Java--HashMap--创建/排序/遍历/重写equels和hashCode

╰+哭是因爲堅強的太久メ 2022-09-17 03:14 98阅读 0赞

原文网址：[Java--HashMap--创建/排序/遍历/重写equels和hashCode\_IT利刃出鞘的博客-CSDN博客][Java--HashMap--_equels_hashCode_IT_-CSDN]

# 简介 #

本文用实例介绍HashMap的操作，包括：方法大全、创建、排序（按照key排序和按照value排序）、按插入顺序存放、遍历方法及其性能、重写equels和hashCode。

# 方法大全 #

<table> 
 <tbody> 
 <tr> 
 <td> 返回类型 </td> 
 <td> 方法和描述 </td> 
 <td>示例</td> 
 </tr> 
 <tr> 
 <td> void </td> 
 <td> <a title="clear" rel="nofollow">clear</a>() 从该Map中删除所有的映射（可选操作）。 </td> 
 <td></td> 
 </tr> 
 <tr> 
 <td> default&nbsp;<a href="https://www.matools.com/file/manual/jdk_api_1.8_google/java/util/Map.html" title="V" rel="nofollow">V</a> </td> 
 <td> <a title="compute" rel="nofollow">compute</a>(<a href="https://www.matools.com/file/manual/jdk_api_1.8_google/java/util/Map.html" title="K" rel="nofollow">K</a>&nbsp;key,&nbsp;<a href="https://www.matools.com/file/manual/jdk_api_1.8_google/java/util/function/BiFunction.html" title="BiFunction" rel="nofollow">BiFunction</a>&lt;? super&nbsp;<a href="https://www.matools.com/file/manual/jdk_api_1.8_google/java/util/Map.html" title="K" rel="nofollow">K</a>,? super&nbsp;<a href="https://www.matools.com/file/manual/jdk_api_1.8_google/java/util/Map.html" title="V" rel="nofollow">V</a>,? extends&nbsp;<a href="https://www.matools.com/file/manual/jdk_api_1.8_google/java/util/Map.html" title="V" rel="nofollow">V</a>&gt;&nbsp;remappingFunction) 通过指定键来进行处理。 若我们返回null,则在map中移除对应的key-value对。 </td> 
 <td> Map&lt;String, Integer&gt; map = new HashMap&lt;&gt;(); map.put("aa", 1); map.put("bb", 1); Integer compute = map.compute("cc", (k, v) -&gt; (v == null) ? 0 : 2 * v); System.out.println(compute);&nbsp; //0 System.out.println(map);&nbsp; &nbsp; //{cc=0, aa=1, bb=1} </td> 
 </tr> 
 <tr> 
 <td> default&nbsp;<a href="https://www.matools.com/file/manual/jdk_api_1.8_google/java/util/Map.html" title="V" rel="nofollow">V</a> </td> 
 <td> <a title="computeIfAbsent" rel="nofollow">computeIfAbsent</a>(<a href="https://www.matools.com/file/manual/jdk_api_1.8_google/java/util/Map.html" title="K" rel="nofollow">K</a>&nbsp;key,&nbsp;<a href="https://www.matools.com/file/manual/jdk_api_1.8_google/java/util/function/Function.html" title="Function" rel="nofollow">Function</a>&lt;? super&nbsp;<a href="https://www.matools.com/file/manual/jdk_api_1.8_google/java/util/Map.html" title="K" rel="nofollow">K</a>,? extends&nbsp;<a href="https://www.matools.com/file/manual/jdk_api_1.8_google/java/util/Map.html" title="V" rel="nofollow">V</a>&gt;&nbsp;mappingFunction) 如果指定的键尚未与值相关联（或映射到&nbsp;null&nbsp;），则尝试使用给定的映射函数计算其值，并将其加入到此map中。 若我们返回null,则在map中移除对应的key-value对。 </td> 
 <td> Map&lt;String, Integer&gt; map = new HashMap&lt;&gt;(); map.put("aa", 1); map.put("bb", 1); Integer computeIfAbsent = map.computeIfAbsent("cc", k -&gt; { &nbsp;&nbsp; &nbsp;return 0; }); System.out.println(computeIfAbsent);&nbsp; //0 System.out.println(map);&nbsp; &nbsp;//{cc=0, aa=1, bb=1} </td> 
 </tr> 
 <tr> 
 <td> default&nbsp;<a href="https://www.matools.com/file/manual/jdk_api_1.8_google/java/util/Map.html" title="V" rel="nofollow">V</a> </td> 
 <td> <a title="computeIfPresent" rel="nofollow">computeIfPresent</a>(<a href="https://www.matools.com/file/manual/jdk_api_1.8_google/java/util/Map.html" title="K" rel="nofollow">K</a>&nbsp;key,&nbsp;<a href="https://www.matools.com/file/manual/jdk_api_1.8_google/java/util/function/BiFunction.html" title="BiFunction" rel="nofollow">BiFunction</a>&lt;? super&nbsp;<a href="https://www.matools.com/file/manual/jdk_api_1.8_google/java/util/Map.html" title="K" rel="nofollow">K</a>,? super&nbsp;<a href="https://www.matools.com/file/manual/jdk_api_1.8_google/java/util/Map.html" title="V" rel="nofollow">V</a>,? extends&nbsp;<a href="https://www.matools.com/file/manual/jdk_api_1.8_google/java/util/Map.html" title="V" rel="nofollow">V</a>&gt;&nbsp;remappingFunction) 如果指定的密钥的值存在且非空，则尝试计算给定密钥及其当前映射值的新映射。 若我们返回null,则在map中移除对应的key-value对。 </td> 
 <td> Map&lt;String, Integer&gt; map = new HashMap&lt;&gt;(); map.put("aa", 1); map.put("bb", 1); Integer computeIfPresent1 = map.computeIfPresent("bb", (k, v) -&gt; { &nbsp;&nbsp; &nbsp;return v * 2; }); System.out.println(computeIfPresent1);&nbsp; //2 System.out.println(map);&nbsp; //{aa=1, bb=2} </td> 
 </tr> 
 <tr> 
 <td> boolean </td> 
 <td> <a title="containsKey" rel="nofollow">containsKey</a>(<a href="https://www.matools.com/file/manual/jdk_api_1.8_google/java/lang/Object.html" title="Object" rel="nofollow">Object</a>&nbsp;key) 如果此映射包含指定键的映射，则返回&nbsp;true&nbsp;。 </td> 
 <td></td> 
 </tr> 
 <tr> 
 <td> boolean </td> 
 <td> <a title="containsValue" rel="nofollow">containsValue</a>(<a href="https://www.matools.com/file/manual/jdk_api_1.8_google/java/lang/Object.html" title="Object" rel="nofollow">Object</a>&nbsp;value) 如果此地图将一个或多个键映射到指定的值，则返回&nbsp;true&nbsp;。 </td> 
 <td></td> 
 </tr> 
 <tr> 
 <td> <a href="https://www.matools.com/file/manual/jdk_api_1.8_google/java/util/Set.html" title="Set" rel="nofollow">Set</a>&lt;<a href="https://www.matools.com/file/manual/jdk_api_1.8_google/java/util/Map.Entry.html" title="Map." rel="nofollow">Map.</a> <a href="https://www.matools.com/file/manual/jdk_api_1.8_google/java/util/Map.Entry.html" title="Entry" rel="nofollow">Entry</a>&lt;<a href="https://www.matools.com/file/manual/jdk_api_1.8_google/java/util/Map.html" title="K" rel="nofollow">K</a>,<a href="https://www.matools.com/file/manual/jdk_api_1.8_google/java/util/Map.html" title="V" rel="nofollow">V</a>&gt;&gt; </td> 
 <td> <a title="entrySet" rel="nofollow">entrySet</a>() 返回此地图中包含的映射的<a href="https://www.matools.com/file/manual/jdk_api_1.8_google/java/util/Set.html" title="Set" rel="nofollow">Set</a>视图。 </td> 
 <td></td> 
 </tr> 
 <tr> 
 <td> boolean </td> 
 <td> <a title="equals" rel="nofollow">equals</a>(<a href="https://www.matools.com/file/manual/jdk_api_1.8_google/java/lang/Object.html" title="Object" rel="nofollow">Object</a>&nbsp;o) 将指定的对象与此映射进行比较以获得相等性。 </td> 
 <td></td> 
 </tr> 
 <tr> 
 <td> default void </td> 
 <td> <a title="forEach" rel="nofollow">forEach</a>(<a href="https://www.matools.com/file/manual/jdk_api_1.8_google/java/util/function/BiConsumer.html" title="BiConsumer" rel="nofollow">BiConsumer</a>&lt;? super&nbsp;<a href="https://www.matools.com/file/manual/jdk_api_1.8_google/java/util/Map.html" title="K" rel="nofollow">K</a>,? super&nbsp;<a href="https://www.matools.com/file/manual/jdk_api_1.8_google/java/util/Map.html" title="V" rel="nofollow">V</a>&gt;&nbsp;action) 对此映射中的每个条目执行给定的操作，直到所有条目都被处理或操作引发异常。 </td> 
 <td></td> 
 </tr> 
 <tr> 
 <td> <a href="https://www.matools.com/file/manual/jdk_api_1.8_google/java/util/Map.html" title="V" rel="nofollow">V</a> </td> 
 <td> <a title="get" rel="nofollow">get</a>(<a href="https://www.matools.com/file/manual/jdk_api_1.8_google/java/lang/Object.html" title="Object" rel="nofollow">Object</a>&nbsp;key) 如果有该key：返回到指定键所映射的值。如果没有该key，返回null； </td> 
 <td></td> 
 </tr> 
 <tr> 
 <td> default&nbsp;<a href="https://www.matools.com/file/manual/jdk_api_1.8_google/java/util/Map.html" title="V" rel="nofollow">V</a> </td> 
 <td> <a title="getOrDefault" rel="nofollow">getOrDefault</a>(<a href="https://www.matools.com/file/manual/jdk_api_1.8_google/java/lang/Object.html" title="Object" rel="nofollow">Object</a>&nbsp;key,&nbsp;<a href="https://www.matools.com/file/manual/jdk_api_1.8_google/java/util/Map.html" title="V" rel="nofollow">V</a>&nbsp;defaultValue) 如果有该key：返回到指定键所映射的值。如果没有该key，返回defaultValue； </td> 
 <td> Map&lt;String, Integer&gt; map = new HashMap&lt;&gt;(); map.put("aa", 1); map.put("bb", 1); Integer getOrDefault = map.getOrDefault("cc", 0); System.out.println(getOrDefault);&nbsp; &nbsp;//0 System.out.println(map);&nbsp; &nbsp; //{aa=1, bb=1} </td> 
 </tr> 
 <tr> 
 <td> int </td> 
 <td> <a title="hashCode" rel="nofollow">hashCode</a>() 返回此地图的哈希码值。 </td> 
 <td></td> 
 </tr> 
 <tr> 
 <td> boolean </td> 
 <td> <a title="isEmpty" rel="nofollow">isEmpty</a>() 如果此地图不包含键值映射，则返回&nbsp;true&nbsp;。 </td> 
 <td></td> 
 </tr> 
 <tr> 
 <td> <a href="https://www.matools.com/file/manual/jdk_api_1.8_google/java/util/Set.html" title="Set" rel="nofollow">Set</a>&lt;<a href="https://www.matools.com/file/manual/jdk_api_1.8_google/java/util/Map.html" title="K" rel="nofollow">K</a>&gt; </td> 
 <td> <a title="keySet" rel="nofollow">keySet</a>() 返回此地图中包含的键的<a href="https://www.matools.com/file/manual/jdk_api_1.8_google/java/util/Set.html" title="Set" rel="nofollow">Set</a>视图。 </td> 
 <td></td> 
 </tr> 
 <tr> 
 <td> default&nbsp;<a href="https://www.matools.com/file/manual/jdk_api_1.8_google/java/util/Map.html" title="V" rel="nofollow">V</a> </td> 
 <td> <a title="merge" rel="nofollow">merge</a>(<a href="https://www.matools.com/file/manual/jdk_api_1.8_google/java/util/Map.html" title="K" rel="nofollow">K</a>&nbsp;key,&nbsp;<a href="https://www.matools.com/file/manual/jdk_api_1.8_google/java/util/Map.html" title="V" rel="nofollow">V</a>&nbsp;value,&nbsp;<a href="https://www.matools.com/file/manual/jdk_api_1.8_google/java/util/function/BiFunction.html" title="BiFunction" rel="nofollow">BiFunction</a>&lt;? super&nbsp;<a href="https://www.matools.com/file/manual/jdk_api_1.8_google/java/util/Map.html" title="V" rel="nofollow">V</a>,? super&nbsp;<a href="https://www.matools.com/file/manual/jdk_api_1.8_google/java/util/Map.html" title="V" rel="nofollow">V</a>,? extends&nbsp;<a href="https://www.matools.com/file/manual/jdk_api_1.8_google/java/util/Map.html" title="V" rel="nofollow">V</a>&gt;&nbsp;remappingFunction) 如果指定的键尚未与值相关联或与null相关联，则将其与给定的非空值相关联。 </td> 
 <td></td> 
 </tr> 
 <tr> 
 <td> <a href="https://www.matools.com/file/manual/jdk_api_1.8_google/java/util/Map.html" title="V" rel="nofollow">V</a> </td> 
 <td> <a title="put" rel="nofollow">put</a>(<a href="https://www.matools.com/file/manual/jdk_api_1.8_google/java/util/Map.html" title="K" rel="nofollow">K</a>&nbsp;key,&nbsp;<a href="https://www.matools.com/file/manual/jdk_api_1.8_google/java/util/Map.html" title="V" rel="nofollow">V</a>&nbsp;value) 将指定的值与该映射中的指定键相关联（可选操作）。 若key已经有value，则覆盖。 </td> 
 <td></td> 
 </tr> 
 <tr> 
 <td> void </td> 
 <td> <a title="putAll" rel="nofollow">putAll</a>(<a href="https://www.matools.com/file/manual/jdk_api_1.8_google/java/util/Map.html" title="Map" rel="nofollow">Map</a>&lt;? extends&nbsp;<a href="https://www.matools.com/file/manual/jdk_api_1.8_google/java/util/Map.html" title="K" rel="nofollow">K</a>,? extends&nbsp;<a href="https://www.matools.com/file/manual/jdk_api_1.8_google/java/util/Map.html" title="V" rel="nofollow">V</a>&gt;&nbsp;m) 将指定map的所有映射复制到此映射（可选操作）。 </td> 
 <td></td> 
 </tr> 
 <tr> 
 <td> default&nbsp;<a href="https://www.matools.com/file/manual/jdk_api_1.8_google/java/util/Map.html" title="V" rel="nofollow">V</a> </td> 
 <td> <a title="putIfAbsent" rel="nofollow">putIfAbsent</a>(<a href="https://www.matools.com/file/manual/jdk_api_1.8_google/java/util/Map.html" title="K" rel="nofollow">K</a>&nbsp;key,&nbsp;<a href="https://www.matools.com/file/manual/jdk_api_1.8_google/java/util/Map.html" title="V" rel="nofollow">V</a>&nbsp;value) 如果指定的键尚未与某个值相关联（或映射到&nbsp;null&nbsp;）将其与给定值相关联并返回&nbsp;null&nbsp;，否则返回当前值。 </td> 
 <td></td> 
 </tr> 
 <tr> 
 <td> <a href="https://www.matools.com/file/manual/jdk_api_1.8_google/java/util/Map.html" title="V" rel="nofollow">V</a> </td> 
 <td> <a title="remove" rel="nofollow">remove</a>(<a href="https://www.matools.com/file/manual/jdk_api_1.8_google/java/lang/Object.html" title="Object" rel="nofollow">Object</a>&nbsp;key) 如果存在（从可选的操作），从该地图中删除一个键的映射。 </td> 
 <td></td> 
 </tr> 
 <tr> 
 <td> default boolean </td> 
 <td> <a title="remove" rel="nofollow">remove</a>(<a href="https://www.matools.com/file/manual/jdk_api_1.8_google/java/lang/Object.html" title="Object" rel="nofollow">Object</a>&nbsp;key,&nbsp;<a href="https://www.matools.com/file/manual/jdk_api_1.8_google/java/lang/Object.html" title="Object" rel="nofollow">Object</a>&nbsp;value) 仅当指定的密钥当前映射到指定的值时删除该条目。 </td> 
 <td></td> 
 </tr> 
 <tr> 
 <td> default&nbsp;<a href="https://www.matools.com/file/manual/jdk_api_1.8_google/java/util/Map.html" title="V" rel="nofollow">V</a> </td> 
 <td> <a title="replace" rel="nofollow">replace</a>(<a href="https://www.matools.com/file/manual/jdk_api_1.8_google/java/util/Map.html" title="K" rel="nofollow">K</a>&nbsp;key,&nbsp;<a href="https://www.matools.com/file/manual/jdk_api_1.8_google/java/util/Map.html" title="V" rel="nofollow">V</a>&nbsp;value) 只有当目标映射到某个值时，才能替换指定键的条目。 </td> 
 <td></td> 
 </tr> 
 <tr> 
 <td> default boolean </td> 
 <td> <a title="replace" rel="nofollow">replace</a>(<a href="https://www.matools.com/file/manual/jdk_api_1.8_google/java/util/Map.html" title="K" rel="nofollow">K</a>&nbsp;key,&nbsp;<a href="https://www.matools.com/file/manual/jdk_api_1.8_google/java/util/Map.html" title="V" rel="nofollow">V</a>&nbsp;oldValue,&nbsp;<a href="https://www.matools.com/file/manual/jdk_api_1.8_google/java/util/Map.html" title="V" rel="nofollow">V</a>&nbsp;newValue) 仅当当前映射到指定的值时，才能替换指定键的条目。 </td> 
 <td></td> 
 </tr> 
 <tr> 
 <td> default void </td> 
 <td> <a title="replaceAll" rel="nofollow">replaceAll</a>(<a href="https://www.matools.com/file/manual/jdk_api_1.8_google/java/util/function/BiFunction.html" title="BiFunction" rel="nofollow">BiFunction</a>&lt;? super&nbsp;<a href="https://www.matools.com/file/manual/jdk_api_1.8_google/java/util/Map.html" title="K" rel="nofollow">K</a>,? super&nbsp;<a href="https://www.matools.com/file/manual/jdk_api_1.8_google/java/util/Map.html" title="V" rel="nofollow">V</a>,? extends&nbsp;<a href="https://www.matools.com/file/manual/jdk_api_1.8_google/java/util/Map.html" title="V" rel="nofollow">V</a>&gt;&nbsp;function) 将每个条目的值替换为对该条目调用给定函数的结果，直到所有条目都被处理或该函数抛出异常。 </td> 
 <td></td> 
 </tr> 
 <tr> 
 <td> int </td> 
 <td> <a title="size" rel="nofollow">size</a>() 返回此地图中键值映射的数量。 </td> 
 <td></td> 
 </tr> 
 <tr> 
 <td> <a href="https://www.matools.com/file/manual/jdk_api_1.8_google/java/util/Collection.html" title="Collection" rel="nofollow">Collection</a>&lt;<a href="https://www.matools.com/file/manual/jdk_api_1.8_google/java/util/Map.html" title="V" rel="nofollow">V</a>&gt; </td> 
 <td> <a title="values" rel="nofollow">values</a>() 返回此地图中包含的值的<a href="https://www.matools.com/file/manual/jdk_api_1.8_google/java/util/Collection.html" title="Collection" rel="nofollow">Collection</a>视图。 </td> 
 <td></td> 
 </tr> 
 </tbody> 
</table>

**Function<T, R> //**只能接受一个参数

java/util/function/Function.java

package org.example.a;
 
 import java.util.function.Function;
 
 public class Demo {
 public static void main(String[] args) {
 Function<Integer, Integer> fun1= arg -> arg * arg;
 Integer apply = fun1.apply(10);
 System.out.println(apply); // 100
 }
 }

运行结果

100

**BiFunction<T, U, R> //可接受两个参数**

java/util/function/BiFunction.java

package org.example.a;
 
 import java.util.function.BiFunction;
 
 public class Demo {
 public static void main(String[] args) {
 BiFunction<Integer, Integer, Integer> fun2 = (arg1, arg2) -> arg1 + arg2;
 Integer sum = fun2.apply(10, 20);
 System.out.println(sum); // 30
 }
 }

运行结果

# 创建 #

**法1：匿名内部类**

HashMap<String, String> map = new HashMap<String, String>() {
 {
         put("name", "test");  
         put("age", "20"); 
    }};

缺点

1.内存泄露隐患

非静态内部类/ 匿名内部类包含了外围实例的引用， 如果拥有比 外部类更长的生命周期，有内存泄露隐患。

2.如果这个对象要串行化，可能会导致串行化失败。

1.  此种方式是匿名内部类的声明方式，所以引用中持有着外部类的引用。所以当时串行化这个集合时外部类也会被不知不觉的串行化，当外部类没有实现serialize接口时，就会报错。
2.  上例中，其实是声明了一个继承自HashMap的子类。然而有些串行化方法，例如要通过Gson串行化为json，或者要串行化为xml时，类库中提供的方式，是无法串行化Hashset或者HashMap的子类的，从而导致串行化失败。

解决办法：重新初始化为一个HashMap对象：

HashMap<String, String> map = new HashMap<String, String>() {
 {
         put("name", "test");  
         put("age", "20"); 
    }};
 HashMap<String, String> newMap = new HashMap(map);

这样就可以正常初始化了。

**法2：静态方法**

public class Demo{
 private static final Map<String, String> myMap;
 static {
 myMap = new HashMap<String, String>();
 myMap.put("a", "b");
 myMap.put("c", "d");
 }
 }

**法3：第三方包Guava**

Map<String, Integer> left = ImmutableMap.of("a", 1, "b", 2, "c", 3);
 //或者
 Map<String, String> test = ImmutableMap.<String, String>builder()
 .put("k1", "v1")
 .put("k2", "v2")
 ...
 .build();

# 排序已有数据 #

## 按key排序 ##

**使用stream进行排序（按key升序/降序）**

package org.example.a;
 
 import java.util.*;
 
 public class Demo {
 public static void main(String[] args) {
 Map<String, String> map = new HashMap<>();
 map.put("ad", "dd");
 map.put("bc", "ee");
 map.put("cb", "ff");
 for (Map.Entry<String, String> entry : map.entrySet()) {
 System.out.println(entry.getKey() + ":" + entry.getValue());
 }
 System.out.println();
 
 Map<String, String> linkedHashMap = new LinkedHashMap<>();
 // 默认按照升序排列
 map.entrySet().stream().sorted(Map.Entry.comparingByKey())
 .forEach(o -> linkedHashMap.put(o.getKey(), o.getValue()));
 for (Map.Entry<String, String> entry : linkedHashMap.entrySet()) {
 System.out.println(entry.getKey() + ":" + entry.getValue());
 }
 System.out.println();
 
 Map<String, String> linkedHashMap1 = new LinkedHashMap<>();
 // 自定义排序（降序）
 map.entrySet().stream().sorted(Map.Entry.comparingByKey(new Comparator<String>() {
 @Override
 public int compare(String o1, String o2) {
 return o2.compareTo(o1);
 }
 })).forEach(o -> linkedHashMap1.put(o.getKey(), o.getValue()));
 for (Map.Entry<String, String> entry : linkedHashMap1.entrySet()) {
 System.out.println(entry.getKey() + ":" + entry.getValue());
 }
 }
 }

执行结果

bc:ee
    ad:dd
    cb:ff
    
    ad:dd
    bc:ee
    cb:ff
    
    cb:ff
    bc:ee
    ad:dd

**HashMap转TreeMap自定义排序（按key升序/降序）**

package org.example.a;
 
 import java.util.Comparator;
 import java.util.HashMap;
 import java.util.Map;
 import java.util.TreeMap;
 
 public class Demo {
 public static void main(String[] args) {
 Map<String, String> map = new HashMap<>();
 map.put("ad", "dd");
 map.put("bc", "ee");
 map.put("cb", "ff");
 for (Map.Entry<String, String> entry : map.entrySet()) {
 System.out.println(entry.getKey() + ":" + entry.getValue());
 }
 System.out.println();
 // 默认按照升序排序
 Map<String, String> map1 = new TreeMap<>();
 map.forEach(map1::put);
 for (Map.Entry<String, String> entry : map1.entrySet()) {
 System.out.println(entry.getKey() + ":" + entry.getValue());
 }
 System.out.println();
 
 // 自定义排序（降序）
 Map<String, String> map2 = new TreeMap<>(new Comparator<String>() {
 @Override
 public int compare(String o1, String o2) {
 return o2.compareTo(o1);
 }
 });
 map.forEach(map2::put);
 for (Map.Entry<String, String> entry : map2.entrySet()) {
 System.out.println(entry.getKey() + ":" + entry.getValue());
 }
 }
 }

执行结果

bc:ee
    ad:dd
    cb:ff
    
    ad:dd
    bc:ee
    cb:ff
    
    cb:ff
    bc:ee
    ad:dd

## 按value排序 ##

**使用stream进行排序（按value升序/降序）**

package org.example.a;
 
 import java.util.*;
 
 public class Demo {
 public static void main(String[] args) {
 Map<String, String> map = new HashMap<>();
 map.put("ad", "dd");
 map.put("bc", "ee");
 map.put("cb", "ff");
 for (Map.Entry<String, String> entry : map.entrySet()) {
 System.out.println(entry.getKey() + ":" + entry.getValue());
 }
 System.out.println();
 
 Map<String, String> linkedHashMap = new LinkedHashMap<>();
 // 默认按照升序排列
 map.entrySet().stream().sorted(Map.Entry.comparingByValue())
 .forEach(o -> linkedHashMap.put(o.getKey(), o.getValue()));
 for (Map.Entry<String, String> entry : linkedHashMap.entrySet()) {
 System.out.println(entry.getKey() + ":" + entry.getValue());
 }
 System.out.println();
 
 Map<String, String> linkedHashMap1 = new LinkedHashMap<>();
 // 自定义排序（降序）
 map.entrySet().stream().sorted(Map.Entry.comparingByValue(new Comparator<String>() {
 @Override
 public int compare(String o1, String o2) {
 return o2.compareTo(o1);
 }
 })).forEach(o -> linkedHashMap1.put(o.getKey(), o.getValue()));
 for (Map.Entry<String, String> entry : linkedHashMap1.entrySet()) {
 System.out.println(entry.getKey() + ":" + entry.getValue());
 }
 }
 }

执行结果

bc:ee
    ad:dd
    cb:ff
    
    ad:dd
    bc:ee
    cb:ff
    
    cb:ff
    bc:ee
    ad:dd

**借助List进行排序（按value升序/降序）**

原理：将待排序Map中的所有元素置于一个列表中，接着使用Collections的一个静态方法 sort(List<T> list, Comparator<? super T> c) 来排序列表，同样是用比较器定义比较规则。排序后的列表中的元素再依次装入Map，为了肯定的保证Map中元素与排序后的List中的元素的顺序一致，使用了LinkedHashMap数据类型。

本处只写升序代码，降序只是调换个顺序而已。

package org.example.a;
 
 import java.util.*;
 
 public class Demo {
 public static void main(String[] args) {
 Map<String, String> map = new HashMap<>();
 map.put("ad", "dd");
 map.put("bc", "ee");
 map.put("cb", "ff");
 for (Map.Entry<String, String> entry : map.entrySet()) {
 System.out.println(entry.getKey() + ":" + entry.getValue());
 }
 System.out.println();
 
 Map<String, String> sortedMap = new LinkedHashMap<>();
 List<Map.Entry<String, String>> entryList = new ArrayList<Map.Entry<String, String>>(
 map.entrySet());
 Collections.sort(entryList, new Comparator<Map.Entry<String, String>>() {
 @Override
 public int compare(Map.Entry<String, String> me1, Map.Entry<String, String> me2) {
 return me1.getValue().compareTo(me2.getValue());
 }
 });
 for (Map.Entry<String, String> stringStringEntry : entryList) {
 sortedMap.put(stringStringEntry.getKey(), stringStringEntry.getValue());
 }
 
 for (Map.Entry<String, String> entry : sortedMap.entrySet()) {
 System.out.println(entry.getKey() + ":" + entry.getValue());
 }
 }
 }

运行结果

bc:ee
    ad:dd
    cb:ff
    
    ad:dd
    bc:ee
    cb:ff

# **按插入顺序存放** #

使用HashMap

package org.example.a;
 
 import java.util.*;
 
 public class Demo{
 public static List arrayList = new ArrayList();
 public static void main(String[] args) {
 Map<String, String> hashMap = new HashMap<String, String>();
 hashMap.put("name1", "josan1");
 hashMap.put("name2", "josan2");
 hashMap.put("name3", "josan3");
 
 Set<Map.Entry<String, String>> set = hashMap.entrySet();
 Iterator<Map.Entry<String, String>> iterator = set.iterator();
 while(iterator.hasNext()) {
 Map.Entry entry = iterator.next();
 String key = (String) entry.getKey();
 String value = (String) entry.getValue();
 System.out.println("key:" + key + ", value:" + value);
 }
 }
 }

执行结果（未按照插入顺序输出）

key:name3, value:josan3  
key:name2, value:josan2  
key:name1, value:josan1

使用LinkedHashMap

package org.example.a;
 
 import java.util.*;
 
 public class Demo{
 public static List arrayList = new ArrayList();
 public static void main(String[] args) {
 Map<String, String> hashMap = new LinkedHashMap<String, String>();
 hashMap.put("name1", "josan1");
 hashMap.put("name2", "josan2");
 hashMap.put("name3", "josan3");
 
 Set<Map.Entry<String, String>> set = hashMap.entrySet();
 Iterator<Map.Entry<String, String>> iterator = set.iterator();
 while(iterator.hasNext()) {
 Map.Entry entry = iterator.next();
 String key = (String) entry.getKey();
 String value = (String) entry.getValue();
 System.out.println("key:" + key + ", value:" + value);
 }
 }
 }

执行结果（按照插入顺序输出）

key:name1, value:josan1  
key:name2, value:josan2  
key:name3, value:josan3

# **遍历方法及其性能** #

**遍历方法**

<table> 
 <tbody> 
 <tr> 
 <td> 方法 </td> 
 <td> 说明/示例 </td> 
 </tr> 
 <tr> 
 <td> for each map.entrySet() </td> 
 <td> Map&lt;String, String&gt; map = new HashMap&lt;String, String&gt;(); for (Entry&lt;String, String&gt; entry : map.entrySet()) { &nbsp;&nbsp;entry.getKey(); &nbsp;&nbsp;entry.getValue(); } </td> 
 </tr> 
 <tr> 
 <td> 调用map.entrySet()的集合迭代器 </td> 
 <td> Iterator&lt;Map.Entry&lt;String, String&gt;&gt; iterator = map.entrySet().iterator(); while (iterator.hasNext()) { &nbsp;&nbsp;entry.getKey(); &nbsp;&nbsp;entry.getValue(); } </td> 
 </tr> 
 <tr> 
 <td> for each map.keySet()，再调用get获取 </td> 
 <td> Map&lt;String, String&gt; map = new HashMap&lt;String, String&gt;(); for (String key : map.keySet()) { &nbsp;&nbsp;map.get(key); } </td> 
 </tr> 
 </tbody> 
</table>

**遍历方法对比**

三种遍历方式的性能测试及对比

测试环境：Windows7 32位系统 3.2G双核CPU 4G内存，Java 7，Eclipse -Xms512m -Xmx512m

测试结果：

<table> 
 <tbody> 
 <tr> 
 <td>map size</td> 
 <td>10,000</td> 
 <td>100,000</td> 
 <td>1,000,000</td> 
 <td>2,000,000</td> 
 </tr> 
 <tr> 
 <td>for each entrySet</td> 
 <td>2ms</td> 
 <td>6ms</td> 
 <td>36ms</td> 
 <td>91ms</td> 
 </tr> 
 <tr> 
 <td>for iterator entrySet</td> 
 <td>0ms</td> 
 <td>4ms</td> 
 <td>35ms</td> 
 <td>89ms</td> 
 </tr> 
 <tr> 
 <td>for each keySet</td> 
 <td>1ms</td> 
 <td>6ms</td> 
 <td>48ms</td> 
 <td>126ms</td> 
 </tr> 
 </tbody> 
</table>

遍历方式结果分析（由上表可知）：

*  for each entrySet与for iterator entrySet性能等价
 *  for each keySet由于要再调用get（key）获取值，比较耗时（若hash散列算法较差，会更加耗时）
 *  在循环过程中若要对map进行删除操作，只能用for iterator entrySet（在HahsMap非线程安全里介绍）。

HashMap entrySet源码

private final class EntryIterator extends HashIterator<Map.Entry<K,V>> {
 public Map.Entry<K,V> next() {
 return nextEntry();
 }
 }

HashMap keySet源码

private final class KeyIterator extends HashIterator<K> {
 public K next() {
 return nextEntry().getKey();
 }
 }

由源码可知：

keySet()与entrySet()都是返回set的迭代器。父类相同，只是返回值不同，因此性能差不多。只是keySet()多了一步根据key get value的操作而已。get的时间复杂度取决于for循环的次数，即hash算法。

public V get(Object key) {
 if (key == null)
 return getForNullKey();
 Entry<K,V> entry = getEntry(key);
 return null == entry ? null : entry.getValue();
 }
 /**
 1. Returns the entry associated with the specified key in the
 2. HashMap. Returns null if the HashMap contains no mapping
 3. for the key.
 */
 final Entry<K,V> getEntry(Object key) {
 int hash = (key == null) ? 0 : hash(key);
 for (Entry<K,V> e = table[indexFor(hash, table.length)];
 e != null;
 e = e.next) {
 Object k;
 if (e.hash == hash &&
 ((k = e.key) == key || (key != null && key.equals(k))))
 return e;
 }
 return null;
 }

**使用场景总结**

<table> 
 <tbody> 
 <tr> 
 <td> 方法 </td> 
 <td> 使用场景 </td> 
 </tr> 
 <tr> 
 <td> for each map.entrySet() </td> 
 <td> 循环中需要key、value，但不对map进行删除操作 </td> 
 </tr> 
 <tr> 
 <td> 调用map.entrySet()的集合迭代器 </td> 
 <td> 循环中需要key、value，且要对map进行删除操作 </td> 
 </tr> 
 <tr> 
 <td> for each map.keySet() </td> 
 <td> 循环中只需要key </td> 
 </tr> 
 </tbody> 
</table>

# 重写equals和hashCode #

**其他网址**

[如何重写hashCode()和equals()方法\_王鸿飞的专栏-CSDN博客][hashCode_equals_-CSDN]  
[如何编写出高质量的 equals 和 hashcode 方法？ - 掘金][equals _ hashcode _ -]

# **其他网址** #

[Java HashMap 初始化赋值 (不建议)\_AlbenXie的博客-CSDN博客\_hashmap 初始化][Java HashMap _ _AlbenXie_-CSDN_hashmap]  
[JAVA构造MAP并初始化MAP\_dujianxiong的博客-CSDN博客\_java map初始化][JAVA_MAP_MAP_dujianxiong_-CSDN_java map]

[java8 map根据key排序和根据value排序\_weixin\_40841731的博客-CSDN博客\_map根据key排序][java8 map_key_value_weixin_40841731_-CSDN_map_key]  
[Java Map 按Key排序和按Value排序\_Ricky-CSDN博客\_java map key 排序][Java Map _Key_Value_Ricky-CSDN_java map key]

[Java HashMap三种循环遍历方式及其性能对比实例分析\_java\_脚本之家][Java HashMap_java]  
[Java迭代器（转）(iterator详解以及和for循环的区别) - redcoatjk - 博客园][Java_iterator_for_ - redcoatjk -]

[Java--HashMap--_equels_hashCode_IT_-CSDN]: https://knife.blog.csdn.net/article/details/120958444
[hashCode_equals_-CSDN]: https://blog.csdn.net/neosmith/article/details/17068365
[equals _ hashcode _ -]: https://juejin.im/post/6844903954006933517
[Java HashMap _ _AlbenXie_-CSDN_hashmap]: https://blog.csdn.net/AlbenXie/article/details/109318659
[JAVA_MAP_MAP_dujianxiong_-CSDN_java map]: https://blog.csdn.net/dujianxiong/article/details/54849079
[java8 map_key_value_weixin_40841731_-CSDN_map_key]: https://blog.csdn.net/weixin_40841731/article/details/89490560
[Java Map _Key_Value_Ricky-CSDN_java map key]: https://blog.csdn.net/top_code/article/details/8528954
[Java HashMap_java]: https://www.jb51.net/article/173079.htm
[Java_iterator_for_ - redcoatjk -]: https://www.cnblogs.com/redcoatjk/articles/4863340.html