Java 容器(四) Map

Java 容器(四) Map
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，不允许多条记录的键为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;
}
@Override
public String toString() {
return "Student [id=" + id + ", name=" + name + "]";
}
@Override
public int hashCode() {
return this.id;
}
@Override
public 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>() {
@Override
public 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:1
key:2
key:3
value:Student [id=3, name=sun]
value:Student [id=1, name=zhao]
value:Student [id=2, name=qian]