有关于Java Map，应该掌握的问题
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内容图文
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：
[plain] view plaincopy
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：
[plain] view plaincopy
[
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
[plain] view plaincopy
for
(
Entry
entry
:
map
.
entrySet
())
{
// get key
K key
entry
.
getKey
();
// get value
V value
entry
.
getValue
();
}
实例代码：
[plain] view plaincopy
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
[plain] view plaincopy
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：
[plain] view plaincopy
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：
[plain] view plaincopy
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
[plain] view plaincopy
List
list
new
ArrayList
(
map
.
entrySet
());
Collections
.
sort
(
list
,
(
Entry
e1
,
Entry
e2
)->
{
return
e1
.
getKey
().
compareTo
(
e2
.
getKey
());
});
实例代码：
[plain] view plaincopy
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
[plain] view plaincopy
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：
[plain] view plaincopy
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：
[plain] view plaincopy
key
:
1999
,
value
:
whx
key
:
2010
,
value
:
jay
key
:
3010
,
value
:
huaxiao
4、如何对Map的values进行排序
[plain] view plaincopy
List
list
new
ArrayList
(
map
.
entrySet
());
Collections
.
sort
(
list
,
(
Entry
e1
,
Entry
e2
)
->{
return
e1
.
getValue
().
compareTo
(
e2
.
getValue
());
});
实例代码：
[plain] view plaincopy
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：
[plain] view plaincopy
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：
[plain] view plaincopy
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：
[plain] view plaincopy
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：
[plain] view plaincopy
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：
[plain] view plaincopy
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
[plain] view plaincopy
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：
[plain] view plaincopy
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：
[plain] view plaincopy
{
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：
[plain] view plaincopy
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：
[plain] view plaincopy
{
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
[plain] view plaincopy
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：
[plain] view plaincopy
{
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
内容总结
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