JAVA的map

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Map的用法

类型介绍

  • HashMap

    最常用的Map,它根据键的HashCode值存储数据，根据键可以直接获取它的值，具有很快的访问速度。HashMap最多只允许一条记录的键为null（多条会覆盖）；允许多条记录的值为null。非同步的。

  • TreeMap

    能够把它保存的记录根据键（key）排序，默认是按升序排序，也可指定排序的比较器，当用Iterator遍历TreeMap时，得到的记录是排过序的。TreeMap不允许key的值为null。非同步的。

  • Hashtable

    与HashMap类似，不同的是：key和value的值均不允许为null;它支持线程同步，即任一时刻只有一个线程能写HashTable,因此也导致了HashTable在写入时会比较慢。

  • LinkedHashMap

    保存了记录的插入顺序，在使用Iterator遍历LinkedHashMap时，先得到的记录肯定是先插入的，在遍历的时候会比HashMap慢。key和value均允许为空。非同步的。

Map用法

用法
Map<String,String> map = new HashMap<String,String>();

插入元素

map.put("key1","value1");

获取元素

map.get("key1");

移除元素

map.remove("key1")

四种常用Map插入与读取性能比较

测试代码

package net.xsoftlab.baike;
import java.util.HashMap;
import java.util.Hashtable;
import java.util.LinkedHashMap;
import java.util.Map;
import java.util.Random;
import java.util.TreeMap;
import java.util.UUID;
public class Test {
    static int hashMapW = 0;
    static int hashMapR = 0;
    static int linkMapW = 0;
    static int linkMapR = 0;
    static int treeMapW = 0;
    static int treeMapR = 0;
    static int hashTableW = 0;
    static int hashTableR = 0;
    public static void main(String[] args) {
        for (int i = 0; i < 10; i++) {
            Test test = new Test();
            test.test(100 * 10000);
            System.out.println();
        }
        System.out.println("hashMapW = " + hashMapW / 10);
        System.out.println("hashMapR = " + hashMapR / 10);
        System.out.println("linkMapW = " + linkMapW / 10);
        System.out.println("linkMapR = " + linkMapR / 10);
        System.out.println("treeMapW = " + treeMapW / 10);
        System.out.println("treeMapR = " + treeMapR / 10);
        System.out.println("hashTableW = " + hashTableW / 10);
        System.out.println("hashTableR = " + hashTableR / 10);
    }
    public void test(int size) {
        int index;
        Random random = new Random();
        String[] key = new String[size];
        // HashMap 插入
        Map<String, String> map = new HashMap<String, String>();
        long start = System.currentTimeMillis();
        for (int i = 0; i < size; i++) {
            key[i] = UUID.randomUUID().toString();
            map.put(key[i], UUID.randomUUID().toString());
        }
        long end = System.currentTimeMillis();
        hashMapW += (end - start);
        System.out.println("HashMap插入耗时 = " + (end - start) + " ms");
        // HashMap 读取
        start = System.currentTimeMillis();
        for (int i = 0; i < size; i++) {
            index = random.nextInt(size);
            map.get(key[index]);
        }
        end = System.currentTimeMillis();
        hashMapR += (end - start);
        System.out.println("HashMap读取耗时 = " + (end - start) + " ms");
        // LinkedHashMap 插入
        map = new LinkedHashMap<String, String>();
        start = System.currentTimeMillis();
        for (int i = 0; i < size; i++) {
            key[i] = UUID.randomUUID().toString();
            map.put(key[i], UUID.randomUUID().toString());
        }
        end = System.currentTimeMillis();
        linkMapW += (end - start);
        System.out.println("LinkedHashMap插入耗时 = " + (end - start) + " ms");
        // LinkedHashMap 读取
        start = System.currentTimeMillis();
        for (int i = 0; i < size; i++) {
            index = random.nextInt(size);
            map.get(key[index]);
        }
        end = System.currentTimeMillis();
        linkMapR += (end - start);
        System.out.println("LinkedHashMap读取耗时 = " + (end - start) + " ms");
        // TreeMap 插入
        key = new String[size];
        map = new TreeMap<String, String>();
        start = System.currentTimeMillis();
        for (int i = 0; i < size; i++) {
            key[i] = UUID.randomUUID().toString();
            map.put(key[i], UUID.randomUUID().toString());
        }
        end = System.currentTimeMillis();
        treeMapW += (end - start);
        System.out.println("TreeMap插入耗时 = " + (end - start) + " ms");
        // TreeMap 读取
        start = System.currentTimeMillis();
        for (int i = 0; i < size; i++) {
            index = random.nextInt(size);
            map.get(key[index]);
        }
        end = System.currentTimeMillis();
        treeMapR += (end - start);
        System.out.println("TreeMap读取耗时 = " + (end - start) + " ms");
        // Hashtable 插入
        key = new String[size];
        map = new Hashtable<String, String>();
        start = System.currentTimeMillis();
        for (int i = 0; i < size; i++) {
            key[i] = UUID.randomUUID().toString();
            map.put(key[i], UUID.randomUUID().toString());
        }
        end = System.currentTimeMillis();
        hashTableW += (end - start);
        System.out.println("Hashtable插入耗时 = " + (end - start) + " ms");
        // Hashtable 读取
        start = System.currentTimeMillis();
        for (int i = 0; i < size; i++) {
            index = random.nextInt(size);
            map.get(key[index]);
        }
        end = System.currentTimeMillis();
        hashTableR += (end - start);
        System.out.println("Hashtable读取耗时 = " + (end - start) + " ms");
    }
}

Map遍历

初始化数据

Map<String,String> map = new HashMap<String,String>();
map.put("key1","value1");
map.put("key2","value2");

增强for循环遍历

使用keySet()遍历

for(String key:map.keySet()){
    System.out.println(key+":"+map.get(key))
}

使用entrySet()遍历

for(Map.Entry<String,String> entry:map.entrySet()){
    System.out.println(entry.getKey()+":"+entry.getValue());
}

迭代器遍历

使用keySet()遍历

Iterator<String> iterator = map.keySet().iterator();
while(iterator.hasNext()){
    String key = iterator.next();
    System.out.println(key+":"+map.get(key));
}

使用entrySet()遍历

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

HashMap四种遍历方式性能比较

package net.xsoftlab.baike;
import java.util.HashMap;
import java.util.Iterator;
import java.util.Map;
import java.util.Map.Entry;
public class TestMap {
    public static void main(String[] args) {
        // 初始化，10W次赋值
        Map<Integer, Integer> map = new HashMap<Integer, Integer>();
        for (int i = 0; i < 100000; i++)
            map.put(i, i);
        /** 增强for循环，keySet迭代 **/
        long start = System.currentTimeMillis();
        for (Integer key : map.keySet()) {
            map.get(key);
        }
        long end = System.currentTimeMillis();
        System.out.println("增强for循环，keySet迭代 -> " + (end - start) + " ms");
        /** 增强for循环，entrySet迭代 */
        start = System.currentTimeMillis();
        for (Entry<Integer, Integer> entry : map.entrySet()) {
            entry.getKey();
            entry.getValue();
        }
        end = System.currentTimeMillis();
        System.out.println("增强for循环，entrySet迭代 -> " + (end - start) + " ms");
        /** 迭代器，keySet迭代 **/
        start = System.currentTimeMillis();
        Iterator<Integer> iterator = map.keySet().iterator();
        Integer key;
        while (iterator.hasNext()) {
            key = iterator.next();
            map.get(key);
        }
        end = System.currentTimeMillis();
        System.out.println("迭代器，keySet迭代 -> " + (end - start) + " ms");
        /** 迭代器，entrySet迭代 **/
        start = System.currentTimeMillis();
        Iterator<Map.Entry<Integer, Integer>> iterator1 = map.entrySet().iterator();
        Map.Entry<Integer, Integer> entry;
        while (iterator1.hasNext()) {
            entry = iterator1.next();
            entry.getKey();
            entry.getValue();
        }
        end = System.currentTimeMillis();
        System.out.println("迭代器，entrySet迭代 -> " + (end - start) + " ms");
    }
}

运行三次，比较结果 第一次
增强for循环，keySet迭代 -> 37 ms
增强for循环，entrySet迭代 -> 19 ms
迭代器，keySet迭代 -> 14 ms
迭代器，entrySet迭代 -> 9 ms

增强for循环，keySet迭代 -> 29 ms
增强for循环，entrySet迭代 -> 22 ms
迭代器，keySet迭代 -> 19 ms
迭代器，entrySet迭代 -> 12 ms

增强for循环，keySet迭代 -> 27 ms
增强for循环，entrySet迭代 -> 19 ms
迭代器，keySet迭代 -> 18 ms
迭代器，entrySet迭代 -> 10 ms

总结：

  1. 增强for循环使用方便，但性能较差，不适合处理超大量级的数据。
  2. 迭代器的遍历速度要比增强for循环快很多，是增强for循环的2倍左右。
  3. 使用entrySet遍历的速度比keySet快很多，是keySet的1.5倍左右。

Map排序

HashMap、HashTable、LinkedHashMap排序

HashMap

Map<String,String> map = new HashMap<String,String>();
map.put("b","b");
map.put("a","c");
map.put("c","a");
//排序
List<Map.Entry<String,String>> list = new ArrayList<Map.Entry<String, String>>(map.entrySet().size());
Collections.sort(list, new Comparator<Map.Entry<String, String>>() {
    @Override
    public int compare(Map.Entry<String, String> o1, Map.Entry<String, String> o2) {
        return o1.getKey().compareTo(o2.getKey());
    }
});

for (Map.Entry<String,String> mapping:
list) {
    System.out.println(mapping.getKey()+":"+mapping.getValue());
}

TreeMap

Map<String, String> map = new TreeMap<String, String>(new Comparator<String>() {
    @Override
    public int compare(String o1, String o2) {
        // 降序排序
        return o1.compareTo(o2);
    }
});
map.put("b", "b");
map.put("a", "c");
map.put("c", "a");
for (String key : map.keySet()) {
    System.out.println(key + " ：" + map.get(key));
}

按value排序(通用)

Map<String, String> map = new TreeMap<String, String>();
map.put("b", "b");
map.put("a", "c");
map.put("c", "a");
// 通过ArrayList构造函数把map.entrySet()转换成list
List<Map.Entry<String, String>> list = new ArrayList<Map.Entry<String, String>>(map.entrySet());
// 通过比较器实现比较排序
Collections.sort(list, new Comparator<Map.Entry<String, String>>() {
    @Override
    public int compare(Map.Entry<String, String> mapping1, Map.Entry<String, String> mapping2) {
        return mapping1.getValue().compareTo(mapping2.getValue());
    }
});
for (String key : map.keySet()) {
    System.out.println(key + " ：" + map.get(key));
}

常用API

扩展List如何一边遍历一边删除

public static void main(String[] args) {
    List<String> platformList = new ArrayList<>();
    platformList.add("博客园");
    platformList.add("CSDN");
    platformList.add("掘金");

    for (String platform : platformList) {
        if (platform.equals("博客园")) {
            platformList.remove(platform);
        }
    }

    System.out.println(platformList);
}

java.util.ConcurrentModificationException异常了，翻译成中文就是：并发修改异常

使用Iterator的remove()方法

public static void main(String[] args) {
    List<String> platformList = new ArrayList<>();
    platformList.add("博客园");
    platformList.add("CSDN");
    platformList.add("掘金");

    Iterator<String> iterator = platformList.iterator();
    while (iterator.hasNext()) {
        String platform = iterator.next();
        if (platform.equals("博客园")) {
            iterator.remove();
        }
    }

    System.out.println(platformList);
}

使用for循环正序遍历

public static void main(String[] args) {
    List<String> platformList = new ArrayList<>();
    platformList.add("博客园");
    platformList.add("CSDN");
    platformList.add("掘金");

    for (int i = 0; i < platformList.size(); i++) {
        String item = platformList.get(i);

        if (item.equals("博客园")) {
            platformList.remove(i);
            i = i - 1;
        }
    }

    System.out.println(platformList);
}

使用for循环倒序遍历

public static void main(String[] args) {
    List<String> platformList = new ArrayList<>();
    platformList.add("博客园");
    platformList.add("CSDN");
    platformList.add("掘金");

    for (int i = platformList.size() - 1; i >= 0; i--) {
        String item = platformList.get(i);

        if (item.equals("掘金")) {
            platformList.remove(i);
        }
    }

    System.out.println(platformList);
}