true} if and only if the argument is not
* {@code null} and is an {@code Integer} object that
* contains the same {@code int} value as this object.
*
* @param obj the object to compare with.
* @return {@code true} if the objects are the same;
* {@code false} otherwise.
*/
public boolean equals(Object obj) {
if (obj instanceof Integer) {
return value == ((Integer)obj).intValue();
}
return false;
}
基础2:两个对象equals相等，hashCode值不一定相等
UserEntity userEntity = new UserEntity("wei", 18);
UserEntity userEntity1 = new UserEntity("wei", 18);
//false 内存地址肯定不相等
System.out.println(userEntity==userEntity1);
//false Object默认实现了equals方法，本质还是==，比较内存地址
System.out.println(userEntity.equals(userEntity1));
Object类的equals
public boolean equals(Object obj) {
return (this == obj);
}
结论：重写equals必须重写hashCode方法。
阿里规范
1.只要重写equals必须重写hashCode方法【强制】
2.Set、Map底层用hashCode和equals进行判断相等
3.String已重写equals和hashCode，所以可以用字符串作为Key【推荐】
不重写equals必须重写hashCode方法会导致内存无法回收（内存泄漏问题），同一个对象，一直创建新的空间存放。
HashMap1.7底层是如何实现的？
HashMapKey为null存放在什么位置？
基础：HashMap与HashTable之间的区别
HashMap底层如何实现
1.ArrayList集合实现，不需要考虑hash碰撞，时间复杂度为O（n）：
2. JDK1.7基于数组和链表(h=（key.hashCode)^h>>>16），不发生冲突为O（1），发生冲突链表为O（n），hashCode相同，对象不同
3. JDK1.8基于数组和链表和红黑树
源码解读
1.默认参数
/**
* The default initial capacity - MUST be a power of two.
*/
//hashmap默认初始大小16,用位移做运算
static final int DEFAULT_INITIAL_CAPACITY = 1 << 4; // aka 16
/**
* The maximum capacity, used if a higher value is implicitly specified
* by either of the constructors with arguments.
* MUST be a power of two <= 1<<30.
*/
//2^30次方
static final int MAXIMUM_CAPACITY = 1 << 30;
/**
* The load factor used when none specified in constructor.
*/
//加载因子
static final float DEFAULT_LOAD_FACTOR = 0.75f;
/**
* The bin count threshold for using a tree rather than list for a
* bin. Bins are converted to trees when adding an element to a
* bin with at least this many nodes. The value must be greater
* than 2 and should be at least 8 to mesh with assumptions in
* tree removal about conversion back to plain bins upon
* shrinkage.
*/
//链表大于8，转换成红黑树
static final int TREEIFY_THRESHOLD = 8;
/**
* The bin count threshold for untreeifying a (split) bin during a
* resize operation. Should be less than TREEIFY_THRESHOLD, and at
* most 6 to mesh with shrinkage detection under removal.
*/
//链表小于6，转换成红黑树
static final int UNTREEIFY_THRESHOLD = 6;
/**
* The smallest table capacity for which bins may be treeified.
* (Otherwise the table is resized if too many nodes in a bin.)
* Should be at least 4 * TREEIFY_THRESHOLD to avoid conflicts
* between resizing and treeification thresholds.
*/
//数组长度大于64，且链表长度大于8才会转换成红黑树
static final int MIN_TREEIFY_CAPACITY = 64;
2.内部静态类
static class Node<K,V> implements Map.Entry<K,V> {
final int hash;
final K key;
V value;
Node<K,V> next;
Node(int hash, K key, V value, Node<K,V> next) {
this.hash = hash;
this.key = key;
this.value = value;
this.next = next;
}
public final K getKey() { return key; }
public final V getValue() { return value; }
public final String toString() { return key + "=" + value; }
public final int hashCode() {
return Objects.hashCode(key) ^ Objects.hashCode(value);
}
public final V setValue(V newValue) {
V oldValue = value;
value = newValue;
return oldValue;
}
public final boolean equals(Object o) {
if (o == this)
return true;
if (o instanceof Map.Entry) {
Map.Entry<?,?> e = (Map.Entry<?,?>)o;
if (Objects.equals(key, e.getKey()) &&
Objects.equals(value, e.getValue()))
return true;
}
return false;
}
}
3.字段
/* ---------------- Fields -------------- */
/**
* The table, initialized on first use, and resized as
* necessary. When allocated, length is always a power of two.
* (We also tolerate length zero in some operations to allow
* bootstrapping mechanics that are currently not needed.)
*/
//数组 transient不能序列化
transient Node<K,V>[] table;
/**
* Holds cached entrySet(). Note that AbstractMap fields are used
* for keySet() and values().
*/
transient Set<Map.Entry<K,V>> entrySet;
/**
* The number of key-value mappings contained in this map.
*/
transient int size;
/**
* The number of times this HashMap has been structurally modified
* Structural modifications are those that change the number of mappings in
* the HashMap or otherwise modify its internal structure (e.g.,
* rehash). This field is used to make iterators on Collection-views of
* the HashMap fail-fast. (See ConcurrentModificationException).
*/
// fail-fast机制
transient int modCount;
/**
* The next size value at which to resize (capacity * load factor).
*
* @serial
*/
// (The javadoc description is true upon serialization.
// Additionally, if the table array has not been allocated, this
// field holds the initial array capacity, or zero signifying
// DEFAULT_INITIAL_CAPACITY.)
int threshold;
/**
* The load factor for the hash table.
*
* @serial
*/
//加载因子，如果不填写则采用默认加载因子
final float loadFactor;
4.forEach修改报错
public final void forEach(Consumer<? super K> action) {
Node<K,V>[] tab;
if (action == null)
throw new NullPointerException();
if (size > 0 && (tab = table) != null) {
int mc = modCount;
for (int i = 0; i < tab.length; ++i) {
for (Node<K,V> e = tab[i]; e != null; e = e.next)
action.accept(e.key);
}
if (modCount != mc)
throw new ConcurrentModificationException();
}
}
}
5.put方法
public V put(K key, V value) {
return putVal(hash(key), key, value, false, true);
}
static final int hash(Object key) {
int h;
// null值放在0号位置，科学方法计算hash值，减少冲突
return (key == null) ? 0 : (h = key.hashCode()) ^ (h >>> 16);
}
/**
* 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 当不存在时，则不更改现有值
* @param evict if false, the table is in creation mode. 如果为false，则退出，表处于创建模式
* @return previous value, or null if none
*/
final V putVal(int hash, K key, V value, boolean onlyIfAbsent,
boolean evict) {
//数组+链表 n 数组长度 i链表存放位置
Node<K,V>[] tab; Node<K,V> p; int n, i;
//数组不存在或者数组长度为0，则调用扩容方法，懒加载
if ((tab = table) == null || (n = tab.length) == 0)
n = (tab = resize()).length;
//hash不冲突，数组直接存放链表
if ((p = tab[i = (n - 1) & hash]) == null)
tab[i] = newNode(hash, key, value, null);
else {
Node<K,V> e; K k;
//如果hash值相等且key值相等，将p赋值给e，等待做更新操作
if (p.hash == hash &&
((k = p.key) == key || (key != null && key.equals(k))))
e = p;
//是红黑树
else if (p instanceof TreeNode)
e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);
else {
for (int binCount = 0; ; ++binCount) {
// 遍历链表，到末尾添加元素
if ((e = p.next) == null) {
p.next = newNode(hash, key, value, null);
//帮链表转换成红黑树，数组容量>=64 && 链表长度大于8
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;
}
}
// 存在则直接修改
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;
}
扩容方法
/**
* Initializes or doubles table size. If null, allocates in
* accord with initial capacity target held in field threshold.
* Otherwise, because we are using power-of-two expansion, the
* elements from each bin must either stay at same index, or move
* with a power of two offset in the new table.
*
* @return the table
*/
final Node<K,V>[] resize() {
//旧的数组
Node<K,V>[] oldTab = table;
//获取旧数组的长度
int oldCap = (oldTab == null) ? 0 : oldTab.length;
// 旧数组的扩容阈值
int oldThr = threshold;
// 新的数组长度和新的扩容阈值
int newCap, newThr = 0;
// 如果旧的数组存在
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 两倍扩容
}
// 旧数组存在，将旧的数组的长度赋值给新数组
else if (oldThr > 0) // initial capacity was placed in threshold
newCap = oldThr;
//使用默认值进行初始化
else { // zero initial threshold signifies using defaults
//初始化数组容量=数组默认初始大小16
newCap = DEFAULT_INITIAL_CAPACITY;
// 初始化数组扩容阈值=链表加载因子*数组默认初始大小12
newThr = (int)(DEFAULT_LOAD_FACTOR * DEFAULT_INITIAL_CAPACITY);
}
//如果新数组扩容阈值为0
if (newThr == 0) {
//ft = 新的数组长度*加载因子，即计算阈值
float ft = (float)newCap * loadFactor;
newThr = (newCap < MAXIMUM_CAPACITY && ft < (float)MAXIMUM_CAPACITY ?
(int)ft : Integer.MAX_VALUE);
}
//将新数组扩容阈值赋值到全局变量
threshold = newThr;
@SuppressWarnings({"rawtypes","unchecked"})
Node<K,V>[] newTab = (Node<K,V>[])new Node[newCap];
table = newTab;
if (oldTab != null) {
for (int j = 0; j < oldCap; ++j) {
Node<K,V> e;
if ((e = oldTab[j]) != null) {
oldTab[j] = null;
if (e.next == null)
newTab[e.hash & (newCap - 1)] = e;
else if (e instanceof TreeNode)
((TreeNode<K,V>)e).split(this, newTab, j, oldCap);
else { // preserve order
//扩容是2的次方，分高位低位进行移动操作，插入新数组，尾插法
Node<K,V> loHead = null, loTail = null;
Node<K,V> hiHead = null, hiTail = null;
Node<K,V> next;
do {
next = e.next;
if ((e.hash & oldCap) == 0) {
if (loTail == null)
loHead = e;
else
loTail.next = e;
loTail = e;
}
else {
if (hiTail == null)
hiHead = e;
else
hiTail.next = e;
hiTail = e;
}
} while ((e = next) != null);
if (loTail != null) {
loTail.next = null;
newTab[j] = loHead;
}
if (hiTail != null) {
hiTail.next = null;
newTab[j + oldCap] = hiHead;
}
}
}
}
}
return newTab;
}
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