学习笔记之JavaSE（35）--JavaAPI详解10

今天学习的内容是集合类之HashMap、LinkedHashMap和TreeMap

常用的映射有HashMap、LinkedHashMap和TreeMap，下面分别介绍一下它们

一、HashMap和LinkedHashMap

HashMap、LinkedHashMap与HashSet、LinkedHashSet的原理基本相同，把映射中的键视作集中的元素即可。注意HashMap的键和值都可以是null。

补充个知识点：HashTable，此类也实现了Map接口，与HashMap类似，区别在于它是同步的，且不能使用null作为键和值（面试要点）。虽然这个类已经过时，但是它的子类Properties很重要，可以和IO技术相结合存储键值对型的配置文件的信息。

示例程序：

public class Test60 {public static void main(String[] args) {// HashMap的键不能重复，且元素是无序的Map<Person, String> hs = new HashMap<>();hs.put(new Person("张三", 21), "云南");hs.put(new Person("张三", 21), "北京");hs.put(new Person("李四", 25), "上海");hs.put(new Person("王五", 27), "天津");hs.put(new Person("赵六", 24), "广州");System.out.println(hs);// HashMap是无序的 {Person [name=王五, age=21]=天津,// Person [name=赵六, age=24]=广州, Person [name=张三,// age=21]=北京, Person [name=李四, age=22]=上海}// LinkedHashMap是有序的Map<Person, String> lhs = new LinkedHashMap<>();lhs.put(new Person("张三", 21), "云南");lhs.put(new Person("张三", 21), "北京");lhs.put(new Person("李四", 25), "上海");lhs.put(new Person("王五", 27), "天津");lhs.put(new Person("赵六", 24), "广州");System.out.println(lhs);// LinkedHashMap是有序的 {Person [name=张三,// age=21]=北京, Person [name=李四, age=25]=上海,// Person [name=王五, age=27]=天津, Person [name=赵六,// age=24]=广州}}}//要覆盖Person的hashCode()与equals()方法 class Person {private String name;private int age;public Person() {super();}public Person(String name, int age) {super();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;}@Overridepublic String toString() {return "Person [name=" + name + ", age=" + age + "]";}@Overridepublic int hashCode() { // 覆盖Object的hashCode()方法final int prime = 31;int result = 1;result = prime * result + age;result = prime * result + ((name == null) ? 0 : name.hashCode());return result;}@Overridepublic boolean equals(Object obj) {// 覆盖Object的equals()方法if (this == obj)return true;if (obj == null)return false;if (getClass() != obj.getClass())return false;Person other = (Person) obj;if (age != other.age)return false;if (name == null) {if (other.name != null)return false;} else if (!name.equals(other.name))return false;return true;}}

二、TreeMap

TreeMap与TreeSet的原理也基本相同。注意由于TreeMap要根据键按照指定规则进行排序，所以TreeMap存储的元素的键不能是null，而值可以是null。

示例程序：

public class Test61 {public static void main(String[] args) {// 使用自然排序Map<Person, String> tm = new TreeMap<>();tm.put(new Person("zhangsan", 21), "天津");tm.put(new Person("lisi", 22), "北京");// 如果Person类没有实现Comparable接口，会发生ClassCastExceptiontm.put(new Person("wangwua", 23), "云南");tm.put(new Person("wangwub", 23), "上海");tm.put(new Person("zhaoliu", 24), "广州");System.out.println(tm);// {Person [name=zhangsan, age=21]=天津, Person// [name=lisi, age=22]=北京, Person [name=wangwua,// age=23]=云南, Person [name=wangwub, age=23]=上海,// Person [name=zhaoliu, age=24]=广州}// 使用比较器排序Map<Person, String> tm_com = new TreeMap<>(new ComparatorByName());tm_com.put(new Person("zhangsan", 21), "天津");tm_com.put(new Person("lisi", 22), "北京");tm_com.put(new Person("wangwua", 23), "云南");tm_com.put(new Person("wangwub", 23), "上海");tm_com.put(new Person("zhaoliu", 24), "广州");System.out.println(tm_com);// {Person [name=lisi, age=22]=北京, Person// [name=wangwua, age=23]=云南, Person// [name=wangwub, age=23]=上海, Person// [name=zhangsan, age=21]=天津, Person// [name=zhaoliu, age=24]=广州}}}class Person implements Comparable<Person> {private String name;private int age;public Person() {super();}public Person(String name, int age) {super();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;}@Overridepublic String toString() {return "Person [name=" + name + ", age=" + age + "]";}@Overridepublic int hashCode() { // 覆盖Object的hashCode()方法final int prime = 31;int result = 1;result = prime * result + age;result = prime * result + ((name == null) ? 0 : name.hashCode());return result;}@Overridepublic boolean equals(Object obj) {// 覆盖Object的equals()方法if (this == obj)return true;if (obj == null)return false;if (getClass() != obj.getClass())return false;Person other = (Person) obj;if (age != other.age)return false;if (name == null) {if (other.name != null)return false;} else if (!name.equals(other.name))return false;return true;}// 本例中按照年龄进行从小到大排序，如果年龄相等，再按照姓名进行自然排序@Overridepublic int compareTo(Person o) {int temp = this.age - o.age;return temp == 0 ? this.name.compareTo(o.name) : temp;}}// 自定义比较器，按照姓名进行自然排序，如果姓名相同，再按照年龄大小进行排序class ComparatorByName implements Comparator<Person> {@Overridepublic int compare(Person o1, Person o2) {int temp = o1.getName().compareTo(o2.getName());return temp == 0 ? o1.getAge() - o2.getAge() : temp;}}

总结：如果要按照特定规则对元素进行排序，建议使用TreeMap；如果想让元素按照添加顺序排序，建议使用LinkedHashMap；其他情况都使用HashMap