HashMap实现原理，源码分析

一、源码分析 

全文可参考知乎上美团官方写的：https://zhuanlan.zhihu.com/p/21673805

定义数组的初始容量。

static final int DEFAULT_INITIAL_CAPACITY = 1 << 4; // aka 16

定义数组最大容量

static final int MAXIMUM_CAPACITY = 1 << 30;

定义负载因子默认值

static final float DEFAULT_LOAD_FACTOR = 0.75f;

定义阈值默认值

static final int TREEIFY_THRESHOLD = 8;

 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);//重写hashCode函数，为键值的hashCode值异或
        }

        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;
        }
    }

static final int hash(Object key) {
        int h;
        return (key == null) ? 0 : (h = key.hashCode()) ^ (h >>> 16);
    }

HashMap#hash()，即上面的函数。

参考：https://blog.csdn.net/fan2012huan/article/details/51097331

 static final int tableSizeFor(int cap) {
        int n = cap - 1;
        n |= n >>> 1;
        n |= n >>> 2;
        n |= n >>> 4;
        n |= n >>> 8;
        n |= n >>> 16;
        return (n < 0) ? 1 : (n >= MAXIMUM_CAPACITY) ? MAXIMUM_CAPACITY : n + 1;
    }

HashMap#tableSizeFor() ：获取大于这个方法用于找到大于等于initialCapacity的最小的2的幂

参考：https://blog.csdn.net/fan2012huan/article/details/51097331

 static Class<?> comparableClassFor(Object x) {
        if (x instanceof Comparable) {
            Class<?> c; Type[] ts, as; ParameterizedType p;
            if ((c = x.getClass()) == String.class) // bypass checks
                return c;
            if ((ts = c.getGenericInterfaces()) != null) {
                for (Type t : ts) {
                    if ((t instanceof ParameterizedType) &&
                        ((p = (ParameterizedType) t).getRawType() ==
                         Comparable.class) &&
                        (as = p.getActualTypeArguments()) != null &&
                        as.length == 1 && as[0] == c) // type arg is c
                        return c;
                }
            }
        }
        return null;
    }

comparableClassFor：HashMap类中有一个comparableClassFor(Object x)方法，当x的类型为X，且X直接实现了Comparable接口（比较类型必须为X类本身）时，返回x的运行时类型；否则返回null。

instanceof

x instanceof Comparable

instanceof 可以理解为某种类型的实例，无论是运行时类型，还是它的父类，它实现的接口，它的父类实现的接口，甚至它的父类的父类的父类实现的接口的父类的父类，总之，只要在继承链上有这个类型就可以了。

getClass()

c = x.getClass()

与instanceof相应对的是getClass()方法，无论该对象如何转型，getClass()返回的只会是它的运行时类型，可以简单的理解为它的实际类型，也就是new它的时候的类型。 
有一种例外情况，匿名对象。当匿名对象调用getClass()时返回的是依赖它的对象的运行时类型，并以1,2,3…的索引区分。

public class Demo {
    public static void main(String[] args) {
        D d = new D();
        System.out.println(new A(){}.getClass());   // class Demo$1
        System.out.println(new B(){}.getClass());   // class Demo$2
        System.out.println(new Comparable<Object>(){    // class Demo$3
            @Override
            public int compareTo(Object o) {
                return 0;
            }}.getClass());
        System.out.println(d.c.getClass()); // class D$1
    }
}
 
abstract class A{}
abstract class B{}
abstract class C{}
class D{
    C c;
    D(){
        c= new C(){};
    }
}

参考：https://blog.csdn.net/qpzkobe/article/details/79533237

resize：初始化table数组或者扩容

 final Node<K,V>[] resize() {
        Node<K,V>[] oldTab = table;
        int oldCap = (oldTab == null) ? 0 : oldTab.length;
        int oldThr = threshold;
        int newCap, newThr = 0;
        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
        }
        else if (oldThr > 0) // initial capacity was placed in threshold
            newCap = oldThr;
        else {               // zero initial threshold signifies using defaults
            newCap = DEFAULT_INITIAL_CAPACITY;
            newThr = (int)(DEFAULT_LOAD_FACTOR * DEFAULT_INITIAL_CAPACITY);
        }
        if (newThr == 0) {
            float ft = (float)newCap * loadFactor;
            newThr = (newCap < MAXIMUM_CAPACITY && ft < (float)MAXIMUM_CAPACITY ?
                      (int)ft : Integer.MAX_VALUE);
        }
        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
                        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;
    }

参考：https://blog.csdn.net/u013494765/article/details/77837338