Java中遍历Map的两种方法：keySet和entrySet

何伟超

import java.util.Calendar;
import java.util.Date;
import java.util.HashMap;
import java.util.Iterator;
import java.util.Map.Entry;
/**
* 测试keySet()与entrySet()的迭代时间
* keySet():迭代后只能通过get()取key
* entrySet()：迭代后可以e.getKey()，e.getValue()取key和value。返回的是Entry接口
* 最后说明下keySet()的速度比entrySet()慢了很多。看来以后要考虑用entrySet()了
* @author JackeenLiu
* @date 2014-4-14 星期一
*/
public class HashMapTest
{
public static void main(String[] args)
{
HashMap<String,String> kmap = new HashMap<String,String>();
HashMap<String, String> emap = new HashMap<String, String>();
//装数据
for (int i = 0; i < 100; i++)
{
kmap.put(""+i, "abcdefghijklmnopqrstuvwxyz");
}
for (int i = 0; i < 100; i++)
{
emap.put(""+i, "abcdefghijklmnopqrstuvwxyz");
}
long stimes = System.currentTimeMillis();
long ctimes = Calendar.getInstance().getTimeInMillis();
long dtimes = new Date().getTime();
//初始时间这里我用了三种取值方式最后发现System.currentTimeMillis();是最直接的取值方法
System.out.println(stimes+" "+ctimes+" "+dtimes);
Iterator<String> ktor = kmap.keySet().iterator();
while(ktor.hasNext())
{
System.out.println(ktor.next());
}
long stimes1 = System.currentTimeMillis();
long ctimes1 = Calendar.getInstance().getTimeInMillis();
long dtimes1 = new Date().getTime();
//结束世界并且也是entrySet的开始时间
System.out.println((stimes1-stimes)+" "+(ctimes1-ctimes)+" "+(dtimes1-dtimes));
System.out.println(stimes1+" "+ctimes1+" "+dtimes1);
Iterator<Entry<String, String>> itor = emap.entrySet().iterator();
while(itor.hasNext())
{
Entry<String, String> e = itor.next();
//System.out.println(e.getKey());
System.out.println(e.getValue());
}
long stimes2 = System.currentTimeMillis();
long ctimes2 = Calendar.getInstance().getTimeInMillis();
long dtimes2 = new Date().getTime();
System.out.println(stimes2+" "+ctimes2+" "+dtimes2);
System.out.println((stimes2-stimes1)+" "+(ctimes2-ctimes1)+" "+(dtimes2-dtimes1));
}
}

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_Water · _Water

/*
对学生数据类的存取
*/

import java.util.*;
class Test
{
public static void main(String[] args)
{
HashMap<Student,String> hs =new HashMap<Student,String>();
hs.put(new Student("zhangsan",11),"bj");
hs.put(new Student("lisi",31),"nj");
hs.put(new Student("wangwu",21),"dj");
//方式1.
Set<Student> keySet=hs.keySet();
for (Iterator<Student> it =keySet.iterator(); it.hasNext(); )
{
Student key =it.next();
String addr =hs.get(key);
sop(key+".."+addr);
}
//方式2
Set<Map.Entry<Student,String>> entry =hs.entrySet();
for (Iterator<Map.Entry<Student,String>> it =entry.iterator();it.hasNext() ; )
{
Map.Entry<Student,String> e =it.next();
//Student key =it.next().getKey();
//String value =it.next().getValue();
Student key =e.getKey();
String value =e.getValue();
sop(key+"----++++-------"+value);
}
}
public static void sop(Object obj)
{
System.out.println(obj);
}
}
class Student implements Comparable<Student>
{
private String name;
private String address;
private int age;
public int compareTo(Student s)
{
int num =new Integer(this.age).compareTo(new Integer(s.age));
if (num ==0)
{
return this.name.compareTo(s.name);
}
return num;
}
public int hashCode()
{
return name.hashCode()+age*11;
}
public boolean equals(Object obj)
{
if(!(obj instanceof Student))
throw new ClassCastException("不是同一类");
Student s =(Student)obj;
return this.name.equals(s.name) && this.age==s.age;
}
Student(String name,int age)
{
this.name=name;
this.age=age;
}
public String getName()
{
return name;
}
public int getAge()
{
return age;
}
public String toString()
{
return name+"---->"+age;
}
}

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_Water · _Water

import java.util.*;
/*
需求：对学生对象的年龄进行升序排序。
因为数据是以键值对形式存在的，
要使用可以进行排序的Map集合。TreeMap
*/
class StuNameCompare implements Comparator<Student>//自定义比较器,先按名字排序
{
public int compare(Student s1,Student s2)
{
int num =s1.getName().compareTo(s2.getName());
if (num ==0)
{
return new Integer (s1.getAge()).compareTo(new Integer(s2.getAge()));
}
return num;
}
}
class Test
{
public static void main(String[] args)
{
TreeMap<Student,String> tm =new TreeMap<Student,String>();
tm.put(new Student("zhangsan",11),"bj");
tm.put(new Student("lisi",31),"nj");
tm.put(new Student("wangwu",21),"dj");
tm.put(new Student("wangwu",21),"极乐世界");
//方式1.
Set<Student> keySet=tm.keySet();
for (Iterator<Student> it =keySet.iterator(); it.hasNext(); )
{
Student key =it.next();
String addr =tm.get(key);
sop(key+".."+addr);
}
//方式2
Set<Map.Entry<Student,String>> entry =tm.entrySet();
for (Iterator<Map.Entry<Student,String>> it =entry.iterator();it.hasNext() ; )
{
Map.Entry<Student,String> e =it.next();
//Student key =it.next().getKey();
//String value =it.next().getValue();
Student key =e.getKey();
String value =e.getValue();
sop(key+"----++++-------"+value);
}
}
public static void sop(Object obj)
{
System.out.println(obj);
}
}
class Student implements Comparable<Student>
{
private String name;
private String address;
private int age;
public int compareTo(Student s)
{
int num =new Integer(this.age).compareTo(new Integer(s.age));
if (num ==0)
{
return this.name.compareTo(s.name);
}
return num;
}
public int hashCode()
{
return name.hashCode()+age*11;
}
public boolean equals(Object obj)
{
if(!(obj instanceof Student))
throw new ClassCastException("不是同一类");
Student s =(Student)obj;
return this.name.equals(s.name) && this.age==s.age;
}
Student(String name,int age)
{
this.name=name;
this.age=age;
}
public String getName()
{
return name;
}
public int getAge()
{
return age;
}
public String toString()
{
return name+"---->"+age;
}
}

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_Water · _Water

import java.util.*;
/*
需求：对字符串出现的次数进行统计：a[2]b[3]:
TreeMap:可以对输出的字符进行排序
*/
class Test
{
public static void main(String[] args)
{
sop(charCount("dsafsadkfljsdfasd"));
sop(avg());
}
public static void sop(Object obj)
{
System.out.println(obj);
}
public static String charCount(String str)
{
char[] chs =str.toCharArray();
TreeMap<Character,Integer> tm =new TreeMap<Character,Integer>();
for (int x =0;x<chs.length ;x++ )
{
if (!(chs[x]>='a' && chs[x]<='z' || chs[x]>='A' && chs[x]<='Z'))
continue;
Integer value =tm.get(chs[x]);
if (!(value==null))
{
value =value +1;
tm.put(chs[x],value);
}
else{
tm.put(chs[x],1);
}
}
StringBuilder sb =new StringBuilder();
Set<Map.Entry<Character,Integer>> entry=tm.entrySet();
for (Iterator<Map.Entry<Character,Integer>> it =entry.iterator();it.hasNext() ; )
{
Map.Entry<Character,Integer> me =it.next();
Character key =me.getKey();
Integer value =me.getValue();
//sop(key+"---+++++----------"+value);
sb.append(key+"["+value+"]\n");
}
return sb.toString();
}
}

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Java中遍历Map的两种方法：keySet和entrySet

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