java中Map的常用方法总结

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java中Map的常用方法总结

1、如何把一个Map转化为List

日常开发中，我们经常遇到这种场景，把一个Map转化为List。map转List有以下三种转化方式：

• 把map的键key转化为list

• 把map的值value转化为list

• 把map的键值key-value转化为list

示例代码：

//语法
/**
key 转list
List keyList = new ArrayList(map.keySet());value转list
List valueList = new ArrayList(map.values());//map转list
List entryList = new ArrayList(map.entrySet());
*/public class Test {public static void main(String[] args) {Map<Integer,String> userMap=new HashMap<>();userMap.put(1, "刘备");userMap.put(2, "诸葛亮");userMap.put(3, "关羽");userMap.put(4, "张飞");userMap.put(5, "赵云");userMap.put(6, "黄忠");userMap.put(7, "马超");//把一个map的键转化成listList<Integer>keyList=new ArrayList<>(userMap.keySet());System.out.println(keyList);//把一个map的值转化成listList<String> valueList = new ArrayList<>(userMap.values());System.out.println(valueList);//把map的键值转化成listList entryList=new ArrayList(userMap.entrySet());System.out.println(entryList);}
}
------------------------------------------------------------------------
// 结果：
[1, 2, 3, 4, 5, 6, 7]
[刘备, 诸葛亮, 关羽, 张飞, 赵云, 黄忠, 马超]
[1=刘备, 2=诸葛亮, 3=关羽, 4=张飞, 5=赵云, 6=黄忠, 7=马超]  

2、如何遍历一个Map

我们经常需要遍历一个map，可以有以下两种方式实现：

通过entrySet+for实现遍历

public class Test {public static void main(String[] args) {Map<Integer,String> userMap=new HashMap<>();userMap.put(1, "刘备");userMap.put(2, "诸葛亮");userMap.put(3, "关羽");userMap.put(4, "张飞");userMap.put(5, "赵云");userMap.put(6, "黄忠");userMap.put(7, "马超");//把一个map的键转化成listList<Integer>keyList=new ArrayList<>(userMap.keySet());//   System.out.println(keyList);//把一个map的值转化成listList<String> valueList = new ArrayList<>(userMap.values());// System.out.println(valueList);//把map的键值转化成listList entryList=new ArrayList(userMap.entrySet());//  System.out.println(entryList);//        for (Object o : entryList) {
//         String[] str=String.valueOf(o).split("=");
//            System.out.println(str[0]);
//            System.out.println(str[1]);
//        }for(Map.Entry entry:userMap.entrySet()){System.out.println(entry);}}
}
-----------------------------------------------
结果：
1=刘备
2=诸葛亮
3=关羽
4=张飞
5=赵云
6=黄忠
7=马超

通过Iterator+while实现遍历

public class Test {public static void main(String[] args) {Map<Integer,String> userMap=new HashMap<>();userMap.put(1, "刘备");userMap.put(2, "诸葛亮");userMap.put(3, "关羽");userMap.put(4, "张飞");userMap.put(5, "赵云");userMap.put(6, "黄忠");userMap.put(7, "马超");Iterator iterator=userMap.entrySet().iterator();while (iterator.hasNext()){Map.Entry entry=(Map.Entry)iterator.next();//get keyInteger key=(Integer) entry.getKey();//get valueString value=(String) entry.getValue();System.out.println("key:"+key+",value:"+value);}}
}
------------------------------------------------------------//结果：
key:1,value:刘备
key:2,value:诸葛亮
key:3,value:关羽
key:4,value:张飞
key:5,value:赵云
key:6,value:黄忠
key:7,value:马超  

3、如何根据Map的keys进行排序

对Map的keys进行排序，在日常开发很常见，主要有以下两种方式实现。

把Map.Entry放进list，再用Comparator对list进行排序

public class Test {public static void main(String[] args) {Map<String,String> userMap=new HashMap<>();userMap.put("104", "诸葛亮");userMap.put("106", "关羽");userMap.put("105", "张飞");userMap.put("107", "赵云");userMap.put("103", "黄忠");userMap.put("102", "马超");userMap.put("101", "刘备");List<Map.Entry<String,String>> list=new ArrayList<>(userMap.entrySet());Collections.sort(list, Comparatorparing((Map.Entry e) -> e.getKey().toString()));for(Map.Entry entry:list){System.out.println("key:"+entry.getKey()+",value:"+entry.getValue());}}
}
------------------------------------------------//结果：
key:101,value:刘备
key:102,value:马超
key:103,value:黄忠
key:104,value:诸葛亮
key:105,value:张飞
key:106,value:关羽
key:107,value:赵云

使用SortedMap+TreeMap+Comparator实现

public class Test {public static void main(String[] args) {Map<String,String> userMap=new HashMap<>();userMap.put("104", "诸葛亮");userMap.put("106", "关羽");userMap.put("105", "张飞");userMap.put("107", "赵云");userMap.put("103", "黄忠");userMap.put("102", "马超");userMap.put("101", "刘备");SortedMap sortedMap=new TreeMap(new Comparator<String>(){@Overridepublic int compare(String k1, String k2) {return k1pareTo(k2);}});sortedMap.putAll(userMap);Iterator  itr=sortedMap.entrySet().iterator();while (itr.hasNext()){Map.Entry entry=(Map.Entry) itr.next();//get keyString key=(String) entry.getKey();//get valueString value=(String) entry.getValue();System.out.println("key:"+key+",value:"+value);}}
}
----------------------------------------------//结果：
key:101,value:刘备
key:102,value:马超
key:103,value:黄忠
key:104,value:诸葛亮
key:105,value:张飞
key:106,value:关羽
key:107,value:赵云

4、如何使用Map中的value进行排序

public class SortValuesMapTest {public static void main(String[] args) {Map<String, String> map = new HashMap<>();map.put("2010", "jay");map.put("1999", "whx");map.put("3010", "huaxiao");List <Map.Entry<String,String>>list = new ArrayList<>(map.entrySet());Collections.sort(list, (Map.Entry e1, Map.Entry e2)-> {return e1.getValue().toString()pareTo(e2.getValue().toString());});for (Map.Entry entry : list) {System.out.println("key:" + entry.getKey() + ",value:" + entry.getValue());}}
}

5、如何初始化一个静态/不可变的Map

//初始化一个静态不可变的map，单单static final+static代码块还是不行的，如下：
public class Test1 {private static final Map <Integer,String>map;static { map = new HashMap<Integer, String>() map.put(1, "one");map.put(2, "two");}public static void main(String[] args) {map.put(3, "three");Iterator itr = map.entrySet().iterator();while(itr.hasNext()) {Map.Entry entry = (Map.Entry) itr.next(); // get keyInteger key = (Integer) entry.getKey(); // get valueString value = (String) entry.getValue();System.out.println("key:"+key+",value:"+value);}}
}
---------------------------------------------
//这里面，map继续添加元素(3,"three")，发现是OK的，运行结果如下：
key:1,value:one
key:2,value:two
key:3,value:three-----------------------------------------------------------------------
//真正实现一个静态不可变的map，需要Collections.unmodifiableMap，代码如下：
public class Test2 {private static final Map<Integer, String> map;static {Map<Integer,String> aMap = new HashMap<>();aMap.put(1, "one");aMap.put(2, "two");map = Collections.unmodifiableMap(aMap);}public static void main(String[] args) { map.put(3, "3");Iterator itr = map.entrySet().iterator();while(itr.hasNext()) { Map.Entry entry = (Map.Entry) itr.next();// get key Integer key = (Integer) entry.getKey();// get valueString value = (String) entry.getValue(); System.out.println("key:"+key+",value:"+value);}}
}----------------------------------------------------
//真正实现一个静态不可变的map，需要Collections.unmodifiableMap，代码如下：
public class Test {private  static final Map<Integer,String> map; //凡是静态变量都得初始化static {Map<Integer,String>aMap=new HashMap<>();aMap.put(1,"one");aMap.put(2,"two");aMap.put(3,"three");aMap.put(4,"four");map = Collections.unmodifiableMap(aMap);}public static void main(String[] args) {map.put(5,"five");}Iterator itr=map.entrySet().iterator();while(itr. hasNext()){Map.Entry entry = (Map.Entry) itr.next();// get keyInteger key = (Integer) entry.getKey();// get valueString value = (String) entry.getValue();System.out.println("key:"+key+",value:"+value);}
}
------------------------------------------------
//结果：
Erro报错

7、如何创建一个空map

如果map是不可变的，可以这样创建：

Map map=Collections.emptyMap()；
or
Map<String,String> map=Collections.<String, String>emptyMap(); 
//map1.put("1", "1"); 运行出错
---------------------------------
//如果你希望你的空map可以添加元素的，可以这样创建
Map map = new HashMap();

8、有关于map的复制

有关于hashmap的复制，在日常开发中，使用也比较多。主要有 =，clone，putAll，但是他们都是浅复制，使用的时候注意啦，可以看一下以下例子：

例子一，使用=复制一个map：

public class CopyMapAssignTest { public static void main(String[] args) { Map<Integer, User> userMap = new HashMap<>();userMap.put(1, new User("jay", 26));userMap.put(2, new User("fany", 25));//Shallow cloneMap<Integer, User> clonedMap = userMap;//Same as userMap System.out.println(clonedMap);System.out.println("\nChanges reflect in both maps \n");//Change a value is clonedMapclonedMap.get(1).setName("test");//Verify content of both mapsSystem.out.println(userMap);System.out.println(clonedMap);}}

运行结果：

{1=User{name='jay', age=26}, 2=User{name='fany', age=25}}
Changes reflect in both maps
{1=User{name='test', age=26}, 2=User{name='fany', age=25}}
{1=User{name='test', age=26}, 2=User{name='fany', age=25}}

从运行结果看出，对cloneMap修改，两个map都改变了，所以=是浅复制。

例子二，使用hashmap的clone复制：

public class CopyCloneMapTest { public static void main(String[] args) {HashMap<Integer, User> userMap = new HashMap<>();userMap.put(1, new User("jay", 26));userMap.put(2, new User("fany", 25));//Shallow cloneHashMap<Integer, User> clonedMap = (HashMap<Integer, User>) userMap.clone();//Same as userMap System.out.println(clonedMap); System.out.println("\nChanges reflect in both maps \n");//Change a value is clonedMapclonedMap.get(1).setName("test");//Verify content of both mapsSystem.out.println(userMap);System.out.println(clonedMap);}
} 

运行结果：

{1=User{name='jay', age=26}, 2=User{name='fany', age=25}}Changes reflect in both maps{1=User{name='test', age=26}, 2=User{name='fany', age=25}}{1=User{name='test', age=26}, 2=User{name='fany', age=25}}

从运行结果看出，对cloneMap修改，两个map都改变了，所以hashmap的clone也是浅复制。

例子三，通过putAll操作

public class CopyPutAllMapTest {public static void main(String[] args) {HashMap<Integer, User> userMap = new HashMap<>();userMap.put(1, new User("jay", 26));userMap.put(2, new User("fany", 25));//Shallow cloneHashMap<Integer, User> clonedMap = new HashMap<>();clonedMap.putAll(userMap);//Same as userMapSystem.out.println(clonedMap);System.out.println("\nChanges reflect in both maps \n");//Change a value is clonedMapclonedMap.get(1).setName("test");//Verify content of both mapsSystem.out.println(userMap);System.out.println(clonedMap);}
}

运行结果：

{1=User{name='jay', age=26}, 2=User{name='fany', age=25}}
Changes reflect in both maps
{1=User{name='test', age=26}, 2=User{name='fany', age=25}}{1=User{name='test', age=26}, 2=User{name='fany', age=25}}

从运行结果看出，对cloneMap修改，两个map都改变了，所以putAll还是浅复制。

那么，如何实现深度复制呢？

可以使用序列化实现，如下为谷歌Gson序列化HashMap，实现深度复制的例子：

public class CopyDeepMapTest {public static void main(String[] args) {HashMap<Integer, User> userMap = new HashMap<>();userMap.put(1, new User("jay", 26));userMap.put(2, new User("fany", 25));//Shallow cloneGson gson = new Gson();String jsonString = gson.toJson(userMap);Type type = new TypeToken<HashMap<Integer, User>>(){}.getType();HashMap<Integer, User> clonedMap = gson.fromJson(jsonString, type);//Same as userMapSystem.out.println(clonedMap);System.out.println("\nChanges reflect in only one map \n");//Change a value is clonedMapclonedMap.get(1).setName("test");//Verify content of both mapsSystem.out.println(userMap);System.out.println(clonedMap);}
}

运行结果：

{1=User{name='jay', age=26}, 2=User{name='fany', age=25}}Changes reflect in only one map{1=User{name='jay', age=26}, 2=User{name='fany', age=25}}{1=User{name='test', age=26}, 2=User{name='fany', age=25}}

从运行结果看出，对cloneMap修改，userMap没有被改变，所以是深度复制。

本文标签： java中Map的常用方法总结