Map接口簡介

今天來看一看map集合，map映射接口，用于存放鍵值對，<key,value>，通過key來查找value,顧名思義key不能為空，唯一且不重復(fù)，不然底層怎么查呢！

可以從圖中看出Map為單獨(dú)的接口，他和Collection有什么區(qū)別呢？

Map和Collection在集合中并列存在。

Map集合是雙列的，鍵值對，而Collection是單列集合

Map存儲(chǔ)元素使用put方法，Collection使用Put方法。

Map遍歷沒有直接取出元素的方法，而是先轉(zhuǎn)成Set集合，再通過迭代獲取元素。

--Map常用方法

–Map應(yīng)用

添加：使用HashMap。立了學(xué)生姓名和年齡之間的映射關(guān)系。并試圖添加重復(fù)的鍵

復(fù)制代碼

??public static void main(String[] args) {

????// 定義一個(gè)Map的容器對象??

????Map<String, Integer > map1 = new HashMap<String, Integer >();??

????map1.put("jack", 20);??

????map1.put("rose", 18);??

????map1.put("lucy", 17);??

????map1.put("java", 25);

???// map1.put("jack", 30)； 在沒有hashCode和equals方式??添加重復(fù)的鍵值（值不同）,會(huì)覆蓋掉前面key值相同的值

????System.out.println(map1);??

????

????Map<String, Integer> map2 = new HashMap<String, Integer>();??

????map2.put("張三豐", 100);??

????map2.put("虛竹", 20);??

????System.out.println("map2:" + map2);??

????// 從指定映射中將所有映射關(guān)系復(fù)制到此映射中。??

????map1.putAll(map2);??

????System.out.println("map1:" + map1);??

??}??

??public static void main(String[] args) {??

// 刪除：??

????// remove() 刪除關(guān)聯(lián)對象，指定key對象??

????// clear() 清空集合對象??

??

????Map<String, Integer> map1 = new HashMap<String, Integer>();??

????map1.put("jack", 20);??

????map1.put("rose", 18);??

????map1.put("lucy", 17);??

????map1.put("java", 25);??

????System.out.println(map1);?????????

????// 指定key，返回刪除的鍵值對映射的值。??

????map1.remove("java");

????System.out.println(map1);??

????map1.clear();??

????System.out.println("map1:" + map1);??

??}??

public static void main(String[] args) {

???// 獲取：??

????// V get(Object key) 通過指定的key對象獲取value對象??

????// int size() 獲取容器的大小??

????Map<String, Integer> map1 = new HashMap<String, Integer>();??

????map1.put("jack", 20);??

????map1.put("rose", 18);??

????map1.put("lucy", 17);??

????map1.put("java", 25);??

????System.out.println(map1);??

????// V get(Object key) 通過指定的key對象獲取value對象??

????System.out.println("value:" + map1.get("jack"));??

????// int size() 獲取容器的大小

????System.out.println("map.size:" + map1.size());?

??}?

public static void main(String[] args) {

????// 判斷：??

????// boolean isEmpty() 判斷集合是否為空??長度為0返回true否則false??

????// boolean containsKey(Object key) 判斷集合中是否包含指定的key??

????// boolean containsValue(Object value)??

??

????Map<String, Integer> map1 = new HashMap<String, Integer>();??

????map1.put("jack", 20);??

????map1.put("rose", 18);??

????map1.put("lucy", 17);??

????map1.put("java", 25);??

????System.out.println(map1);??

????System.out.println("isEmpty:" + map1.isEmpty());??

????System.out.println("containskey:" + map1.containsKey("jack"));??

????System.out.println("containsvalues:" + map1.containsValue(100));??

??}??

遍歷Map的4中方式：

第一種：

public static void main(String[] args) {

????//遍歷Map 第一種方式

????Map<String, Integer> map1 = new HashMap<String, Integer>();??

????map1.put("jack", 20);??

????map1.put("rose", 18);??

????map1.put("lucy", 17);??

????map1.put("java", 25);??

?????

????//通過 map1.keySet() 獲取key?通過key 找到value

????for (String key : map1.keySet()) {

??????Integer value = map1.get(key);

??????System.out.println("key : "+key+" value : "+value);

????}

??}

第二種：

public static void main(String[] args) {

????//遍歷Map 第二種方式

????Map<String, Integer> map1 = new HashMap<String, Integer>();??

????map1.put("jack", 20);??

????map1.put("rose", 18);??

????map1.put("lucy", 17);??

????map1.put("java", 25);??

?????

????//通過Map.Entry(String,Integer) 獲取，然后使用entry.getKey()獲取到鍵，通過entry.getValue()獲取到值

????for(Map.Entry<String, Integer> entry : map1.entrySet()){

??????System.out.println("鍵 key ："+entry.getKey()+" 值value ："+entry.getValue());

????}

??}

第三種：

//遍歷Map 第三種方式

????Map<String, Integer> map1 = new HashMap<String, Integer>();??

????map1.put("jack", 20);??

????map1.put("rose", 18);??

????map1.put("lucy", 17);??

????map1.put("java", 25);??

????//第三種只遍歷鍵或者值，通過加強(qiáng)for循環(huán)

????for(String s1:map1.keySet()){//遍歷map的鍵

??????System.out.println("鍵key ："+s1);

????}

????for(Integer s2:map1.values()){//遍歷map的值

??????System.out.println("值value ："+s2);

????}

??????System.out.println("====================================");???

??}

第四種：

public static void main(String[] args) {

????//遍歷Map 第一種方式

????Map<String, Integer> map1 = new HashMap<String, Integer>();??

????map1.put("jack", 20);??

????map1.put("rose", 18);??

????map1.put("lucy", 17);??

????map1.put("java", 25);??

?????

????//第四種Iterator遍歷獲取，然后獲取到Map.Entry<String, String>，再得到getKey()和getValue()

????Iterator<Map.Entry<String, Integer>> it=map1.entrySet().iterator();

????while(it.hasNext()){

??????Map.Entry<String, Integer> entry=it.next();?

??????System.out.println("鍵key ："+entry.getKey()+" value ："+entry.getValue());

????}

????

??}

底層是哈希表數(shù)據(jù)結(jié)構(gòu)，線程是不同步的，可以存入null鍵，null值。要保證鍵的唯一性，需要覆蓋hashCode方法，和equals方法。

案例：自定義對象作為Map的鍵。

??public class Demo3 {??

????public static void main(String[] args) {??

??????HashMap<Person, String> hm = new HashMap<Person, String>();??

??????hm.put(new Person("jack", 20), "1001");??

??????hm.put(new Person("rose", 18), "1002");??

??????hm.put(new Person("lucy", 19), "1003");??

??????hm.put(new Person("hmm", 17), "1004");??

??????hm.put(new Person("ll", 25), "1005");??

??????System.out.println(hm);??

??????System.out.println(hm.put(new Person("rose", 18), "1006"));?//重寫hashCode和equalse后key相同不會(huì)覆蓋

????

??????Set<Entry<Person, String>> entrySet = hm.entrySet();??

??????Iterator<Entry<Person, String>> it = entrySet.iterator();??

??????while (it.hasNext()) {??

????????Entry<Person, String> next = it.next();??

????????Person key = next.getKey();??

????????String value = next.getValue();??

????????System.out.println(key + " = " + value);??

??????}??

????}??

??}??

????

??class Person {??

????private String name;??

????private int age;??

????

????Person() {??

????

????}??

????

????public Person(String name, int age) {??

??????this.name = name;??

??????this.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 int hashCode() {??

????

??????return this.name.hashCode() + age * 37;??

????}??

????

????@Override??

????public boolean equals(Object obj) {??

??????if (obj instanceof Person) {??

????????Person p = (Person) obj;??

????????return this.name.equals(p.name) && this.age == p.age;??

??????} else {??

????????return false;??

??????}??

????}??

????

????@Override??

????public String toString() {??

????

??????return "Person@name:" + this.name + " age:" + this.age;??

????}??

????

??}??

??}??

TreeMap的排序，TreeMap可以對集合中的鍵進(jìn)行排序。如何實(shí)現(xiàn)鍵的排序？

方式一：元素自身具備比較性

和TreeSet一樣原理，需要讓存儲(chǔ)在鍵位置的對象實(shí)現(xiàn)Comparable接口，重寫compareTo方法，也就是讓元素自身具備比較性，這種方式叫做 元素的自然排序也叫做默認(rèn)排序。

方式二：容器具備比較性

當(dāng)元素自身不具備比較性，或者自身具備的比較性不是所需要的。那么此時(shí)可以讓容器自身具備。需要定義一個(gè)類實(shí)現(xiàn)接口Comparator，重 寫compare方法，并將該接口的子類實(shí)例對象作為參數(shù)傳遞給TreeMap集合的構(gòu)造方法。

注意：當(dāng)Comparable比較方式和Comparator比較方式同時(shí)存在時(shí)，以Comparator的比較方式為主；

注意：在重寫compareTo或者compare方法時(shí)，必須要明確比較的主要條件相等時(shí)要比較次要條件。（假設(shè)姓名和年齡一致的人為相同的人， 如果想要對人按照年齡的大小來排序，如果年齡相同的人，需要如何處理？不能直接return 0，以為可能姓名不同（年齡相同姓名不同的人 是不同的人）。此時(shí)就需要進(jìn)行次要條件判斷（需要判斷姓名），只有姓名和年齡同時(shí)相等的才可以返回0.）

通過return 0來判斷唯一性。

public class Demo4 {?

??public static void main(String[] args) {?

????TreeMap<String, Integer> tree = new TreeMap<String, Integer>();?

????tree.put("張三", 19);?

????tree.put("李四", 20);?

????tree.put("王五", 21);?

????tree.put("趙六", 22);?

????tree.put("周七", 23);?

????tree.put("張三", 24);?

????System.out.println(tree);?

????System.out.println("張三".compareTo("李四"));

??}?

}?

??public class Demo3 {??

????public static void main(String[] args) {??

??????TreeMap<Person, String> hm = new TreeMap<Person, String>(??

??????????new MyComparator());??

??????hm.put(new Person("jack", 20), "1001");??

??????hm.put(new Person("rose", 18), "1002");??

??????hm.put(new Person("lucy", 19), "1003");??

??????hm.put(new Person("hmm", 17), "1004");??

??????hm.put(new Person("ll", 25), "1005");??

??????System.out.println(hm);??

??????System.out.println(hm.put(new Person("rose", 18), "1006"));??

????

??????Set<Entry<Person, String>> entrySet = hm.entrySet();??

??????Iterator<Entry<Person, String>> it = entrySet.iterator();??

??????while (it.hasNext()) {??

????????Entry<Person, String> next = it.next();??

????????Person key = next.getKey();??

????????String value = next.getValue();??

????????System.out.println(key + " = " + value);??

??????}??

????}??

??}??

????

??class MyComparator implements Comparator<Person> {??

????

????@Override??

????public int compare(Person p1, Person p2) {??

??????if (p1.getAge() > p2.getAge()) {??

????????return -1;??

??????} else if (p1.getAge() < p2.getAge()) {??

????????return 1;??

??????}??

??????return p1.getName().compareTo(p2.getName());??

????}??

????

??}??

????

??class Person implements Comparable<Person> {??

????private String name;??

????private int age;??

????

????Person() {??

????

????}??

????

????public Person(String name, int age) {??

????

??????this.name = name;??

??????this.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 int hashCode() {??

????

??????return this.name.hashCode() + age * 37;??

????}??

????@Override??

????public boolean equals(Object obj) {??

??????if (obj instanceof Person) {??

????????Person p = (Person) obj;??

????????return this.name.equals(p.name) && this.age == p.age;??

??????} else {??

????????return false;??

??????}??

????}??

????

????@Override??

????public String toString() {??

????

??????return "Person@name:" + this.name + " age:" + this.age;??

????}??

????

????@Override??

????public int compareTo(Person p) {??

????

??????if (this.age > p.age) {??

????????return 1;??

??????} else if (this.age < p.age) {??

????????return -1;??

??????}??

??????return this.name.compareTo(p.name);??

????}??

????

??}??

注意：Set的元素不可重復(fù)，Map的鍵不可重復(fù)，如果存入重復(fù)元素如何處理

Set元素重復(fù)元素不能存入add方法返回false

Map的重復(fù)健將覆蓋舊鍵，將舊值返回。