有关于Java Map，应该掌握的8个问题

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最近几天看了几篇有关于Java Map的外国博文，写得非常不错，所以整理了Java map 应该掌握的8个问题，都是日常开发司空见惯的问题，希望对大家有帮助；如果有不正确的地方，欢迎提出，万分感谢哈~

本章节所有代码demo已上传github

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

  伪代码如下：

// key list
List
 keyList 
=

new

ArrayList
(
map
.
keySet
());
// value list
List
 valueList 
=

new

ArrayList
(
map
.
values
());
// key-value list
List
 entryList 
=

new

ArrayList
(
map
.
entrySet
());

示例代码：

public

class

Test

{

public

static

void
 main
(
String
[]
 args
)

{

Map
<
Integer
,

String
>
 map 
=

new

HashMap
<>();
        map
.
put
(
2
,

"jay"
);
        map
.
put
(
1
,

"whx"
);
        map
.
put
(
3
,

"huaxiao"
);

//把一个map的键转化为list

List
<
Integer
>
 keyList 
=

new

ArrayList
<>(
map
.
keySet
());

System
.
out
.
println
(
keyList
);

//把map的值转化为list

List
<
String
>
 valueList 
=

new

ArrayList
<>(
map
.
values
());

System
.
out
.
println
(
valueList
);

把
map
的键值转化为
list

List
 entryList 
=

new

ArrayList
(
map
.
entrySet
());

System
.
out
.
println
(
entryList
);

}
}

运行结果：

[
1
,

2
,

3
]
[
whx
,
 jay
,
 huaxiao
]
[
1
=
whx
,

2
=
jay
,

3
=
huaxiao
]

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

for
(
Entry
 entry
:
 map
.
entrySet
())

{

// get key
  K key 
=
 entry
.
getKey
();

// get value
  V value 
=
 entry
.
getValue
();
}

实例代码：

public

class

EntryMapTest

{

public

static

void
 main
(
String
[]
 args
)

{

Map
<
Integer
,

String
>
 map 
=

new

HashMap
<>();
        map
.
put
(
2
,

"jay"
);
        map
.
put
(
1
,

"whx"
);
        map
.
put
(
3
,

"huaxiao"
);

for
(
Map
.
Entry
 entry
:
 map
.
entrySet
())

{

// get key

Integer
 key 
=

(
Integer
)
 entry
.
getKey
();

// get value

String
 value 
=

(
String
)
 entry
.
getValue
();

System
.
out
.
println
(
"key:"
+
key
+
",value:"
+
value
);

}

}
}

通过Iterator+while实现遍历

Iterator
 itr 
=
 map
.
entrySet
().
iterator
();
while
(
itr
.
hasNext
())

{

Entry
 entry 
=
 itr
.
next
();

// get key
  K key 
=
 entry
.
getKey
();

// get value
  V value 
=
 entry
.
getValue
();
}

实例代码：

public

class

IteratorMapTest

{

public

static

void
 main
(
String
[]
 args
)

{

Map
<
Integer
,

String
>
 map 
=

new

HashMap
<>();
        map
.
put
(
2
,

"jay"
);
        map
.
put
(
1
,

"whx"
);
        map
.
put
(
3
,

"huaxiao"
);

Iterator
 itr 
=
 map
.
entrySet
().
iterator
();

while
(
itr
.
hasNext
())

{

Map
.
Entry
 entry 
=

(
Map
.
Entry
)
 itr
.
next
();

// get key

Integer
 key 
=

(
Integer
)
 entry
.
getKey
();

// get value

String
 value 
=

(
String
)
 entry
.
getValue
();

System
.
out
.
println
(
"key:"
+
key
+
",value:"
+
value
);

}

}
}

运行结果：

key
:
1
,
value
:
whx
key
:
2
,
value
:
jay
key
:
3
,
value
:
huaxiao

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

List
 list 
=

new

ArrayList
(
map
.
entrySet
());
Collections
.
sort
(
list
,

(
Entry
 e1
,

Entry
 e2
)->

{

return
 e1
.
getKey
().
compareTo
(
e2
.
getKey
());
});

实例代码：

public

class

SortKeysMapTest

{

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
.
getKey
().
toString
().
compareTo
(
e2
.
getKey
().
toString
());

});

for

(
Map
.
Entry
 entry 
:
 list
)

{

System
.
out
.
println
(
"key:"

+
 entry
.
getKey
()

+

",value:"

+
 entry
.
getValue
());

}

}
}

使用SortedMap+TreeMap+Comparator实现

SortedMap
 sortedMap 
=

new

TreeMap
(
new

Comparator
()

{

@Override

public

int
 compare
(
K k1
,
 K k2
)

{

return
 k1
.
compareTo
(
k2
);

}
});
sortedMap
.
putAll
(
map
);

实例代码：

public

class

SortKeys2MapTest

{

public

static

void
 main
(
String
[]
 args
)

{

Map
<
String
,

String
>
 map 
=

new

HashMap
<>();
        map
.
put
(
"2010"
,

"jay"
);
        map
.
put
(
"1999"
,

"whx"
);
        map
.
put
(
"3010"
,

"huaxiao"
);

SortedMap
 sortedMap 
=

new

TreeMap
(
new

Comparator
<
String
>()

{

@Override

public

int
 compare
(
String
 k1
,

String
 k2
)

{

return
 k1
.
compareTo
(
k2
);

}

});
        sortedMap
.
putAll
(
map
);

Iterator
 itr 
=
 sortedMap
.
entrySet
().
iterator
();

while
(
itr
.
hasNext
())

{

Map
.
Entry
 entry 
=

(
Map
.
Entry
)
 itr
.
next
();

// get key

String
 key 
=

(
String
)
 entry
.
getKey
();

// get value

String
 value 
=

(
String
)
 entry
.
getValue
();

System
.
out
.
println
(
"key:"
+
key
+
",value:"
+
value
);

}

}
}

运行结果：

key
:
1999
,
value
:
whx
key
:
2010
,
value
:
jay
key
:
3010
,
value
:
huaxiao

4、如何对Map的values进行排序

List
 list 
=

new

ArrayList
(
map
.
entrySet
());
Collections
.
sort
(
list
,

(
Entry
 e1
,

Entry
 e2
)

->{

return
 e1
.
getValue
().
compareTo
(
e2
.
getValue
());

});

实例代码：

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
().
compareTo
(
e2
.
getValue
().
toString
());

}

);

for

(
Map
.
Entry
 entry 
:
 list
)

{

System
.
out
.
println
(
"key:"

+
 entry
.
getKey
()

+

",value:"

+
 entry
.
getValue
());

}

}
}

运行结果：

key
:
3010
,
value
:
huaxiao
key
:
2010
,
value
:
jay
key
:
1999
,
value
:
whx

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 key

Integer
 key 
=

(
Integer
)
 entry
.
getKey
();

// get value

String
 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 value

String
 value 
=

(
String
)
 entry
.
getValue
();

System
.
out
.
println
(
"key:"
+
key
+
",value:"
+
value
);

}

}

}

运行结果如下：

可以发现，继续往map添加元素是会报错的，实现真正不可变的map。

6、HashMap, TreeMap, and Hashtable，ConcurrentHashMap的区别

HashMap TreeMap Hashtable ConcurrentHashMap
有序性 否 是 否 否
null k-v 是-是 否-是 否-否 否-否
线性安全 否 否 是 是
时间复杂度 O（1） O（log n） O(1) O（log n）
底层结构 数组+链表 红黑树 数组+链表 红黑树

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 clone

Map
<
Integer
,

User
>
 clonedMap 
=
 userMap
;

//Same as userMap

System
.
out
.
println
(
clonedMap
);

System
.
out
.
println
(
"\nChanges reflect in both maps \n"
);

//Change a value is clonedMap
        clonedMap
.
get
(
1
).
setName
(
"test"
);

//Verify content of both maps

System
.
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 clone

HashMap
<
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 clonedMap
        clonedMap
.
get
(
1
).
setName
(
"test"
);

//Verify content of both maps

System
.
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 clone

HashMap
<
Integer
,

User
>
 clonedMap 
=

new

HashMap
<>();
        clonedMap
.
putAll
(
userMap
);

//Same as userMap

System
.
out
.
println
(
clonedMap
);

System
.
out
.
println
(
"\nChanges reflect in both maps \n"
);

//Change a value is clonedMap
        clonedMap
.
get
(
1
).
setName
(
"test"
);

//Verify content of both maps

System
.
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 clone

Gson
 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 userMap

System
.
out
.
println
(
clonedMap
);

System
.
out
.
println
(
"\nChanges reflect in only one map \n"
);

//Change a value is clonedMap
        clonedMap
.
get
(
1
).
setName
(
"test"
);

//Verify content of both maps

System
.
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。

参考与感谢

• Top 9 questions about Java Maps
• Best way to create an empty map in Java
• How to clone HashMap – Shallow and Deep Copy

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有关于Java Map，应该掌握的8个问题

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原文地址：https://blog.51cto.com/14989534/2547470