The Map Entry understanding and application

概述

Map.Entry是Map接口的一个内部接口,用于获取Mapelement key-value pair.可以通过map.entrySet()的返回值是一个Set,其类型为Map.Entry,常用的有以下几个方法：

• getKey()：get the key of the element
• getValue()：获取元素的值
• setValue()：设置元素的值

实例

MapHow to traverse type data

package com.linfanchen.springboot.lab.map;

import org.junit.Test;
import org.junit.runner.RunWith;
import org.springframework.boot.test.context.SpringBootTest;
import org.springframework.test.context.junit4.SpringRunner;
import java.util.HashMap;
import java.util.Iterator;
import java.util.Map;

@RunWith(SpringRunner.class)
@SpringBootTest
public class MapEntryTest {
    

    @Test
    public void carPriceTest() {
    
        Map<String, Integer> carPriceMap = new HashMap<>();
        carPriceMap.put("bmw", 500000);
        carPriceMap.put("benz", 550000);
        carPriceMap.put("lexus", 440000);

        // 遍历方式一：使用Map.keySet()
        for (String key : carPriceMap.keySet()) {
    
            System.out.println("key=" + key + ", value=" + carPriceMap.get(key));
        }

        // 遍历方式二（推荐）：使用Map.entrySet()的iterator()迭代器
        Iterator<Map.Entry<String, Integer>> iterator = carPriceMap.entrySet().iterator();
        while (iterator.hasNext()) {
    
            Map.Entry<String, Integer> entry = iterator.next();
            System.out.println("key=" + entry.getKey() + ", value=" + entry.getValue());
        }

        // 遍历方式三（推荐）：entrySet()
        for (Map.Entry<String, Integer> entry : carPriceMap.entrySet()) {
    
            System.out.println("key=" + entry.getKey() + ", value=" + entry.getValue());
        }

        // 遍历方式四：使用map.values()只能获取值
        for (Integer val : carPriceMap.values()) {
    
            System.out.println("value=" + val);
        }


    }

}

遍历效率

代码

写个Case验证一下：

package com.linfanchen.springboot.lab.map;

import org.junit.Test;
import org.junit.runner.RunWith;
import org.springframework.boot.test.context.SpringBootTest;
import org.springframework.test.context.junit4.SpringRunner;

import java.util.*;

@RunWith(SpringRunner.class)
@SpringBootTest
public class MapEntryTest {
    

    @Test
    public void carPriceTest() {
    
        Map<String, Integer> carPriceMap = new HashMap<>();
        carPriceMap.put("bmw", 500000);
        carPriceMap.put("benz", 550000);
        carPriceMap.put("lexus", 440000);

        // 遍历方式一：使用Map.keySet()
        for (String key : carPriceMap.keySet()) {
    
            System.out.println("key=" + key + ", value=" + carPriceMap.get(key));
        }

        // 遍历方式二（推荐）：使用Map.entrySet()的iterator()迭代器
        Iterator<Map.Entry<String, Integer>> iterator = carPriceMap.entrySet().iterator();
        while (iterator.hasNext()) {
    
            Map.Entry<String, Integer> entry = iterator.next();
            System.out.println("key=" + entry.getKey() + ", value=" + entry.getValue());
        }

        // 遍历方式三（推荐）：entrySet()
        for (Map.Entry<String, Integer> entry : carPriceMap.entrySet()) {
    
            System.out.println("key=" + entry.getKey() + ", value=" + entry.getValue());
        }

        // 遍历方式四：使用map.values()只能获取值
        for (Integer val : carPriceMap.values()) {
    
            System.out.println("value=" + val);
        }



    }

    @Test
    public void speedTest() {
    
        {
    
            // 构造500w的数据源
            HashMap<Integer, String> hashmap = new HashMap<Integer, String>();
            for (int i = 0; i < 5000000; i++) {
    
                hashmap.put(i, "lfc");
            }

            long bs = Calendar.getInstance().getTimeInMillis();
            Iterator iterator = hashmap.keySet().iterator();
            while (iterator.hasNext()) {
    
                hashmap.get(iterator.next());
            }
            System.out.print("keyset:");
            System.out.println(Calendar.getInstance().getTimeInMillis() - bs);

        }

        {
    
            // 构造500w的数据源
            HashMap<Integer,String> hashmap = new HashMap<Integer,String>();
            for (int i = 0; i < 5000000; i++ ) {
    
                hashmap.put(i, "lfc");
            }
            long bs = Calendar.getInstance().getTimeInMillis();
            Iterator it = hashmap.entrySet().iterator();
            while (it.hasNext()) {
    
                Map.Entry entry = (Map.Entry) it.next();
                entry.getValue();
            }
            System.out.print("entryset:");
            System.out.println(Calendar.getInstance().getTimeInMillis() - bs);
        }

    }

}

输出：

keyset:120
entryset:98

从输出结果看出entrysetThe convenience and performance will be much better,那为什么呢？
Find the answer from the source code：

public V get(Object key) {
    
        if (key == null)
            return getForNullKey();
        Entry<K,V> entry = getEntry(key);
 
        return null == entry ? null : entry.getValue();
    }

• hashmap.entryset：在set集合中存放的是entry对象.而在hashmap中的key 和 value 是存放在entry对象里面的;然后用迭代器,遍历set集合,you can get everyentry对象;得到entryobjects can be accessed directly fromentry拿到value了;
• hashmap.keyset：只是把hashmap中key放到一个set集合中去,Or through the iterator to traverse,然后再通过 hashmap.get(key)方法拿到value;

源码

位于 java.util.Map：

public interface Entry<K, V> {
    
		/* * 获取键 */
        K getKey();

		/* * 获取值 */
        V getValue();

		/* * 设置值 */
        V setValue(V var1);

        boolean equals(Object var1);

        int hashCode();

		/* * Compare by key,Specific comparative logic */
        static <K extends Comparable<? super K>, V> Comparator<Map.Entry<K, V>> comparingByKey() {
    
            return (Comparator)((Serializable)((c1, c2) -> {
    
                return ((Comparable)c1.getKey()).compareTo(c2.getKey());
            }));
        }

		/* * Compare by value,Specific comparative logic */
        static <K, V extends Comparable<? super V>> Comparator<Map.Entry<K, V>> comparingByValue() {
    
            return (Comparator)((Serializable)((c1, c2) -> {
    
                return ((Comparable)c1.getValue()).compareTo(c2.getValue());
            }));
        }

		/* * Compare by key,Specific comparative logic */
        static <K, V> Comparator<Map.Entry<K, V>> comparingByKey(Comparator<? super K> cmp) {
    
            Objects.requireNonNull(cmp);
            return (Comparator)((Serializable)((c1, c2) -> {
    
                return cmp.compare(c1.getKey(), c2.getKey());
            }));
        }

		/* * Compare by value,Specific comparative logic */
        static <K, V> Comparator<Map.Entry<K, V>> comparingByValue(Comparator<? super V> cmp) {
    
            Objects.requireNonNull(cmp);
            return (Comparator)((Serializable)((c1, c2) -> {
    
                return cmp.compare(c1.getValue(), c2.getValue());
            }));
        }
    }

总结

在日常的开发中,要注意mapType traversal efficiency.可以使用entrySet()高效遍历,Save program running time.PS:Why is Apple's operating system so smooth?,It is to optimize the efficiency of each line of code as much as possible,We have to follow the same philosophy.