深度分享:面试阿里,字节跳动,美团90%会被问到的HashMap知识

一,HashTable

哈希表,它相比于hashMap结构简单点,它没有涉及红黑树,直接使用链表的方式解决哈希冲突。

我们看它的字段,和hashMap差不多,使用table存放元素

private transient Entry[] table;
private transient int count;
private int threshold;
private float loadFactor;
private transient int modCount = 0;

它没有常量字段,默认值是在构造方法里面直接体现的,我们看一下无参构造:

public Hashtable() {
    this(11, 0.75f);
}

1.get()方法

根据key获得value

public synchronized V get(Object key) {
    Entry tab[] = table;
//计算下标
    int hash = key.hashCode();
    int index = (hash & 0x7FFFFFFF) % tab.length;
//遍历查找,e=e.next
    for (Entry e = tab[index] ; e != null ; e = e.next) {
        if ((e.hash == hash) && e.key.equals(key)) {
            return (V)e.value;
        }
    }
    return null;
}

2.put()方法

与get()方法类似,也是遍历table,然后调用addEntry()实现添加。

public synchronized V put(K key, V value) {
    if (value == null) {
        throw new NullPointerException();
    }
    Entry tab[] = table;
    int hash = key.hashCode();
    int index = (hash & 0x7FFFFFFF) % tab.length;
    @SuppressWarnings("unchecked")
    Entry entry = (Entry)tab[index];
//如果已经存在,则覆盖,返回老的值
    for(; entry != null ; entry = entry.next) {
        if ((entry.hash == hash) && entry.key.equals(key)) {
            V old = entry.value;
            entry.value = value;
            return old;
        }
    }
//不存在,直接添加
    addEntry(hash, key, value, index);
    return null;
}

addEntry()

private void addEntry(int hash, K key, V value, int index) {
    modCount++;
    Entry tab[] = table;
    if (count >= threshold) {    //大小超过阈值,要扩容
        // Rehash the table if the threshold is exceeded
        rehash();
        tab = table;
        hash = key.hashCode();
        index = (hash & 0x7FFFFFFF) % tab.length;
    }
//添加
    @SuppressWarnings("unchecked")
    Entry e = (Entry) tab[index];
    tab[index] = new Entry<>(hash, key, value, e);
    count++;
}

注意这里的手法,直接将新来的节点,放到头部,这样就可以不管后面是否存在节点,都不会出现问题

protected Entry(int hash, K key, V value, Entry next) {
    this.hash = hash;
    this.key =  key;
    this.value = value;
    this.next = next;
}

二,HashMap

1.常量字段介绍

static final int DEFAULT_INITIAL_CAPACITY = 1 << 4; // aka 16 2的四次方,初始化默认的容量
static final int MAXIMUM_CAPACITY = 1 << 30; 最大的容量值
static final float DEFAULT_LOAD_FACTOR = 0.75f; //容量 负载因子,
static final int TREEIFY_THRESHOLD = 8;        //链表转换为数的阈值
static final int UNTREEIFY_THRESHOLD = 6;    //树转坏为链表的阈值
static final int MIN_TREEIFY_CAPACITY = 64;    //桶中的数据采用红黑树存储时,整个table的最小容量

字段:

transient Node[] table; //存储主干,节点数组
transient Set> entrySet;
transient int size;        //元素数量
transient int modCount;        //修改次数
//The next size value at which to resize (capacity * load factor).
int threshold;  //下一次扩容的大小,
final float loadFactor;    //负载因子

2.构造函数

2.1常用的无参构造:

默认构造方法,就直接给负载因子赋值,其他没有操作,其他字段都是默认的。

// Constructs an empty 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
}

2.2带初始化容器大小,和负载因子的构造方法:

首先要判断传入参数的正确性,然后赋值。

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);
}

2.3带集合的构造方法:

传入一个Map集合,调用put方法进行初始化。

public HashMap(Map m) {
    this.loadFactor = DEFAULT_LOAD_FACTOR;
    putMapEntries(m, false);
}

从上面的代码中可以看到,在构造方法中并没有初始化table,具体的table初始化是在put操作上进行的。

3.添加

3.1 put()

是一个入口方法,实际调用的是putVal()方法,其中通过hash()方法计算了key对应的 值

public V put(K key, V value) {
    return putVal(hash(key), key, value, false, true);
}
//异或运算,保证存储位置尽量均匀分布。
static final int hash(Object key) {
    int h;
    return (key == null) ? 0 : (h = key.hashCode()) ^ (h >>> 16);
}

具体的putVal()方法,内容很长

final V putVal(int hash, K key, V value, boolean onlyIfAbsent,
               boolean evict) {
    Node[] tab; Node p; int n, i;  //变量初始化,n表示table的长度
    if ((tab = table) == null || (n = tab.length) == 0)    //容器初始化
        n = (tab = resize()).length;            //通过resize()方法获取分配空间。
    if ((p = tab[i = (n - 1) & hash]) == null)    //如果新的位置是空的,则直接放入,否者要解决冲突
        tab[i] = newNode(hash, key, value, null);        //将value封装成新的node
    else {    //解决冲突
        Node e; K k;
        // 注意p的赋值在 第二个if里面,它表示的是冲突位置所存放的节点。如果新传入的节点和当前node的hash和key相同,则下面再处理
        if (p.hash == hash &&
            ((k = p.key) == key || (key != null && key.equals(k))))
            e = p;
        else if (p instanceof TreeNode)    //基于红黑树的插入
            e = ((TreeNode)p).putTreeVal(this, tab, hash, key, value);
        else {        //基于链表的插入
            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;
            }
        }
            //更新当前传入的值到当前node中。返回之前的oldValue
        if (e != null) { // existing mapping for key
            V oldValue = e.value;
            if (!onlyIfAbsent || oldValue == null)
                e.value = value;
            afterNodeAccess(e);
            return oldValue;
        }
    }
    ++modCount;    //修改次数+1,
    if (++size > threshold)    //空间大小,如果超过了阈值,要扩容
        resize();
    afterNodeInsertion(evict);
    return null;
}

其中涉及的重点方法:resize()方法,返回新分配的空间。

final Node[] resize() {
    Node[] oldTab = table;//获取老的table空间
    int oldCap = (oldTab == null) ? 0 : oldTab.length; //获取老的容量
    int oldThr = threshold;    //获取老的阈值
    int newCap, newThr = 0;
    if (oldCap > 0) {
        if (oldCap >= MAXIMUM_CAPACITY) {
            threshold = Integer.MAX_VALUE;
            return oldTab;
        }    //扩容两倍
        else if ((newCap = oldCap << 1) < MAXIMUM_CAPACITY &&
                 oldCap >= DEFAULT_INITIAL_CAPACITY)
            newThr = oldThr << 1; // double threshold
    }
    else if (oldThr > 0) // initial capacity was placed in threshold,构造函数指定了阈值
        newCap = oldThr;
    else {               // 第一次初始化 oldCap=0,oldThr=0
        newCap = DEFAULT_INITIAL_CAPACITY;    //默认大小,16
        newThr = (int)(DEFAULT_LOAD_FACTOR * DEFAULT_INITIAL_CAPACITY);    //默认计算方法,16*0.75,12
    }
    if (newThr == 0) {    //初始化阈值
        float ft = (float)newCap * loadFactor;
        newThr = (newCap < MAXIMUM_CAPACITY && ft < (float)MAXIMUM_CAPACITY ?
                  (int)ft : Integer.MAX_VALUE);
    }
    threshold = newThr;    //将新计算出来的值,赋值
    @SuppressWarnings({"rawtypes","unchecked"})
        Node[] newTab = (Node[])new Node[newCap]; //重新分配空间。
    table = newTab;    //扩容后的空间赋值(此时,空间还是空的)
    if (oldTab != null) {    //如果老的空间,不是空的,那么需要元素转移
        for (int j = 0; j < oldCap; ++j) {    //遍历进行转移
            Node e;
            if ((e = oldTab[j]) != null) {    //将元数取出来
                oldTab[j] = null;
                if (e.next == null)
                    newTab[e.hash & (newCap - 1)] = e;
                else if (e instanceof TreeNode)
                    ((TreeNode)e).split(this, newTab, j, oldCap);
                else { // preserve order
                    Node loHead = null, loTail = null;
                    Node hiHead = null, hiTail = null;
                    Node 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;
}

4.Node结构

static class Node implements Map.Entry {
    final int hash;    //节点的hash值
    final K key;        //存入的key值
    V value;            //存放的值
    Node next;    //下一个节点
....
}

注意hashMap和LinkedList的区别,后者是双向的,而hashMap中的Node是单向的。

5.get()操作

public V get(Object key) {
    Node e;
    return (e = getNode(hash(key), key)) == null ? null : e.value; //代码内容在这里
}

调用getNode()方法,计算hash(key)值,通过Hash来获得node

final Node getNode(int hash, Object key) {
    Node[] tab; Node first, e; int n; K k;
    if ((tab = table) != null && (n = tab.length) > 0 &&(first = tab[(n - 1) & hash]) != null) {
        //第一个 检测数组中的hash定位获得第一个Node
        if (first.hash == hash &&((k = first.key) == key || (key != null && key.equals(k))))
            return first;
        //第一个不是,那么就是后续节点中,可能是链表形式,可能是红黑树
        if ((e = first.next) != null) {
            if (first instanceof TreeNode) //如果是红黑树,通过getTreeNode()方法获得
                return ((TreeNode)first).getTreeNode(hash, key);
            //链表形式,直接循环遍历获得。
            do {
                if (e.hash == hash &&((k = e.key) == key || (key != null && key.equals(k))))
                    return e;
            } while ((e = e.next) != null);
        }
    }
    return null;
}

链表形式的获取比较简单,红黑树的获得,我们放在下面红黑树单独进行介绍。

三,TreeMap

TreeMap和之前的两个map就不同了,它没有使用哈希表,而是直接使用红黑树解决,它的字段只保存了根节点

private final Comparator comparator; //排序比较器
private transient Entry root;    //根节点
private transient int size = 0;
private transient int modCount = 0;

1.get()

public V get(Object key) {
    Entry p = getEntry(key);
    return (p==null ? null : p.value);
}

getEntry()

final Entry getEntry(Object key) {
    // Offload comparator-based version for sake of performance
    if (comparator != null)
        return getEntryUsingComparator(key);
    if (key == null)
        throw new NullPointerException();
    @SuppressWarnings("unchecked")
        Comparable k = (Comparable) key;
    Entry p = root;
    while (p != null) {
        int cmp = k.compareTo(p.key);
        //左右分流
        if (cmp < 0)
            p = p.left;
        else if (cmp > 0)
            p = p.right;
        else
            return p;
    }
    return null;
}

**2.put() **涉及红黑树的操作,所以代码比较长

public V put(K key, V value) {
    Entry t = root;
    if (t == null) {
        compare(key, key); // type (and possibly null) check
 
        root = new Entry<>(key, value, null);
        size = 1;
        modCount++;
        return null;
    }
    int cmp;
    Entry parent;
    // split comparator and comparable paths
    Comparator cpr = comparator;
    if (cpr != null) {
        do {
            parent = t;
            cmp = cpr.compare(key, t.key);
            if (cmp < 0)
                t = t.left;
            else if (cmp > 0)
                t = t.right;
            else
                return t.setValue(value);
        } while (t != null);
    }
    else {
        if (key == null)
            throw new NullPointerException();
        @SuppressWarnings("unchecked")
            Comparable k = (Comparable) key;
        do {
            parent = t;
            cmp = k.compareTo(t.key);
            if (cmp < 0)
                t = t.left;
            else if (cmp > 0)
                t = t.right;
            else
                return t.setValue(value);
        } while (t != null);
    }
    Entry e = new Entry<>(key, value, parent);
    if (cmp < 0)
        parent.left = e;
    else
        parent.right = e;
    fixAfterInsertion(e);
    size++;
    modCount++;
    return null;
}

3.remove()

public V remove(Object key) {
    Entry p = getEntry(key);
    if (p == null)
        return null;
 
    V oldValue = p.value;
    deleteEntry(p);  //实际方法
    return oldValue;
}

总结:

HashMap表现得更像TreeMap和HashTable的结合体。

最后

感谢你看到这里,看完有什么的不懂的可以在评论区问我,觉得文章对你有帮助的话记得给我点个赞,每天都会分享java相关技术文章或行业资讯,欢迎大家关注和转发文章!

你可能感兴趣的:(Java,程序员,链表,java,hashmap,数据结构)