map的三种遍历方法!

时间:2022-08-10 05:27:42

map的三种遍历方法!

 

  集合的一个很重要的操作---遍历,学习了三种遍历方法,三种方法各有优缺点~~

/*
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package cn.tsp2c.liubao;

import java.util.Collection;
import java.util.HashMap;
import java.util.Iterator;
import java.util.Map;
import java.util.Set;
import java.util.TreeMap;

/**
 *
 * @author Administrator
 */
public class TestMap {

public static void main(String[] args) {
        Map<String, Student> map = new HashMap<String, Student>();
        Student s1 = new Student("宋江", "1001", 38);
        Student s2 = new Student("卢俊义", "1002", 35);
        Student s3 = new Student("吴用", "1003", 34);
        
        map.put("1001", s1);
        map.put("1002", s2);
        map.put("1003", s3);

Map<String, Student> subMap = new HashMap<String, Student>();
        subMap.put("1008", new Student("tom", "1008", 12));
        subMap.put("1009", new Student("jerry", "1009", 10));
        map.putAll(subMap);

work(map);
        workByKeySet(map);
        workByEntry(map);
    }

  //最常规的一种遍历方法,最常规就是最常用的,虽然不复杂,但很重要,这是我们最熟悉的,就不多说了!!

public static void work(Map<String, Student> map) {
        Collection<Student> c = map.values();
        Iterator it = c.iterator();
        for (; it.hasNext();) {
            System.out.println(it.next());
        }
    }

  //利用keyset进行遍历,它的优点在于可以根据你所想要的key值得到你想要的 values,更具灵活性!!

public static void workByKeySet(Map<String, Student> map) {
        Set<String> key = map.keySet();
        for (Iterator it = key.iterator(); it.hasNext();) {
            String s = (String) it.next();
            System.out.println(map.get(s));
        }
    }

  //比较复杂的一种遍历在这里,呵呵~~他很暴力哦,它的灵活性太强了,想得到什么就能得到什么~~

public static void workByEntry(Map<String, Student> map) {
        Set<Map.Entry<String, Student>> set = map.entrySet();
        for (Iterator<Map.Entry<String, Student>> it = set.iterator(); it.hasNext();) {
            Map.Entry<String, Student> entry = (Map.Entry<String, Student>) it.next();
            System.out.println(entry.getKey() + "--->" + entry.getValue());
        }
    }
}

class Student {

private String name;
    private String id;
    private int age;

public Student(String name, String id, int age) {
        this.name = name;
        this.id = id;
        this.age = age;
    }

@Override
    public String toString() {
        return "Student{" + "name=" + name + "id=" + id + "age=" + age + '}';
    }
}

 
 

bufferedReader和bufferedWriter的执行读写文件

 

  令人煎熬的周末又算熬过去了,比较轻松的周一开始了,蜗居在宿舍一上午,下午睡起来,没事做,决定看看这周学的东西,看了看io,看了看bufferedReader和bufferedWriter

  很明显bufferedreader的用法比inputstream要复杂,复杂的存在必然会导致优势的存在!我们都知道inputstream是一个字节一个字节的读取,每次读取都会执行一次IO,我们知道io的操作是很费时间的,这就必然会导致程序的效率,而bufferedreader很好的解决这一问题,它可以一次读取大量的数据,大大减少了io次数,效率也就上去了,这就像有辆能乘坐一百人的大巴,从热力输送学生到理工本部,司机脑残,学生没睡醒,非要一次只坐一个同学,大巴的来回跑一百趟才能把这一百人全部送到学校,这就类似inputstream,另一个司机是清华毕业,智商当然高了,他让这一百人全部上车,一次九ok了,虽然在学生上车时多用了点时间,但总时间要远比那个脑残司机要少的多!!!当然在计算机中不会有这么大的时间差!!哔哔了这么多,应该表述清楚了,下面是一个bufferedreader的例子,本想写个关于bufferedreader比inputstream快的例子,可能是本人人品太好了吧,运行的结果每次都是0毫秒~~~

package cn.tsp2s.liu.liubao;

import java.io.BufferedReader;
import java.io.BufferedWriter;
import java.io.FileNotFoundException;
import java.io.FileReader;
import java.io.FileWriter;
import java.io.IOException;
import java.util.logging.Level;
import java.util.logging.Logger;

/**
 *
 * @author Administrator
 */
public class TestBufferedReader {
    public static void main(String[] args){
        FileReader in=null;
        BufferedReader read=null;
        String s=null;
        BufferedWriter writer=null;
        try {
            in = new FileReader("d:\\java\\TestLeap.java");
            read=new BufferedReader(in);
            writer=new BufferedWriter(new FileWriter("d:\\java\\leap.txt"));
            while ((s = read.readLine()) != null) {
               // System.out.println(s);
                writer.write(s);
                //这里调用newline()方法是让它输出和读取的完全一致,理由不解释
                writer.newLine();
                //这里一定要调用flush()方法,如果不调用,文件中将会显示不全或者压根就不显示任何东西,理由不解释,你肯定知道
                writer.flush();     
            }
        } catch (FileNotFoundException ex) {
            System.out.println("找不到指定文件!!");
        }catch (IOException e) {
            System.out.println("文件读取有误!");
        }finally{
            try {
                writer.close();
                read.close();
            } catch (IOException ex) {
                System.out.println(ex.getMessage());
            }
        }
    }
}