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：