Java 容器(四) Map

纸上得来终觉浅

1.Map的使用估计是最常见的了： 

1）HashMap：存储的值可以重复，键不能；不能使用Comparator和Comparable接口排序；HashMap最多只允许一条记录的键为null，不允许多条记录的值为null

2）TreeMap：存储的值可以重复，键不能；自动排序或手动排序；

3）LinkedHashMap：存储的值可以重复，键不能；保持输入的顺序

其中它们的用法和Set对比，几乎是一一对应：

package roadArchitectWeb.Test;import java.util.Comparator;import java.util.HashMap;import java.util.LinkedHashMap;import java.util.Map;import java.util.TreeMap;class Student{private Integer id;private String name;public Integer getId() {return id;}public void setId(Integer id) {this.id = id;}public String getName() {return name;}public void setName(String name) {this.name = name;}public Student(Integer id, String name) {super();this.id = id;this.name = name;}@Overridepublic String toString() {return "Student [id=" + id + ", name=" + name + "]";}@Overridepublic int hashCode() {return this.id;}@Overridepublic boolean equals(Object obj) {if(obj instanceof Student){Student stu = (Student)obj;if(stu.name == this.name && stu.id == this.id)return true;}return false;}}public class Test5 {protected static Map<String,Student> hashMap = new HashMap<String,Student>();/*Comparable的排序会失效,因为map只能按键排列*/protected static Map<String,Student> treeMapComparable = new TreeMap<String,Student>();protected static Map<String,Student> treeMapComparator = new TreeMap<String,Student>(new Comparator<String>() {@Overridepublic int compare(String o1, String o2) {return o2.compareTo(o1);}});protected static Map<String,Student> linkedHashMap = new LinkedHashMap<String,Student>();public static void main(String[] args) {hashMap.put("1",new Student(3,"sun"));hashMap.put("2",new Student(1,"zhao"));hashMap.put("3",new Student(2,"qian"));System.out.println("hashMap:"+hashMap);/* *这里使用Comparable */treeMapComparable.put("3",new Student(3,"sun"));treeMapComparable.put("2",new Student(1,"zhao"));treeMapComparable.put("1",new Student(2,"qian"));System.out.println("treeMapComparable:"+treeMapComparable);/*这里使用Comparator */treeMapComparator.put("3",new Student(3,"sun"));treeMapComparator.put("1",new Student(1,"zhao"));treeMapComparator.put("2",new Student(2,"qian"));System.out.println("treeMapComparator:"+treeMapComparator);linkedHashMap.put("3",new Student(3,"sun"));linkedHashMap.put("1",new Student(1,"zhao"));linkedHashMap.put("2",new Student(2,"qian"));System.out.println("linkedHashMap:"+linkedHashMap);}}

结果如下：

hashMap:{1=Student [id=3, name=sun], 2=Student [id=1, name=zhao], 3=Student [id=2, name=qian]}treeMapComparable:{1=Student [id=2, name=qian], 2=Student [id=1, name=zhao], 3=Student [id=3, name=sun]}treeMapComparator:{3=Student [id=3, name=sun], 2=Student [id=2, name=qian], 1=Student [id=1, name=zhao]}linkedHashMap:{3=Student [id=3, name=sun], 1=Student [id=1, name=zhao], 2=Student [id=2, name=qian]}

2.注意

1）同Set，hashMap和LinkedHashMap不支持排序；

2）hashTable：基本上与hashMap相同，它不允许记录的键或者值为空；线程同步；

3）HashSet是通过HashMap实现的,TreeSet是通过TreeMap实现的,只不过Set用的只是Map的key；

4）另外注意到，treeMap当key是基础类，如String时候，似乎只能使用Comparator接口，基础类的Comparable接口是内置的（TreeMap排序使用Comparable没有实现过）

3.Map中迭代器使用：

/*遍历key*/Iterator it1 = hashMap.keySet().iterator();while(it1.hasNext()){System.out.println("key:"+it1.next());}/*遍历entry，从而可以获得key或者value的值*/Iterator it2 = hashMap.entrySet().iterator();while(it2.hasNext()){Map.Entry<String, Student> entry = null;entry = (Entry<String, Student>) it2.next();System.out.println("value:"+entry.getValue());}

结果如下：

key:1key:2key:3value:Student [id=3, name=sun]value:Student [id=1, name=zhao]value:Student [id=2, name=qian]