一、双例集合

1. Map 接口介绍

Map 接口特点： Map 接口定义了双例集合的存储特征，它并不是 Collection 接口的子接口。双例集合的存储特征是以 key 与 value 结构为单位进行存储。体现的是数学中的函数 y=f(x)感念。

Map 与 Collecton 的区别：

1.Collection 中的容器，元素是孤立存在的（理解为单身），向集合中存储元素采用一个个元素的方式存储。

2.Map 中的容器，元素是成对存在的(理解为现代社会的夫妻)。每个元素由键与值两部分组成，通过键可以找对所对应的值。

3.Collection 中的容器称为单列集合，Map 中的容器称为双列集合。

4.Map 中的集合不能包含重复的键，值可以重复；每个键只能对应一个值。

5.Map 中常用的容器为 HashMap，TreeMap 等。

2. HashMap 容器类

HashMap 是 Map 接口的接口实现类，它采用哈希算法实现，是 Map 接口最常用的实现类。 由于底层采用了哈希表存储数据，所以要求键不能重复，如果发生重复，新的值会替换旧的值。 HashMap 在查找、删除、修改方面都有非常高的效率。

2.1 添加元素 ：

 package cn.pxy.test;

import java.util.HashMap;
import java.util.Map;

public class HashMapTest {
    public static void main(String[] args) {
        //实例化HashMap容器
        Map<String,String> map=new HashMap<>();
        //输入元素
        map.put("a","A");
        System.out.println(map.get("a"));
        map.put("a","B");
        String value=map.get("a");
        System.out.println(value);
    }
} 

运行结果：

2.2 获取元素 方式：

方式一：通过get获取

 String value=map.get("a");
Syetem.out.println(value); 

方式二：通过keySet获取

 //获取 HashMap 容器中所有的元素，可以使用 keySet 方法与 get 方法一并完成。 
package cn.pxy.test;

import java.util.HashMap;
import java.util.Map;
import java.util.Set;

public class HashMapTest {
    public static void main(String[] args) {
        //实例化HashMap容器
        Map<String,String> map=new HashMap<>();
        //输入元素
        map.put("a","A");
        System.out.println(map.get("a"));
        map.put("a","B");
        String value=map.get("a");
        System.out.println(value);
        map.put("b","C");
        map.put("c", "D");
    //获取 HashMap 容器中所有的元素，可以使用 keySet 方法与 get 方法一并完成。
    //获取所有的key
        Set<String> keys=map.keySet();
        for(String key:keys) {
            String value=map.get(key);
            System.out.println(key+"-----"+value);
        }
    }
} 

运行效果：

方式三： 通过 entrySet 方法获取 Map.Entry 类型获取元素

 package cn.pxy.test;

import java.util.HashMap;
import java.util.Map;
import java.util.Set;

public class HashMapTest {
    public static void main(String[] args) {
        //实例化HashMap容器
        Map<String,String> map=new HashMap<>();
        //输入元素
        map.put("a","A");
        System.out.println(map.get("a"));
        map.put("a","B");
        String value=map.get("a");
        System.out.println(value);
        map.put("b","C");
        map.put("c", "D");
        Set<String> keys=map.keySet();
        for(String key:keys) {
            String value=map.get(key);
            System.out.println(key+"-----"+value);
        }
        System.out.println("********");
        //通过 entrySet 方法获取 Map.Entry 类型获取元素
        //返回一个 Set 基于 Map.Entry 类型包含 Map 中所有映射
        Set<Map.Entry<String,String>> entrySet = map.entrySet(); 
        for(Map.Entry<String,String> entry:entrySet){ 
            String key = entry.getKey(); 
            String v = entry.getValue(); 
            System.out.println(key+" ---------- "+v); 
        }
    }
} 

运行效果：

2.3 Map 容器的并集操作：

 package cn.pxy.test;

import java.util.HashMap;
import java.util.Map;
import java.util.Set;

public class HashMapTest {
    public static void main(String[] args) {
        //实例化HashMap容器
        Map<String,String> map=new HashMap<>();
        //输入元素;若key相同，则value被覆盖
        System.out.println("***输入元素;若key相同，则value被覆盖**");
        map.put("a","A");
        System.out.println(map.get("a"));
        map.put("a","B");
        //通过get获取元素
        String value=map.get("a");
        System.out.println(value);
        //通过keySet获取元素
        System.out.println("***通过keySet获取元素***");
        map.put("b","C");
        map.put("c", "D");
        Set<String> keys=map.keySet();
        for(String key:keys) {
            String value=map.get(key);
            System.out.println(key+"-----"+value);
        }

        //通过 entrySet 方法获取 Map.Entry 类型获取元素
        //返回一个 Set 基于 Map.Entry 类型包含 Map 中所有映射
        System.out.println("***通过 entrySet 方法获取 Map.Entry 类型获取元素***");
        Set<Map.Entry<String,String>> entrySet = map.entrySet(); 
        for(Map.Entry<String,String> entry:entrySet){ 
            String key = entry.getKey(); 
            String v = entry.getValue(); 
            System.out.println(key+" ---------- "+v); 
        }
        //并集操作
        System.out.println("*****并集操作*****");
        //实例化Map容器
        Map<String,String> map=new HashMap<>();
        map.put("e", "cc");
        map.put("f", "dd");
        //并集操作
        map.putAll(map);
        Set<String> keys=map2.keySet();
        for(String key:keys) {
            String value=map2.get(key);
            System.out.println("Key:"+key+"----"+" Value:"+value);
        }
    }
} 

运行结果：

2.4 删除元素：

 package cn.pxy.test;

import java.util.HashMap;
import java.util.Map;
import java.util.Set;

public class HashMapTest {
    public static void main(String[] args) {
        //实例化HashMap容器
        Map<String,String> map=new HashMap<>();
        //输入元素;若key相同，则value被覆盖
        System.out.println("***输入元素;若key相同，则value被覆盖**");
        map.put("a","A");
        System.out.println(map.get("a"));
        map.put("a","B");
        //通过get获取元素
        String value=map.get("a");
        System.out.println(value);
        //通过keySet获取元素
        System.out.println("***通过keySet获取元素***");
        map.put("b","C");
        map.put("c", "D");
        Set<String> keys=map.keySet();
        for(String key:keys) {
            String value=map.get(key);
            System.out.println(key+"-----"+value);
        }

        //通过 entrySet 方法获取 Map.Entry 类型获取元素
        //返回一个 Set 基于 Map.Entry 类型包含 Map 中所有映射
        System.out.println("***通过 entrySet 方法获取 Map.Entry 类型获取元素***");
        Set<Map.Entry<String,String>> entrySet = map.entrySet(); 
        for(Map.Entry<String,String> entry:entrySet){ 
            String key = entry.getKey(); 
            String v = entry.getValue(); 
            System.out.println(key+" ---------- "+v); 
        }
        //并集操作
        System.out.println("*****并集操作*****");
        //实例化Map容器
        Map<String,String> map=new HashMap<>();
        map.put("e", "cc");
        map.put("f", "dd");
        //并集操作
        map.putAll(map);
        Set<String> keys=map2.keySet();
        for(String key:keys) {
            String value=map2.get(key);
            System.out.println("Key:"+key+"----"+" Value:"+value);
        }

    //删除元素
        System.out.println("***删除元素***");
        String v=map.remove("e");//删除 e 对应的 value
        System.out.println(v);
        Set<String> keys=map2.keySet();
        for(String key:keys) {
            String value=map2.get(key);
            System.out.println("Key:"+key+" Value:"+value);
        }
    }
} 

运行结果：

2.5 判断 key 或 value 是否存在：

 package cn.pxy.test;

import java.util.HashMap;
import java.util.Map;
import java.util.Set;

public class HashMapTest {
    public static void main(String[] args) {
        //实例化HashMap容器
        Map<String,String> map=new HashMap<>();
        //输入元素;若key相同，则value被覆盖
        System.out.println("***输入元素;若key相同，则value被覆盖**");
        map.put("a","A");
        System.out.println(map.get("a"));
        map.put("a","B");
        //通过get获取元素
        String value=map.get("a");
        System.out.println(value);
        //通过keySet获取元素
        System.out.println("***通过keySet获取元素***");
        map.put("b","C");
        map.put("c", "D");
        Set<String> keys=map.keySet();
        for(String key:keys) {
            String value=map.get(key);
            System.out.println(key+"-----"+value);
        }

        //通过 entrySet 方法获取 Map.Entry 类型获取元素
        //返回一个 Set 基于 Map.Entry 类型包含 Map 中所有映射
        System.out.println("***通过 entrySet 方法获取 Map.Entry 类型获取元素***");
        Set<Map.Entry<String,String>> entrySet = map.entrySet(); 
        for(Map.Entry<String,String> entry:entrySet){ 
            String key = entry.getKey(); 
            String v = entry.getValue(); 
            System.out.println(key+" ---------- "+v); 
        }
        //并集操作
        System.out.println("*****并集操作*****");
        //实例化Map容器
        Map<String,String> map=new HashMap<>();
        map.put("e", "cc");
        map.put("f", "dd");
        //并集操作
        map.putAll(map);
        Set<String> keys=map2.keySet();
        for(String key:keys) {
            String value=map2.get(key);
            System.out.println("Key:"+key+"----"+" Value:"+value);
        }
        //删除元素
        System.out.println("***删除元素***");
        String v=map.remove("e");//删除 e 对应的 value
        System.out.println(v);
        Set<String> keys=map2.keySet();
        for(String key:keys) {
            String value=map2.get(key);
            System.out.println("Key:"+key+" Value:"+value);
        }

        System.out.println("****判断key或value是否存在****");
        //判断Key是否存在
        boolean flag=map.containsKey("a");
        System.out.println(flag);
        //判断value是否存在
        boolean flag=map2.containsValue("kk");
        System.out.println(flag);
    }
} 

运行结果：

2.6 HashMap 的底层源码分析：

底层存储： HashMap 底层实现采用了哈希表，这是一种非常重要的数据结构。对于我们以后理解很多技术都非常有帮助，因此，非常有必要让详细的理解。

数据结构中由数组和链表来实现对数据的存储，他们各有特点：

(1) 数组：占用空间连续。 寻址容易，查询速度快。但是，增加和删除效率非常低。

(2) 链表：占用空间不连续。 寻址困难，查询速度慢。但是，增加和删除效率非常高。

哈希表结合了数组和链表的优点（即查询快，增删效率也高），它的本质就是“数组+链表”。

成员变量：

 /**
* The default initial capacity - MUST be a power of two.
*/ 
static final int DEFAULT_INITIAL_CAPACITY = << 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 <=<<30. 
*/ 
static final int MAXIMUM_CAPACITY = << 30; 
/**
* The load factor used when none specified in constructor.
*/ 
static final float DEFAULT_LOAD_FACTOR =.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 and should be at least 8 to mesh with assumptions in 
* tree removal about conversion back to plain bins upon * shrinkage. 
*/ 
static final int TREEIFY_THRESHOLD =; 
/**
* The bin count threshold for untreeifying a (split) bin during a 
* resize operation. Should be less than TREEIFY_THRESHOLD, and at 
* most to mesh with shrinkage detection under removal.
*/ 
static final int UNTREEIFY_THRESHOLD =;
/**
* 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 * TREEIFY_THRESHOLD to avoid conflicts
* between resizing and treeification thresholds. 
*/ 
static final int MIN_TREEIFY_CAPACITY =; 
/**
* The number of key-value mappings contained in this map. 
*/ 
transient int size; 
/**
* 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.) 
*/ 
transient Node<K,V>[] table; 

HashMap 中存储元素的节点类型：

Node类：

 /**
     * Basic hash bin node, used for most entries.  (See below for
     * TreeNode subclass, and in LinkedHashMap for its Entry subclass.)
     */    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;
        }
    } 

TreeNode类：

 /**
     * Entry for Tree bins. Extends LinkedHashMap.Entry (which in turn
     * extends Node) so can be used as extension of either regular or
     * linked node.
     */    static final class TreeNode<K,V> extends LinkedHashMap.Entry<K,V> {
        TreeNode<K,V> parent;  // red-black tree links
        TreeNode<K,V> left;
        TreeNode<K,V> right;
        TreeNode<K,V> prev;    // needed to unlink next upon deletion
        boolean red;
        TreeNode(int hash, K key, V val, Node<K,V> next) {
            super(hash, key, val, next);
        }

        /**
         * Returns root of tree containing this node.
         */        final TreeNode<K,V> root() {
            for (TreeNode<K,V> r = this, p;;) {
                if ((p = r.parent) == null)
                    return r;
                r = p;
            }
        } 

继承关系：

数组初始化：

在 JDK1.8 的 HashMap 中对于数组的初始化采用的是延迟初始化方式。通过 resize 方法实现初始化处理。resize 方法既实现数组初始化，也实现数组扩容处理。

 /**
     * 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() {
        Node<K,V>[] oldTab = table;
        int oldCap = (oldTab == null) ? : oldTab.length;
        int oldThr = threshold;
        int newCap, newThr =;
        if (oldCap >) {
            if (oldCap >= MAXIMUM_CAPACITY) {
                threshold = Integer.MAX_VALUE;
                return oldTab;
            }
            else if ((newCap = oldCap <<) < MAXIMUM_CAPACITY &&
                     oldCap >= DEFAULT_INITIAL_CAPACITY)
                newThr = oldThr <<; // double threshold
        }
        else if (oldThr >) // 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 ==) {
            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 =; j < oldCap; ++j) {
                Node<K,V> e;
                if ((e = oldTab[j]) != null) {
                    oldTab[j] = null;
                    if (e.next == null)
                        newTab[e.hash & (newCap -)] = 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) ==) {
                                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;
    } 

计算 Hash 值：

(1) 获得 key 对象的 hashcode

首先调用 key 对象的 hashcode()方法，获得 key 的 hashcode 值。

(2) 根据 hashcode 计算出 hash 值(要求在[0, 数组长度-1]区间)

hashcode 是一个整数，我们需要将它转化成[0, 数组长度-1]的范围。我们要求转化后的 hash 值尽量均匀地分布在[0,数组长度-1]这个区间，减少“hash 冲突”

i. 一种极端简单和低下的算法是：

hash 值 = hashcode/hashcode;

也就是说，hash 值总是 1。意味着，键值对对象都会存储到数组索引 1位置，这样就形成一个非常长的链表。相当于每存储一个对象都会发生“hash冲突”，HashMap 也退化成了一个“链表”。

ii. 一种简单和常用的算法是（相除取余算法）：

hash 值 = hashcode%数组长度

这种算法可以让 hash 值均匀的分布在[0,数组长度-1]的区间。但是，这种算法由于使用了“除法”，效率低下。JDK 后来改进了算法。首先约定数组长度必须为 2 的整数幂，这样采用位运算即可实现取余的效果：hash 值 =hashcode&(数组长度-1)。

 /**
     * Computes key.hashCode() and spreads (XORs) higher bits of hash
     * to lower.  Because the table uses power-of-two masking, sets of
     * hashes that vary only in bits above the current mask will
     * always collide. (Among known examples are sets of Float keys
     * holding consecutive whole numbers in small tables.)  So we
     * apply a transform that spreads the impact of higher bits
     * downward. There is a tradeoff between speed, utility, and
     * quality of bit-spreading. Because many common sets of hashes
     * are already reasonably distributed (so don't benefit from
     * spreading), and because we use trees to handle large sets of
     * collisions in bins, we just XOR some shifted bits in the
     * cheapest possible way to reduce systematic lossage, as well as
     * to incorporate impact of the highest bits that would otherwise
     * never be used in index calculations because of table bounds.
     */    static final int hash(Object key) {
        int h;
        return (key == null) ? : (h = key.hashCode()) ^ (h >>> 16);
    } 
 /**
     * Associates the specified value with the specified key in this map.
     * If the map previously contained a mapping for the key, the old
     * value is replaced.
     *
     * @param key key with which the specified value is to be associated
     * @param value value to be associated with the specified key
     * @return the previous value associated with {@code key}, or
     *         {@code null} if there was no mapping for {@code key}.
     *         (A {@code null} return can also indicate that the map
     *         previously associated {@code null} with {@code key}.)
     */    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) ==)
            n = (tab = resize()).length;
        if ((p = tab[i = (n -) & 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 =; ; ++binCount) {
                    if ((e = p.next) == null) {
                        p.next = newNode(hash, key, value, null);
                        if (binCount >= TREEIFY_THRESHOLD -) // -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;
    } 

添加元素：

 /**
     * Associates the specified value with the specified key in this map.
     * If the map previously contained a mapping for the key, the old
     * value is replaced.
     *
     * @param key key with which the specified value is to be associated
     * @param value value to be associated with the specified key
     * @return the previous value associated with {@code key}, or
     *         {@code null} if there was no mapping for {@code key}.
     *         (A {@code null} return can also indicate that the map
     *         previously associated {@code null} with {@code key}.)
     */    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) ==)
            n = (tab = resize()).length;
        if ((p = tab[i = (n -) & 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 =; ; ++binCount) {
                    if ((e = p.next) == null) {
                        p.next = newNode(hash, key, value, null);
                        if (binCount >= TREEIFY_THRESHOLD -) // -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;
    } 

数组扩容：

 /**
     * 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() {
        Node<K,V>[] oldTab = table;
        int oldCap = (oldTab == null) ? : oldTab.length;
        int oldThr = threshold;
        int newCap, newThr =;
        if (oldCap >) {
            if (oldCap >= MAXIMUM_CAPACITY) {
                threshold = Integer.MAX_VALUE;
                return oldTab;
            }
            else if ((newCap = oldCap <<) < MAXIMUM_CAPACITY &&
                     oldCap >= DEFAULT_INITIAL_CAPACITY)
                newThr = oldThr <<; // double threshold
        }
        else if (oldThr >) // 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 ==) {
            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 =; j < oldCap; ++j) {
                Node<K,V> e;
                if ((e = oldTab[j]) != null) {
                    oldTab[j] = null;
                    if (e.next == null)
                        newTab[e.hash & (newCap -)] = 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) ==) {
                                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;
    } 

3. TreeMap 容器类

TreeMap 和 HashMap 同样实现了 Map 接口，所以，对于 API 的用法来说是没有区别的。HashMap 效率高于 TreeMap；TreeMap 是可以对键进行排序的一种容器，在需要对键排序时可选用 TreeMap。TreeMap 底层是基于红黑树实现的。

在使用 TreeMap 时需要给定排序规则：1.元素自身实现比较规则；2.通过比较器实现比较规则

元素自身实现比较规则：

 package cn.pxy.test;

public class Users implements Comparable<Users>{
    private String username;
    private int userage;
    public Users(String username,int userage) {
        this.username=username;
        this.userage=userage;
    }
    public Users() {

    }

    @Override
    public boolean equals(Object o) {
        System.out.println("equals...");
        if(this==o) {
            return true;
        }
        if(o==null||getClass()!=o.getClass()) {
            return false;
        }
        Users users=(Users) o;

        if(userage!=users.userage) {
            return false;
        }

        return username!=null?username.equals(users.username):users.username==null;
    }

    @Override
    public int hashCode() {
        int result=username!=null?username.hashCode():;
        result=*result+userage;
        return result;
    }



    public String getUsername() {
        return username;
    }
    public void setUsername(String username) {
        this.username = username;
    }
    public int getUserage() {
        return userage;
    }
    public void setUserage(int userage) {
        this.userage = userage;
    }

    @Override
    public String toString() {
    return "Users{"+"username='"+username+'''+",userage="+userage+'}';
    }
    //定义比较规则
    //正数：大；负数：小；：相等
    @Override
    public int compareTo(Users o) {
        if(this.userage>o.getUserage()) {
            return;
        }
        if(this.userage==o.getUserage()) {
            return this.username.compareTo(o.getUsername());
        }
        return -;
    }
} 
 package cn.pxy.test;

import java.util.Map;
import java.util.Set;
import java.util.TreeMap;

public class TreeMapTest {
    public static void main(String[] args) {
        Map<Users,String> map=new TreeMap<>();
        Users u=new Users("pxy",18);
        Users u=new Users("dhy",22);
        Users u=new Users("pxyxs",19);
        map.put(u, "pxy");
        map.put(u,"dhy");
        map.put(u, "pxyxs");
        Set<Users> keys=map.keySet();
        for(Users key:keys) {
            System.out.println(key+"******"+map.get(key));
        }
    }


} 

运行结果：

通过比较器实现比较规则：

 package cn.pxy.test;

public class Student {
    private String name;
    private int age;
    public Student(String name,int age) {
        this.name=name;
        this.age=age;
    }
    public Student() {

    }
    @Override
    public String toString() {

    return "Student {" + "name='" + name + ''' + ", age=" + age + '}';
    }
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    public int getAge() {
        return age;
    }
    public void setAge(int age) {
        this.age = age;
    }
    @Override
    public boolean equals(Object obj) {
        if(this==obj) {
            return true;
        }
        if(obj==null||getClass()!=obj.getClass()) {
            return false;
        }
        Student student=(Student) obj;
        if(age!=student.age) {
            return false;
        }

        return name!=null?name.equals(student.name):student.name==null;
    }
    @Override
    public int hashCode() {
        int result=name!=null?name.hashCode():;
        result=*result+age;
        return result;
    }
} 
 package cn.pxy.test;

import java.util.Comparator;
//定义比较规则
public class StudentComparator implements Comparator<Student>{
    public int compare(Student o,Student o2) {
        if(o.getAge()>o2.getAge()) {
            return;
        }
        if(o.getAge()==o2.getAge()) {
            return o.getName().compareTo(o2.getName());
        }
        return -;
    }
} 
 package cn.pxy.test;

import java.util.Map;
import java.util.Set;
import java.util.TreeMap;

public class TreeMapTest {
    public static void main(String[] args) {
        Map<Student,String> treeMap=new TreeMap<>(new StudentComparator());
        Student u=new Student("pxy",18);
        Student u=new Student("dhy",22);
        Student u=new Student("pxyxs",19);
        treeMap.put(u, "pxy");
        treeMap.put(u,"dhy");
        treeMap.put(u, "pxyxs");
        Set<Student> keys=treeMap.keySet();
        for(Student key:keys) {
            System.out.println(key+"******"+treeMap.get(key));
        }
    }


} 

运行结果：

4. Iterator 迭代器

4.1 Iterator 迭代器接口介绍

Collection接口继承了Iterable接口，在该接口中包含一个名为iterator的抽象方法，所有实现了Collection接口的容器类对该方法做了具体实现。iterator方法会返回一个Iterator接口类型的迭代器对象，在该对象中包含了三个方法用于实现对单例容器的迭代处理。

Iterator对象的工作原理：

Iterator接口定义了如下方法：

1.boolean hasNext(); //判断游标当前位置是否有元素，如果有返回true，否则返回false；

2.Object next();//获取当前游标所在位置的元素，并将游标移动到下一个位置；

3.void remove();//删除游标当前位置的元素，在执行完next后该操作只能执行一次；

4.2 迭代器的使用

使用 Iterator 迭代 List 接口类型容器：

 package cn.pxy.test;

import java.util.ArrayList;
import java.util.Iterator;
import java.util.List;

public class IteratorListTest {
    public static void main(String[] args) {
        //实例化容器
        List<String> list = new ArrayList<>();
        list.add("a"); 
        list.add("b"); 
        list.add("c"); 
        //获取元素 
        //获取迭代器对象 
        Iterator<String> iterator = list.iterator(); 
        //方式一:在迭代器中，通过 while 循环获取元素 
        while(iterator.hasNext()){ 
            String value = iterator.next(); 
            System.out.println(value); 
        }
        System.out.println("-------------------------------"); 
        //方法二：在迭代器中，通过 for 循环获取元素 
        for(Iterator<String> it = list.iterator();it.hasNext();){ 
            String value = it.next(); 
            System.out.println(value); 
        }
    }
} 

运行效果：

使用 Iterator 迭代 Set 接口类型容器：

 package cn.pxy.test;

import java.util.HashSet;
import java.util.Iterator;
import java.util.Set;

public class IteratorSetTest {
    public static void main(String[] args) {
        //实例化 Set 类型的容器 
        Set<String> set = new HashSet<>(); 
        set.add("a"); 
        set.add("b"); 
        set.add("c"); 
        //方式一：通过 while 循环 
        //获取迭代器对象 
        Iterator<String> iterator = set.iterator(); 
        while(iterator.hasNext()){ 
            String value = iterator.next();
            System.out.println(value); 
        }
        System.out.println("-------------------------"); 
        //方式二：通过 for 循环 
        for(Iterator<String> it = set.iterator();it.hasNext();){ 
            String value = it.next(); 
            System.out.println(value); 
        }
    }
} 

运行结果：

在迭代器中删除元素：

 package cn.pxy.test;

import java.util.ArrayList;
import java.util.Iterator;
import java.util.List;

public class IteratorRemoveTest {
    public static void main(String[] args) {
        List<String> list = new ArrayList<>(); 
        list.add("a"); 
        list.add("b"); 
        list.add("c"); 
        list.add("d"); 
        Iterator<String> iterator = list.iterator(); 
        while(iterator.hasNext()){ 
            //不要在一次循环中多次调用 next 方法。 
            String value = iterator.next(); 
            if("c".equals(value)){ 
                iterator.remove(); 
            } 
        }System.out.println("----------------"); 
        for(Iterator<String> it = list.iterator();it.hasNext();){ 
            System.out.println(it.next()); 
            list.add("dddd");
        }
    }
} 

5. Collections 工具类

Collections 是一个工具类，它提供了对 Set、List、Map 进行排序、填充、查找元素的辅助方法。该类中所有的方法都为静态方法。

常用方法：

1. void sort(List) //对 List 容器内的元素排序，排序的规则是按照升序进行排序。
2. void shuffle(List) //对 List 容器内的元素进行随机排列。
3. void reverse(List) //对 List 容器内的元素进行逆续排列 。
4. void fill(List, Object) //用一个特定的对象重写整个 List 容器。
5. int binarySearch(List, Object)//对于顺序的 List 容器，采用折半查找的方法查找特定对象

5.1 对 List 类型容器进行排序处理：

 package cn.pxy.test;

import java.util.ArrayList;
import java.util.Collections;
import java.util.List;

public class CollectionsSortTest {
    public static void main(String[] args) {
        List<String> list = new ArrayList<>(); 
        list.add("c"); 
        list.add("b"); 
        list.add("d"); 
        list.add("a"); 
        //通过 Collections 工具类中的 sort 方法完成排序
        Collections.sort(list); 
        for(String str:list){ 
            System.out.println(str);
        }
    }
} 

运行结果：

5.2 对 List 类型容器进行随机排序：

 package cn.pxy.test;

import java.util.ArrayList;
import java.util.Collections;
import java.util.List;

public class CollectionsSortTest {
    public static void main(String[] args) {
        List<String> list = new ArrayList<>(); 
        list.add("c"); 
        list.add("b"); 
        list.add("d"); 
        list.add("a"); 
        //通过 Collections 工具类中的 sort 方法完成排序
        Collections.sort(list); 
        for(String str:list){ 
            System.out.println(str);
        }
        System.out.println("*******");
        List<String> list = new ArrayList<>(); 
        list.add("a"); 
        list.add("b"); 
        list.add("c"); 
        list.add("d"); 
        //洗牌处理 
        Collections.shuffle(list); 
        for(String str:list){ 
            System.out.println(str); 
        }
    }
} 

运行结果：

系列文章：

Java基础——容器（上）

Java基础——容器（下）