week07_day06_Map

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Map接口概述：

将键映射到值的对象。(我们可以根据键快速地查找到值)
Map 中键是唯一的 (不能包含重复的键)
每个键最多只能映射到一个值。

关于映射:

Map接口API：

• V put(K key， V value)
  将指定的值与此映射中的指定键关联（可选操作）。如果此映射以前包含一个该键的映射关系，则用指定值替换旧值（当且仅当 m.containsKey(k) 返回 true 时，才能说映射 m 包含键 k 的映射关系）。
  返回以前与 key 关联的值，如果没有针对 key 的映射关系，则返回 null。（如果该实现支持 null 值，则返回 null 也可能表示此映射以前将 null 与 key 关联）。
• void putAll(Map<? extends K,? extends V> m)
  从指定映射中将所有映射关系复制到此映射中（可选操作）。
  如果key已经存在就会更新value，key不存在就会添加进去。
• V remove(Object key)
  如果存在一个键的映射关系，则将其从此映射中移除（可选操作）。
  返回此映射中以前关联该键的值，如果此映射不包含该键的映射关系，则返回 null。

key对应的value可以为null，所以不能通过get(key) != null来判断key是否存在，应当通过containsKey(Object key) 来判断。

• Set<Map.Entry<K,V>> entrySet()
  返回此映射中包含的映射关系的 Set 视图。
  关于Map.Entry<K,V>，是Map接口的内部接口：
  
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HashMap概述：
基于哈希表的Map接口实现。
允许null键和null值。
不保证映射的顺序，特别是它不保证该顺序恒久不变。
不同步。

HashMap和HashSet的构造方法一样：

HashMap是基于哈希表的 Map 接口的实现。此实现提供所有可选的映射操作，并允许使用 null 值和 null 键。（除了非同步和允许使用 null 之外，HashMap 类与 Hashtable 大致相同。）此类不保证映射的顺序，特别是它不保证该顺序恒久不变。

HashMap VS Hashtable
相同点：
底层的数据结构都是哈希表
不同点：
a. HashMap是不同步的, Hashtable是同步的
b. HashMap可以允许null键和null值，Hashtable不允许null键和null值
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LinkedHashMap概述：
HashMap的子类
Map 接口的哈希表和链表实现，具有可预知的迭代顺序.
链表定义了迭代顺序，该迭代顺序就是键值对的插入顺序。
不同步。

LinkedHashMap是Map 接口的哈希表和链接列表实现，具有可预知的迭代顺序。此实现与 HashMap 的不同之处在于，后者维护着一个运行于所有键值对的双重链接列表。此链接列表定义了迭代顺序，该迭代顺序通常就是将键插入到映射中的顺序（插入顺序）。注意，如果在映射中重新插入 键，则插入顺序不受影响。（如果在调用 m.put(k, v) 前 m.containsKey(k) 返回了 true，则调用时会将键 k 重新插入到映射 m 中。）

为什么要注意这一点呢？
在week06_day02中讲过LRU算法，

改进：能不能够在O(1)的时间复杂度内完成所有操作？
LRU：哈希表+双向链表
但LRU应当将最近使用的结点删除，然后重新插入到head位置，而LinkedHashMap仅仅只是改变结点的value值，并不会改变结点的位置，所以，不能直接使用LinkedHashMap当做LRU的缓存。

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TreeMap概述：
底层的数据结构是红黑树。
如果创建对象时，没有传入 Comparator 对象，键将按自然顺序进行排序。
如果创建对象时，传入了 Comparator 对象，键将按 Comparator 进行排序。
不同步。

和TreeSet一样，我们将它的两个构造方法

• TreeMap()
  使用键的自然顺序构造一个新的、空的树映射。
• TreeMap(Comparator<? super K> comparator)
  构造一个新的、空的树映射，该映射根据给定比较器进行排序。

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练习：存储自定义对象

代码一：String类为key，Student类为value

Student类：

public class Student01 {private String name;private int age;public Student01() {}public Student01(String name, int age) {this.name = name;this.age = age;}public String getName() {return name;}public int getAge() {return age;}@Overridepublic String toString() {return "Student01{" +"name='" + name + '\'' +", age=" + age +'}';}}

测试类：

public class TreeMapDemo1 {public static void main(String[] args) {TreeMap<String, Student01> map = new TreeMap<>();Student01 s1 = new Student01("Allen", 22);Student01 s2 = new Student01("Beyonce", 23);Student01 s3 = new Student01("Catalina", 24);Student01 s4 = new Student01("Diana", 25);map.put("Allen",s1);map.put("Beyonce",s2);map.put("Catalina",s3);map.put("Diana",s4);System.out.println(map.size()); // 4System.out.println(map);}
}

之前写TreeSet的代码时，会发现不在Student类中写implements Comparable的话就没法存入TreeSet，但是现在却存入了，为啥？
因为TreeSet的add方法存入的是TreeMap的key，key必须唯一，可比较，但是现在的代码中，key存的是String，String本身就实现了Comparable接口，而Student是value，value可以相同，也就不需要实现Comparable接口。

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代码二：
Student类：

public class Student02 implements Comparable<Student02> {private String name;private int age;public Student02() {}public Student02(String name, int age) {this.name = name;this.age = age;}public String getName() {return name;}public int getAge() {return age;}@Overridepublic String toString() {return "Student01{" +"name='" + name + '\'' +", age=" + age +'}';}@Overridepublic int compareTo(Student02 s) {int cmp = this.age - s.age;cmp = cmp != 0 ? cmp : this.name.compareTo(s.name);return cmp;}
}

测试类：

public class TreeMapDemo02 {public static void main(String[] args) {Map<Student02, String> map = new TreeMap<>();Student02 s1 = new Student02("Allen", 22);Student02 s2 = new Student02("Beyonce", 23);Student02 s3 = new Student02("Catalina", 24);Student02 s4 = new Student02("Diana", 25);map.put(s1, "Allen");map.put(s2, "Beyonce");map.put(s3, "Catalina");map.put(s4, "Diana");System.out.println(map.size()); // 4System.out.println(map);}
}

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代码三：
Student类同代码一
测试类：

public class TreeMapDemo03 {public static void main(String[] args) {Map<Student01, String> map = new TreeMap<>(new Comparator<Student01>() {@Overridepublic int compare(Student01 s1, Student01 s2) {int cmp = s1.getAge() - s2.getAge();cmp = cmp != 0 ? cmp : s1.getName().compareTo(s2.getName());return cmp;}});Student01 s1 = new Student01("Allen", 22);Student01 s2 = new Student01("Beyonce", 23);Student01 s3 = new Student01("Catalina", 24);Student01 s4 = new Student01("Diana", 25);map.put(s1, "Allen");map.put(s2, "Beyonce");map.put(s3, "Catalina");map.put(s4, "Diana");System.out.println(map.size());System.out.println(map);}
}

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TreeMap特有的API：

• K ceilingKey(K key)
  返回大于等于给定键的最小键；如果不存在这样的键，则返回 null。

• K floorKey(K key)
  返回小于等于给定键的最大键；如果不存在这样的键，则返回 null。

• K higherKey(K key)
  返回严格大于给定键的最小键；如果不存在这样的键，则返回 null。

• K lowerKey(K key)
  返回严格小于给定键的最大键；如果不存在这样的键，则返回 null。

• K firstKey()
  返回此映射中当前第一个（最低）键。

• K lastKey()
  返回映射中当前最后一个（最高）键

• Map.Entry<K,V> pollFirstEntry()
  移除并返回与此映射中的最小键关联的键-值映射关系；如果映射为空，则返回 null。

• Map.Entry<K,V> pollLastEntry()
  移除并返回与此映射中的最大键关联的键-值映射关系；如果映射为空，则返回 null。

• NavigableMap<K,V> subMap(K fromKey, boolean fromInclusive, K toKey, boolean toInclusive)
  返回此映射的部分视图，其键的范围从 fromKey 到 toKey。

代码演示

public class TreeMapDemo04 {public static void main(String[] args) {TreeMap<Character, String> map = new TreeMap<>();map.put('A', "Allen");map.put('B', "Beyonce");map.put('C', "Catalina");map.put('D', "Diana");map.put('F', "Frances");map.put('X', "Xenia");// K ceilingKey(K key)/*System.out.println(map.ceilingKey('C')); // 'C'System.out.println(map.ceilingKey('E')); // 'F'System.out.println(map.ceilingKey('Z')); // null*/// K floorKey(K key)/*System.out.println(map.floorKey('C')); // CSystem.out.println(map.floorKey('E')); // DSystem.out.println(map.floorKey('0')); // null*///    K higherKey(K key)/*System.out.println(map.higherKey('C')); // 'D'System.out.println(map.higherKey('E')); // 'F'System.out.println(map.higherKey('Z')); // null*/// K lowerKey(K key)/* System.out.println(map.lowerKey('C')); // BSystem.out.println(map.lowerKey('E')); // DSystem.out.println(map.lowerKey('0')); // null*/// K firstKey()/* System.out.println(map.firstKey());System.out.println(map);*/// K lastKey()/*System.out.println(map.lastKey());System.out.println(map);*/// Map.Entry<K,V> pollFirstEntry()/*System.out.println(map.pollFirstEntry());System.out.println(map);*/// Map.Entry<K,V> pollLastEntry()/*System.out.println(map.pollLastEntry());System.out.println(map);*/// 视图技术// NavigableMap<K,V> subMap(K fromKey, boolean fromInclusive, K toKey, boolean toInclusive)NavigableMap<Character, String> subMap = map.subMap('C', true, 'X', false);System.out.println(subMap);subMap.pollFirstEntry();System.out.println(subMap);System.out.println(map);}
}

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Properties概述：
Hashtable<Object, Object> 的子类
Properties 类表示了一个可持久的属性集。
Properties 可保存在流中或从流中加载。
Properties 中每个键及其对应值都是一个字符串（Sring类型）。

注意事项：不要使用Hashtable里面定义的方法添加键值对！因为它们可以插入不是String 类型的数据。
如果一个Properties中含有非String的键值对，那么这样的Properties是”不安全”的。调用 store 或者 save 方法将失败。

构造方法：

• Properties()
  创建一个无默认值的空属性列表。
• Properties(Properties defaults)
  创建一个带有指定默认值的空属性列表。

API：

• String getProperty(String key)
  用指定的键在此属性列表中搜索属性。
• String getProperty(String key, String defaultValue)
  用指定的键在属性列表中搜索属性。
  key存在的话返回对应的value，不存在的话返回defaultValue
• Object setProperty(String key, String value)
  调用 Hashtable 的方法 put。
• Set<String> stringPropertyNames()
  返回此属性列表中的键集，其中该键及其对应值是字符串，如果在主属性列表中未找到同名的键，则还包括默认属性列表中不同的键。

public class PropertiesDemo1 {public static void main(String[] args) {Properties info = new Properties();info.setProperty("刘强东", "章泽天");info.setProperty("文章", "马伊利");info.setProperty("贾乃亮", "李小璐");info.setProperty("罗志祥", "周扬青");// System.out.println(info);/*Properties properties = new Properties(info);System.out.println(properties);System.out.println(properties.size());*//*System.out.println(properties.containsKey("刘强东"));System.out.println(properties.getProperty("刘强东"));System.out.println(properties.getProperty("文章"));System.out.println(properties.getProperty("谢霆锋"));*//*properties.setProperty("文章", "");System.out.println(properties.getProperty("文章"));*/// String getProperty(String key)/*System.out.println(info.getProperty("刘强东"));System.out.println(info.getProperty("谢霆锋"));// String getProperty(String key, String defaultValue)System.out.println(info.getProperty("刘强东", "default"));System.out.println(info.getProperty("谢霆锋", "default"));*/Set<String> names = info.stringPropertyNames();for(String key : names) {String value = info.getProperty(key);System.out.println(key + "=" + value);}}
}

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• void load(InputStream inStream)
  从输入流中读取属性列表（键和元素对）。
• void load(Reader reader)
  按简单的面向行的格式从输入字符流中读取属性列表（键和元素对）。
• void loadFromXML(InputStream in)
  将指定输入流中由 XML 文档所表示的所有属性加载到此属性表中。
• void store(OutputStream out, String comments)
  以适合使用 load(InputStream) 方法加载到 Properties 表中的格式，将此 Properties 表中的属性列表（键和元素对）写入输出流。
• void store(Writer writer, String comments)
  以适合使用 load(Reader) 方法的格式，将此 Properties 表中的属性列表（键和元素对）写入输出字符。
• void storeToXML(OutputStream os, String comment)
  发出一个表示此表中包含的所有属性的 XML 文档。
• void storeToXML(OutputStream os, String comment, String encoding)
  使用指定的编码发出一个表示此表中包含的所有属性的 XML 文档。

package com.cskaoyan.map;import java.io.*;
import java.util.Properties;/*
和流相关的API// void store(OutputStream out, String comments)void store(Writer out, String comments)// void load(InputStream inStream)void load(Reader read)注意事项：字节流默认使用 ISO 8859-1 字符编码(无法表示汉字)，所以不建议使用第一个和第三个API。void storeToXML(OutputStream os, String comment)void loadFromXML(InputStream in)注意事项：默认采用UTF-8编码。*/
public class PropertiesDemo2 {public static void main(String[] args) {/*Properties info = new Properties();info.setProperty("刘强东", "章泽天");info.setProperty("文章", "马伊利");info.setProperty("贾乃亮", "李小璐");info.setProperty("罗志祥", "周扬青");// void store(Writer out, String comments)try(Writer writer = new FileWriter("a.txt")) {info.store(writer, "...");} catch (IOException e) {e.printStackTrace();}*/// void load(Reader read)/*Properties info = new Properties();try (Reader reader = new FileReader("a.txt")) {info.load(reader);} catch (IOException e) {e.printStackTrace();}System.out.println(info);*/// void storeToXML(OutputStream os, String comment)/*Properties info = new Properties();info.setProperty("刘强东", "章泽天");info.setProperty("文章", "马伊利");info.setProperty("贾乃亮", "李小璐");info.setProperty("罗志祥", "周扬青");try(OutputStream out = new FileOutputStream("b.xml")) {info.storeToXML(out, "...");} catch (IOException e) {e.printStackTrace();}*/// void loadFromXML(InputStream in)Properties info = new Properties();try(InputStream in = new FileInputStream("b.xml")) {info.loadFromXML(in);} catch (IOException e) {e.printStackTrace();}System.out.println(info);}
}

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PPT上的三道题：

1. “aababcabcdabcde”,获取字符串中每一个字母出现的次数要求结果:a(5)b(4)c(3)d(2)e(1)
   大致思路同LeetCode之根据字符出现频率排序

public class Homework01 {public static void main(String[] args) {String s = "aababcabcdabcde";String ret = charCount(s);System.out.println(ret);}public static String charCount(String s) {Map<Character, Integer> map = new TreeMap<>();for (int i = 0; i < s.length(); i++) {//if (map.containsKey(c)) map.put(c, map.get(c) + 1);//else map.put(c, 1);map.put(s.charAt(i), map.getOrDefault(s.charAt(i), 0) + 1);}StringBuilder sb = new StringBuilder();Set<Map.Entry<Character, Integer>> entries = map.entrySet();for (Map.Entry<Character, Integer> e : entries) {Character key = e.getKey();Integer value = e.getValue();sb.append(key).append("(").append(value).append(")");}return sb.toString();}
}

2. LeetCode之两数之和
   这道题用到了哈希表，工作中更常用的是HashMap，底层是哈希表，增删查时间复杂度都是O(1)，而TreeMap底层是红黑树，增删查的时间复杂度是O(logN)，也就是树的高度。
3. 请设计一个猜数字小游戏，可以试玩5次。试玩结束之后，给出提示：游戏试玩结束，请付费。

package com.cskaoyan.exercise;import java.io.*;
import java.util.Properties;
import java.util.Scanner;/*** @author shihao* @create 2020-05-24 17:33*/
public class ex01 {public static void main(String[] args) {Properties properties = new Properties();try (Reader r = new FileReader("count.txt")) {properties.load(r);} catch (Exception e) {e.printStackTrace();}String value = properties.getProperty("count");int num = Integer.valueOf(value);if (num > 5) {System.out.println("试玩结束，请付费");System.exit(0);} else {num++;properties.setProperty("count",String.valueOf(num));try(Writer w = new FileWriter("count.txt")){properties.store(w,null);} catch (IOException e) {e.printStackTrace();}GuessNumber.start();}}
}class GuessNumber {public static void start() {int count = 0;while (true) {//一定要将(Math.random() * 100)强换为int//而不是只讲Math.random()强转为intint num = (int) (Math.random() * 100) + 1;Scanner in = new Scanner(System.in);System.out.println("请输入数据(1-100):");int guessNumber = in.nextInt();count++;if (guessNumber > num) {System.out.println("你猜的数字" + guessNumber + "大了");} else if (guessNumber < num) {System.out.println("你猜的数字" + guessNumber + "小了");} else {System.out.println("恭喜你," + count + "次猜中了");break;}}}
}

count.txt

#Sun May 24 18:12:45 CST 2020
count=1

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Map总结：

Map|-- HashMap|-- LinkedHashMap|-- Hashtable|-- Properties|-- TreeMap

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