遍历map的四种方法

1.获取Entity集合，使用foreach遍历

2.获取key集合，value集合，分别遍历

3.获取Entity迭代器进行遍历

4.获取key集合，通过get方法获取value

推荐使用第一种，效率高，key和value都有，第四种不推荐，效率较低。

SystemTest.java

import java.util.Iterator;
import java.util.Map;
import java.util.Properties;
import java.util.Set;

/**
 * Created by gxf on 2016/12/8.
 */
public class SystemTest {
    public static void main(String args[]){
        getEnv();
    }

    public static void getProperties(){
        Properties properties = System.getProperties();
        Iterator iterator = properties.entrySet().iterator();
        while(iterator.hasNext()){
            Map.Entry e = (Map.Entry)iterator.next();
            System.out.println(e.getKey() + "," + e.getValue());
        }
    }

    public static void getEnv(){
        Map<String, String> mapOfEnv = System.getenv();
        //first
        for(Map.Entry<String, String> entry : mapOfEnv.entrySet()){
            System.out.println(entry.getKey() + ", " + entry.getValue());
        }

        //second
        for(String key : mapOfEnv.keySet()){
            System.out.println("key = " + key);
        }
        for(String value : mapOfEnv.values()){
            System.out.println("value = " + value);
        }

        //third
        Iterator<Map.Entry<String, String>> iterator = mapOfEnv.entrySet().iterator();
        while(iterator.hasNext()){
            Map.Entry<String, String> entry = iterator.next();
            System.out.println(entry.getKey() + ", " + entry.getValue());
        }

        //fourth
        Set<String> setOfKey = mapOfEnv.keySet();
        for(String key : setOfKey){
            System.out.println(key + ", " + mapOfEnv.get(key));
        }
    }
}