HashMap的四种访问方式

第一种：通过Map.entrySet使用iterator遍历key和value

public void visit_1(HashMap<String,Integer> hm){
        Iterator<Map.Entry<String,Integer>> it = hm.entrySet().iterator();
while(it.hasNext()){
Map.Entry<String ,Integer> entry = it.next();
String  key = entry.getKey();
Integer value = entry.getValue();
        }
    }

第二种：通过Key来遍历value

public void visit_2(HashMap<String,Integer> hm){
for (String key:hm.keySet()){
            Integer value = hm.get(key);
        }
    }

第三种：通过Map.Entry遍历key和value

 public void visit_3(HashMap<String,Integer> hm){
        for(Map.Entry<String,Integer> entry:hm.entrySet()){
String key = entry.getKey();
Integer value = entry.getValue();
        }
    }

第四种：通过Map.keySet使用iterator遍历key和value

public void visit_4(HashMap<Integer,String> hm){
long startTime = System.currentTimeMillis();
        Iterator<Integer> it = hm.keySet().iterator();
while(it.hasNext()){
            Integer key = it.next();
            String value = hm.get(key);
        }
        System.out.println("visit_4 10000000 entry:"
                + (System.currentTimeMillis()-startTime) + " milli seconds");
    }

四种方法比较：

package chapter08.c86.c862;

import java.util.HashMap;
import java.util.Iterator;
import java.util.Map;

/**
 * Created by ceoicac on 2017/8/12 10:22.
 */
public class HashMapVisitTest {
public void visit_1(HashMap<Integer,String> hm){
long startTime = System.currentTimeMillis();
        Iterator<Map.Entry<Integer,String>> it = hm.entrySet().iterator();
while(it.hasNext()){
            Map.Entry<Integer,String> entry = it.next();
            Integer  key = entry.getKey();
            String value = entry.getValue();
        }
        System.out.println("visit_1 10000000 entry:"
                + (System.currentTimeMillis()-startTime) + " milli seconds");
    }
public void visit_2(HashMap<Integer,String> hm){
long startTime = System.currentTimeMillis();
for (Integer key:hm.keySet()){
            String value = hm.get(key);
        }
        System.out.println("visit_1 10000000 entry:"
                + (System.currentTimeMillis()-startTime) + " milli seconds");
    }
public void visit_3(HashMap<Integer,String> hm){
long startTime = System.currentTimeMillis();
for(Map.Entry<Integer,String> entry : hm.entrySet()){
            Integer key = entry.getKey();
            String value = entry.getValue();
        }
        System.out.println("visit_1 10000000 entry:"
                + (System.currentTimeMillis()-startTime) + " milli seconds");
    }
public void visit_4(HashMap<Integer,String> hm){
long startTime = System.currentTimeMillis();
        Iterator<Integer> it = hm.keySet().iterator();
while(it.hasNext()){
            Integer key = it.next();
            String value = hm.get(key);
        }
        System.out.println("visit_1 10000000 entry:"
                + (System.currentTimeMillis()-startTime) + " milli seconds");
    }
public static void main(String [] args){
        HashMap<Integer,String> hm = new HashMap<>();
for(int i = 1;i <= 10000000;++i){
            hm.put(i,"num: " + i);
        }
new HashMapVisitTest().visit_1(hm);
new HashMapVisitTest().visit_2(hm);
new HashMapVisitTest().visit_3(hm);
new HashMapVisitTest().visit_4(hm);

    }
}

结果：