关于Java Map应该掌握的问题

Posted 2020-12-21 zifei399

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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：
  [plain] view plain copy
  1. // key list
  2. List
  3. keyList
  4. =
  6. new
  8. ArrayList
  9. (
  10. map
  11. .
  12. keySet
  13. ());
  14. // value list
  15. List
  16. valueList
  17. =
  19. new
  21. ArrayList
  22. (
  23. map
  24. .
  25. values
  26. ());
  27. // key-value list
  28. List
  29. entryList
  30. =
  32. new
  34. ArrayList
  35. (
  36. map
  37. .
  38. entrySet
  39. ());
  示例代码：
  [plain] view plain copy
  1. public
  3. class
  5. Test
  7. {
  9. public
  11. static
  13. void
  14. main
  15. (
  16. String
  17. []
  18. args
  19. )
  21. {
  23. Map
  24. <
  25. Integer
  26. ,
  28. String
  29. >
  30. map
  31. =
  33. new
  35. HashMap
  36. <>();
  37. map
  38. .
  39. put
  40. (
  41. 2
  42. ,
  44. "jay"
  45. );
  46. map
  47. .
  48. put
  49. (
  50. 1
  51. ,
  53. "whx"
  54. );
  55. map
  56. .
  57. put
  58. (
  59. 3
  60. ,
  62. "huaxiao"
  63. );
  65. //把一个map的键转化为list
  67. List
  68. <
  69. Integer
  70. >
  71. keyList
  72. =
  74. new
  76. ArrayList
  77. <>(
  78. map
  79. .
  80. keySet
  81. ());
  83. System
  84. .
  85. out
  86. .
  87. println
  88. (
  89. keyList
  90. );
  92. //把map的值转化为list
  94. List
  95. <
  96. String
  97. >
  98. valueList
  99. =
  101. new
  103. ArrayList
  104. <>(
  105. map
  106. .
  107. values
  108. ());
  110. System
  111. .
  112. out
  113. .
  114. println
  115. (
  116. valueList
  117. );
  119. 把
  120. map
  121. 的键值转化为
  122. list
  124. List
  125. entryList
  126. =
  128. new
  130. ArrayList
  131. (
  132. map
  133. .
  134. entrySet
  135. ());
  137. System
  138. .
  139. out
  140. .
  141. println
  142. (
  143. entryList
  144. );
  146. }
  147. }
  运行结果：
  [plain] view plain copy
  1. [
  2. 1
  3. ,
  5. 2
  6. ,
  8. 3
  9. ]
  10. [
  11. whx
  12. ,
  13. jay
  14. ,
  15. huaxiao
  16. ]
  17. [
  18. 1
  19. =
  20. whx
  21. ,
  23. 2
  24. =
  25. jay
  26. ,
  28. 3
  29. =
  30. huaxiao
  31. ]
  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 plain copy
  1. for
  2. (
  3. Entry
  4. entry
  5. :
  6. map
  7. .
  8. entrySet
  9. ())
  11. {
  13. // get key
  14. K key
  15. =
  16. entry
  17. .
  18. getKey
  19. ();
  21. // get value
  22. V value
  23. =
  24. entry
  25. .
  26. getValue
  27. ();
  28. }
  实例代码：
  [plain] view plain copy
  1. public
  3. class
  5. EntryMapTest
  7. {
  9. public
  11. static
  13. void
  14. main
  15. (
  16. String
  17. []
  18. args
  19. )
  21. {
  23. Map
  24. <
  25. Integer
  26. ,
  28. String
  29. >
  30. map
  31. =
  33. new
  35. HashMap
  36. <>();
  37. map
  38. .
  39. put
  40. (
  41. 2
  42. ,
  44. "jay"
  45. );
  46. map
  47. .
  48. put
  49. (
  50. 1
  51. ,
  53. "whx"
  54. );
  55. map
  56. .
  57. put
  58. (
  59. 3
  60. ,
  62. "huaxiao"
  63. );
  65. for
  66. (
  67. Map
  68. .
  69. Entry
  70. entry
  71. :
  72. map
  73. .
  74. entrySet
  75. ())
  77. {
  79. // get key
  81. Integer
  82. key
  83. =
  85. (
  86. Integer
  87. )
  88. entry
  89. .
  90. getKey
  91. ();
  93. // get value
  95. String
  96. value
  97. =
  99. (
  100. String
  101. )
  102. entry
  103. .
  104. getValue
  105. ();
  107. System
  108. .
  109. out
  110. .
  111. println
  112. (
  113. "key:"
  114. +
  115. key
  116. +
  117. ",value:"
  118. +
  119. value
  120. );
  122. }
  124. }
  125. }
  通过Iterator+while实现遍历
  [plain] view plain copy
  1. Iterator
  2. itr
  3. =
  4. map
  5. .
  6. entrySet
  7. ().
  8. iterator
  9. ();
  10. while
  11. (
  12. itr
  13. .
  14. hasNext
  15. ())
  17. {
  19. Entry
  20. entry
  21. =
  22. itr
  23. .
  24. next
  25. ();
  27. // get key
  28. K key
  29. =
  30. entry
  31. .
  32. getKey
  33. ();
  35. // get value
  36. V value
  37. =
  38. entry
  39. .
  40. getValue
  41. ();
  42. }
  实例代码：
  [plain] view plain copy
  1. public
  3. class
  5. IteratorMapTest
  7. {
  9. public
  11. static
  13. void
  14. main
  15. (
  16. String
  17. []
  18. args
  19. )
  21. {
  23. Map
  24. <
  25. Integer
  26. ,
  28. String
  29. >
  30. map
  31. =
  33. new
  35. HashMap
  36. <>();
  37. map
  38. .
  39. put
  40. (
  41. 2
  42. ,
  44. "jay"
  45. );
  46. map
  47. .
  48. put
  49. (
  50. 1
  51. ,
  53. "whx"
  54. );
  55. map
  56. .
  57. put
  58. (
  59. 3
  60. ,
  62. "huaxiao"
  63. );
  65. Iterator
  66. itr
  67. =
  68. map
  69. .
  70. entrySet
  71. ().
  72. iterator
  73. ();
  75. while
  76. (
  77. itr
  78. .
  79. hasNext
  80. ())
  82. {
  84. Map
  85. .
  86. Entry
  87. entry
  88. =
  90. (
  91. Map
  92. .
  93. Entry
  94. )
  95. itr
  96. .
  97. next
  98. ();
  100. // get key
  102. Integer
  103. key
  104. =
  106. (
  107. Integer
  108. )
  109. entry
  110. .
  111. getKey
  112. ();
  114. // get value
  116. String
  117. value
  118. =
  120. (
  121. String
  122. )
  123. entry
  124. .
  125. getValue
  126. ();
  128. System
  129. .
  130. out
  131. .
  132. println
  133. (
  134. "key:"
  135. +
  136. key
  137. +
  138. ",value:"
  139. +
  140. value
  141. );
  143. }
  145. }
  146. }
  运行结果：
  [plain] view plain copy
  1. key
  2. :
  3. 1
  4. ,
  5. value
  6. :
  7. whx
  8. key
  9. :
  10. 2
  11. ,
  12. value
  13. :
  14. jay
  15. key
  16. :
  17. 3
  18. ,
  19. value
  20. :
  21. 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 plain copy
  1. List
  2. list
  3. =
  5. new
  7. ArrayList
  8. (
  9. map
  10. .
  11. entrySet
  12. ());
  13. Collections
  14. .
  15. sort
  16. (
  17. list
  18. ,
  20. (
  21. Entry
  22. e1
  23. ,
  25. Entry
  26. e2
  27. )->
  29. {
  31. return
  32. e1
  33. .
  34. getKey
  35. ().
  36. compareTo
  37. (
  38. e2
  39. .
  40. getKey
  41. ());
  42. });
  实例代码：
  [plain] view plain copy
  1. public
  3. class
  5. SortKeysMapTest
  7. {
  9. public
  11. static
  13. void
  14. main
  15. (
  16. String
  17. []
  18. args
  19. )
  21. {
  23. Map
  24. <
  25. String
  26. ,
  28. String
  29. >
  30. map
  31. =
  33. new
  35. HashMap
  36. <>();
  37. map
  38. .
  39. put
  40. (
  41. "2010"
  42. ,
  44. "jay"
  45. );
  46. map
  47. .
  48. put
  49. (
  50. "1999"
  51. ,
  53. "whx"
  54. );
  55. map
  56. .
  57. put
  58. (
  59. "3010"
  60. ,
  62. "huaxiao"
  63. );
  65. List
  66. <
  67. Map
  68. .
  69. Entry
  70. <
  71. String
  72. ,
  73. String
  74. >>
  75. list
  76. =
  78. new
  80. ArrayList
  81. <>(
  82. map
  83. .
  84. entrySet
  85. ());
  87. Collections
  88. .
  89. sort
  90. (
  91. list
  92. ,
  94. (
  95. Map
  96. .
  97. Entry
  98. e1
  99. ,
  101. Map
  102. .
  103. Entry
  104. e2
  105. )->
  107. {
  109. return
  110. e1
  111. .
  112. getKey
  113. ().
  114. toString
  115. ().
  116. compareTo
  117. (
  118. e2
  119. .
  120. getKey
  121. ().
  122. toString
  123. ());
  125. });
  127. for
  129. (
  130. Map
  131. .
  132. Entry
  133. entry
  134. :
  135. list
  136. )
  138. {
  140. System
  141. .
  142. out
  143. .
  144. println
  145. (
  146. "key:"
  148. +
  149. entry
  150. .
  151. getKey
  152. ()
  154. +
  156. ",value:"
  158. +
  159. entry
  160. .
  161. getValue
  162. ());
  164. }
  166. }
  167. }
  使用SortedMap+TreeMap+Comparator实现
  [plain] view plain copy
  1. SortedMap
  2. sortedMap
  3. =
  5. new
  7. TreeMap
  8. (
  9. new
  11. Comparator
  12. ()
  14. {
  16. @Override
  18. public
  20. int
  21. compare
  22. (
  23. K k1
  24. ,
  25. K k2
  26. )
  28. {
  30. return
  31. k1
  32. .
  33. compareTo
  34. (
  35. k2
  36. );
  38. }
  39. });
  40. sortedMap
  41. .
  42. putAll
  43. (
  44. map
  45. );
  实例代码：
  [plain] view plain copy
  1. public
  3. class
  5. SortKeys2MapTest
  7. {
  9. public
  11. static
  13. void
  14. main
  15. (
  16. String
  17. []
  18. args
  19. )
  21. {
  23. Map
  24. <
  25. String
  26. ,
  28. String
  29. >
  30. map
  31. =
  33. new
  35. HashMap
  36. <>();
  37. map
  38. .
  39. put
  40. (
  41. "2010"
  42. ,
  44. "jay"
  45. );
  46. map
  47. .
  48. put
  49. (
  50. "1999"
  51. ,
  53. "whx"
  54. );
  55. map
  56. .
  57. put
  58. (
  59. "3010"
  60. ,
  62. "huaxiao"
  63. );
  65. SortedMap
  66. sortedMap
  67. =
  69. new
  71. TreeMap
  72. (
  73. new
  75. Comparator
  76. <
  77. String
  78. >()
  80. {
  82. @Override
  84. public
  86. int
  87. compare
  88. (
  89. String
  90. k1
  91. ,
  93. String
  94. k2
  95. )
  97. {
  99. return
  100. k1
  101. .
  102. compareTo
  103. (
  104. k2
  105. );
  107. }
  109. });
  110. sortedMap
  111. .
  112. putAll
  113. (
  114. map
  115. );
  117. Iterator
  118. itr
  119. =
  120. sortedMap
  121. .
  122. entrySet
  123. ().
  124. iterator
  125. ();
  127. while
  128. (
  129. itr
  130. .
  131. hasNext
  132. ())
  134. {
  136. Map
  137. .
  138. Entry
  139. entry
  140. =
  142. (
  143. Map
  144. .
  145. Entry
  146. )
  147. itr
  148. .
  149. next
  150. ();
  152. // get key
  154. String
  155. key
  156. =
  158. (
  159. String
  160. )
  161. entry
  162. .
  163. getKey
  164. ();
  166. // get value
  168. String
  169. value
  170. =
  172. (
  173. String
  174. )
  175. entry
  176. .
  177. getValue
  178. ();
  180. System
  181. .
  182. out
  183. .
  184. println
  185. (
  186. "key:"
  187. +
  188. key
  189. +
  190. ",value:"
  191. +
  192. value
  193. );
  195. }
  197. }
  198. }
  运行结果：
  [plain] view plain copy
  1. key
  2. :
  3. 1999
  4. ,
  5. value
  6. :
  7. whx
  8. key
  9. :
  10. 2010
  11. ,
  12. value
  13. :
  14. jay
  15. key
  16. :
  17. 3010
  18. ,
  19. value
  20. :
  21. huaxiao
  4、如何对Map的values进行排序
  [plain] view plain copy
  1. List
  2. list
  3. =
  5. new
  7. ArrayList
  8. (
  9. map
  10. .
  11. entrySet
  12. ());
  13. Collections
  14. .
  15. sort
  16. (
  17. list
  18. ,
  20. (
  21. Entry
  22. e1
  23. ,
  25. Entry
  26. e2
  27. )
  29. ->{
  31. return
  32. e1
  33. .
  34. getValue
  35. ().
  36. compareTo
  37. (
  38. e2
  39. .
  40. getValue
  41. ());
  43. });
  实例代码：
  [plain] view plain copy
  1. public
  3. class
  5. SortValuesMapTest
  7. {
  9. public
  11. static
  13. void
  14. main
  15. (
  16. String
  17. []
  18. args
  19. )
  21. {
  23. Map
  24. <
  25. String
  26. ,
  28. String
  29. >
  30. map
  31. =
  33. new
  35. HashMap
  36. <>();
  37. map
  38. .
  39. put
  40. (
  41. "2010"
  42. ,
  44. "jay"
  45. );
  46. map
  47. .
  48. put
  49. (
  50. "1999"
  51. ,
  53. "whx"
  54. );
  55. map
  56. .
  57. put
  58. (
  59. "3010"
  60. ,
  62. "huaxiao"
  63. );
  65. List
  67. <
  68. Map
  69. .
  70. Entry
  71. <
  72. String
  73. ,
  74. String
  75. >>
  76. list
  77. =
  79. new
  81. ArrayList
  82. <>(
  83. map
  84. .
  85. entrySet
  86. ());
  88. Collections
  89. .
  90. sort
  91. (
  92. list
  93. ,
  95. (
  96. Map
  97. .
  98. Entry
  99. e1
  100. ,
  102. Map
  103. .
  104. Entry
  105. e2
  106. )->
  108. {
  110. return
  111. e1
  112. .
  113. getValue
  114. ().
  115. toString
  116. ().
  117. compareTo
  118. (
  119. e2
  120. .
  121. getValue
  122. ().
  123. toString
  124. ());
  126. }
  128. );
  130. for
  132. (
  133. Map
  134. .
  135. Entry
  136. entry
  137. :
  138. list
  139. )
  141. {
  143. System
  144. .
  145. out
  146. .
  147. println
  148. (
  149. "key:"
  151. +
  152. entry
  153. .
  154. getKey
  155. ()
  157. +
  159. ",value:"
  161. +
  162. entry
  163. .
  164. getValue
  165. ());
  167. }
  169. }
  170. }
  运行结果：
  [plain] view plain copy
  1. key
  2. :
  3. 3010
  4. ,
  5. value
  6. :
  7. huaxiao
  8. key
  9. :
  10. 2010
  11. ,
  12. value
  13. :
  14. jay
  15. key
  16. :
  17. 1999
  18. ,
  19. value
  20. :
  21. 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 plain copy
  1. public
  3. class
  5. Test1
  7. {
  9. private
  11. static
  13. final
  15. Map
  17. <
  18. Integer
  19. ,
  20. String
  21. >
  22. map
  23. ;
  25. static
  27. {
  28. map
  29. =
  31. new
  33. HashMap
  34. <
  35. Integer
  36. ,
  38. String
  39. >();
  40. map
  41. .
  42. put
  43. (
  44. 1
  45. ,
  47. "one"
  48. );
  49. map
  50. .
  51. put
  52. (
  53. 2
  54. ,
  56. "two"
  57. );
  59. }
  61. public
  63. static
  65. void
  66. main
  67. (
  68. String
  69. []
  70. args
  71. )
  73. {
  74. map
  75. .
  76. put
  77. (
  78. 3
  79. ,
  81. "three"
  82. );
  84. Iterator
  85. itr
  86. =
  87. map
  88. .
  89. entrySet
  90. ().
  91. iterator
  92. ();
  94. while
  95. (
  96. itr
  97. .
  98. hasNext
  99. ())
  101. {
  103. Map
  104. .
  105. Entry
  106. entry
  107. =
  109. (
  110. Map
  111. .
  112. Entry
  113. )
  114. itr
  115. .
  116. next
  117. ();
  119. // get key
  121. Integer
  122. key
  123. =
  125. (
  126. Integer
  127. )
  128. entry
  129. .
  130. getKey
  131. ();
  133. // get value
  135. String
  136. value
  137. =
  139. (
  140. String
  141. )
  142. entry
  143. .
  144. getValue
  145. ();
  147. System
  148. .
  149. out
  150. .
  151. println
  152. (
  153. "key:"
  154. +
  155. key
  156. +
  157. ",value:"
  158. +
  159. value
  160. );
  162. }
  164. }
  165. }
  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 plain copy
  1. key
  2. :
  3. 1
  4. ,
  5. value
  6. :
  7. one
  8. key
  9. :
  10. 2
  11. ,
  12. value
  13. :
  14. two
  15. key
  16. :
  17. 3
  18. ,
  19. value
  20. :
  21. three
  真正实现一个静态不可变的map，需要Collections.unmodifiableMap，代码如下：
  [plain] view plain copy
  1. public
  3. class
  5. Test2
  7. {
  9. private
  11. static
  13. final
  15. Map
  16. <
  17. Integer
  18. ,
  20. String
  21. >
  22. map
  23. ;
  25. static
  27. {
  29. Map
  30. <
  31. Integer
  32. ,
  33. String
  34. >
  35. aMap
  36. =
  38. new
  40. HashMap
  41. <>();
  42. aMap
  43. .
  44. put
  45. (
  46. 1
  47. ,
  49. "one"
  50. );
  51. aMap
  52. .
  53. put
  54. (
  55. 2
  56. ,
  58. "two"
  59. );
  60. map
  61. =
  63. Collections
  64. .
  65. unmodifiableMap
  66. (
  67. aMap
  68. );
  70. }
  72. public
  74. static
  76. void
  77. main
  78. (
  79. String
  80. []
  81. args
  82. )
  84. {
  85. map
  86. .
  87. put
  88. (
  89. 3
  90. ,
  92. "3"
  93. );
  95. Iterator
  96. itr
  97. =
  98. map
  99. .
  100. entrySet
  101. ().
  102. iterator
  103. ();
  105. while
  106. (
  107. itr
  108. .
  109. hasNext
  110. ())
  112. {
  114. Map
  115. .
  116. Entry
  117. entry
  118. =
  120. (
  121. Map
  122. .
  123. Entry
  124. )
  125. itr
  126. .
  127. next
  128. ();
  130. // get key
  132. Integer
  133. key
  134. =
  136. (
  137. Integer
  138. )
  139. entry
  140. .
  141. getKey
  142. ();
  144. // get value
  146. String
  147. value
  148. =
  150. (
  151. String
  152. )
  153. entry
  154. .
  155. getValue
  156. ();
  158. System
  159. .
  160. out
  161. .
  162. println
  163. (
  164. "key:"
  165. +
  166. key
  167. +
  168. ",value:"
  169. +
  170. value
  171. );
  173. }
  175. }
  177. }
  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 plain copy
  1. Map
  2. map
  3. =
  4. Collections
  5. .
  6. emptyMap
  7. ()；
  8. or
  9. Map
  10. <
  11. String
  12. ,
  13. String
  14. >
  15. map
  16. =
  17. Collections
  18. .<
  19. String
  20. ,
  22. String
  23. >
  24. emptyMap
  25. ();
  26. //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 plain copy
  1. Map
  2. map
  3. =
  5. new
  7. HashMap
  8. ();
  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 plain copy
  1. public
  3. class
  5. CopyMapAssignTest
  7. {
  9. public
  11. static
  13. void
  14. main
  15. (
  16. String
  17. []
  18. args
  19. )
  21. {
  23. Map
  24. <
  25. Integer
  26. ,
  28. User
  29. >
  30. userMap
  31. =
  33. new
  35. HashMap
  36. <>();
  38. userMap
  39. .
  40. put
  41. (
  42. 1
  43. ,
  45. new
  47. User
  48. (
  49. "jay"
  50. ,
  52. 26
  53. ));
  54. userMap
  55. .
  56. put
  57. (
  58. 2
  59. ,
  61. new
  63. User
  64. (
  65. "fany"
  66. ,
  68. 25
  69. ));
  71. //Shallow clone
  73. Map
  74. <
  75. Integer
  76. ,
  78. User
  79. >
  80. clonedMap
  81. =
  82. userMap
  83. ;
  85. //Same as userMap
  87. System
  88. .
  89. out
  90. .
  91. println
  92. (
  93. clonedMap
  94. );
  96. System
  97. .
  98. out
  99. .
  100. println
  101. (
  102. " Changes reflect in both maps "
  103. );
  105. //Change a value is clonedMap
  106. clonedMap
  107. .
  108. get
  109. (
  110. 1
  111. ).
  112. setName
  113. (
  114. "test"
  115. );
  117. //Verify content of both maps
  119. System
  120. .
  121. out
  122. .
  123. println
  124. (
  125. userMap
  126. );
  128. System
  129. .
  130. out
  131. .
  132. println
  133. (
  134. clonedMap
  135. );
  137. }
  138. }
  运行结果：
  [plain] view plain copy
  1. {
  2. 1
  3. =
  4. User
  5. {
  6. name
  7. =
  8. ‘jay‘
  9. ,
  10. age
  11. =
  12. 26
  13. },
  15. 2
  16. =
  17. User
  18. {
  19. name
  20. =
  21. ‘fany‘
  22. ,
  23. age
  24. =
  25. 25
  26. }}
  28. Changes
  29. reflect
  30. in
  31. both maps
  33. {
  34. 1
  35. =
  36. User
  37. {
  38. name
  39. =
  40. ‘test‘
  41. ,
  42. age
  43. =
  44. 26
  45. },
  47. 2
  48. =
  49. User
  50. {
  51. name
  52. =
  53. ‘fany‘
  54. ,
  55. age
  56. =
  57. 25
  58. }}
  59. {
  60. 1
  61. =
  62. User
  63. {
  64. name
  65. =
  66. ‘test‘
  67. ,
  68. age
  69. =
  70. 26
  71. },
  73. 2
  74. =
  75. User
  76. {
  77. name
  78. =
  79. ‘fany‘
  80. ,
  81. age
  82. =
  83. 25
  84. }}
  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 plain copy
  1. public
  3. class
  5. CopyCloneMapTest
  7. {
  9. public
  11. static
  13. void
  14. main
  15. (
  16. String
  17. []
  18. args
  19. )
  21. {
  23. HashMap
  24. <
  25. Integer
  26. ,
  28. User
  29. >
  30. userMap
  31. =
  33. new
  35. HashMap
  36. <>();
  38. userMap
  39. .
  40. put
  41. (
  42. 1
  43. ,
  45. new
  47. User
  48. (
  49. "jay"
  50. ,
  52. 26
  53. ));
  54. userMap
  55. .
  56. put
  57. (
  58. 2
  59. ,
  61. new
  63. User
  64. (
  65. "fany"
  66. ,
  68. 25
  69. ));
  71. //Shallow clone
  73. HashMap
  74. <
  75. Integer
  76. ,
  78. User
  79. >
  80. clonedMap
  81. =
  83. (
  84. HashMap
  85. <
  86. Integer
  87. ,
  89. User
  90. >)
  91. userMap
  92. .
  93. clone
  94. ();
  96. //Same as userMap
  98. System
  99. .
  100. out
  101. .
  102. println
  103. (
  104. clonedMap
  105. );
  107. System
  108. .
  109. out
  110. .
  111. println
  112. (
  113. " Changes reflect in both maps "
  114. );
  116. //Change a value is clonedMap
  117. clonedMap
  118. .
  119. get
  120. (
  121. 1
  122. ).
  123. setName
  124. (
  125. "test"
  126. );
  128. //Verify content of both maps
  130. System
  131. .
  132. out
  133. .
  134. println
  135. (
  136. userMap
  137. );
  139. System
  140. .
  141. out
  142. .
  143. println
  144. (
  145. clonedMap
  146. );
  148. }
  149. }
  运行结果：
  [plain] view plain copy
  1. {
  2. 1
  3. =
  4. User
  5. {
  6. name
  7. =
  8. ‘jay‘
  9. ,
  10. age
  11. =
  12. 26
  13. },
  15. 2
  16. =
  17. User
  18. {
  19. name
  20. =
  21. ‘fany‘
  22. ,
  23. age
  24. =
  25. 25
  26. }}
  28. Changes
  29. reflect
  30. in
  31. both maps
  33. {
  34. 1
  35. =
  36. User
  37. {
  38. name
  39. =
  40. ‘test‘
  41. ,
  42. age
  43. =
  44. 26
  45. },
  47. 2
  48. =
  49. User
  50. {
  51. name
  52. =
  53. ‘fany‘
  54. ,
  55. age
  56. =
  57. 25
  58. }}
  59. {
  60. 1
  61. =
  62. User
  63. {
  64. name
  65. =
  66. ‘test‘
  67. ,
  68. age
  69. =
  70. 26
  71. },
  73. 2
  74. =
  75. User
  76. {
  77. name
  78. =
  79. ‘fany‘
  80. ,
  81. age
  82. =
  83. 25
  84. }}
  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 plain copy
  1. public
  3. class
  5. CopyPutAllMapTest
  7. {
  9. public
  11. static
  13. void
  14. main
  15. (
  16. String
  17. []
  18. args
  19. )
  21. {
  23. HashMap
  24. <
  25. Integer
  26. ,
  28. User
  29. >
  30. userMap
  31. =
  33. new
  35. HashMap
  36. <>();
  38. userMap
  39. .
  40. put
  41. (
  42. 1
  43. ,
  45. new
  47. User
  48. (
  49. "jay"
  50. ,
  52. 26
  53. ));
  54. userMap
  55. .
  56. put
  57. (
  58. 2
  59. ,
  61. new
  63. User
  64. (
  65. "fany"
  66. ,
  68. 25
  69. ));
  71. //Shallow clone
  73. HashMap
  74. <
  75. Integer
  76. ,
  78. User
  79. >
  80. clonedMap
  81. =
  83. new
  85. HashMap
  86. <>();
  87. clonedMap
  88. .
  89. putAll
  90. (
  91. userMap
  92. );
  94. //Same as userMap
  96. System
  97. .
  98. out
  99. .
  100. println
  101. (
  102. clonedMap
  103. );
  105. System
  106. .
  107. out
  108. .
  109. println
  110. (
  111. " Changes reflect in both maps "
  112. );
  114. //Change a value is clonedMap
  115. clonedMap
  116. .
  117. get
  118. (
  119. 1
  120. ).
  121. setName
  122. (
  123. "test"
  124. );
  126. //Verify content of both maps
  128. System
  129. .
  130. out
  131. .
  132. println
  133. (
  134. userMap
  135. );
  137. System
  138. .
  139. out
  140. .
  141. println
  142. (
  143. clonedMap
  144. );
  146. }
  147. }
  运行结果：
  [plain] view plain copy
  1. {
  2. 1
  3. =
  4. User
  5. {
  6. name
  7. =
  8. ‘jay‘
  9. ,
  10. age
  11. =
  12. 26
  13. },
  15. 2
  16. =
  17. User
  18. {
  19. name
  20. =
  21. ‘fany‘
  22. ,
  23. age
  24. =
  25. 25
  26. }}
  28. Changes
  29. reflect
  30. in
  31. both maps
  33. {
  34. 1
  35. =
  36. User
  37. {
  38. name
  39. =
  40. ‘test‘
  41. ,
  42. age
  43. =
  44. 26
  45. },
  47. 2
  48. =
  49. User
  50. {
  51. name
  52. =
  53. ‘fany‘
  54. ,
  55. age
  56. =
  57. 25
  58. }}
  59. {
  60. 1
  61. =
  62. User
  63. {
  64. name
  65. =
  66. ‘test‘
  67. ,
  68. age
  69. =
  70. 26
  71. },
  73. 2
  74. =
  75. User
  76. {
  77. name
  78. =
  79. ‘fany‘
  80. ,
  81. age
  82. =
  83. 25
  84. }}
  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：
  [plain] view plain copy
  1. public
  3. class
  5. CopyDeepMapTest
  7. {
  9. public
  11. static
  13. void
  14. main
  15. (
  16. String
  17. []
  18. args
  19. )
  21. {
  23. HashMap
  24. <
  25. Integer
  26. ,
  28. User
  29. >
  30. userMap
  31. =
  33. new
  35. HashMap
  36. <>();
  38. userMap
  39. .
  40. put
  41. (
  42. 1
  43. ,
  45. new
  47. User
  48. (
  49. "jay"
  50. ,
  52. 26
  53. ));
  54. userMap
  55. .
  56. put
  57. (
  58. 2
  59. ,
  61. new
  63. User
  64. (
  65. "fany"
  66. ,
  68. 25
  69. ));
  71. //Shallow clone
  73. Gson
  74. gson
  75. =
  77. new
  79. Gson
  80. ();
  82. String
  83. jsonString
  84. =
  85. gson
  86. .
  87. toJson
  88. (
  89. userMap
  90. );
  92. Type
  93. type
  94. =
  96. new
  98. TypeToken
  99. <
  100. HashMap
  101. <
  102. Integer
  103. ,
  105. User
  106. >>(){}.
  107. getType
  108. ();
  110. HashMap
  111. <
  112. Integer
  113. ,
  115. User
  116. >
  117. clonedMap
  118. =
  119. gson
  120. .
  121. fromJson
  122. (
  123. jsonString
  124. ,
  125. type
  126. );
  128. //Same as userMap
  130. System
  131. .
  132. out
  133. .
  134. println
  135. (
  136. clonedMap
  137. );
  139. System
  140. .
  141. out
  142. .
  143. println
  144. (
  145. " Changes reflect in only one map "
  146. );
  148. //Change a value is clonedMap
  149. clonedMap
  150. .
  151. get
  152. (
  153. 1
  154. ).
  155. setName
  156. (
  157. "test"
  158. );
  160. //Verify content of both maps
  162. System
  163. .
  164. out
  165. .
  166. println
  167. (
  168. userMap
  169. );
  171. System
  172. .
  173. out
  174. .
  175. println
  176. (
  177. clonedMap
  178. );
  180. }
  181. }
  运行结果：
  [plain] view plain copy
  1. {
  2. 1
  3. =
  4. User
  5. {
  6. name
  7. =
  8. ‘jay‘
  9. ,
  10. age
  11. =
  12. 26
  13. },
  15. 2
  16. =
  17. User
  18. {
  19. name
  20. =
  21. ‘fany‘
  22. ,
  23. age
  24. =
  25. 25
  26. }}
  28. Changes
  29. reflect
  30. in
  31. only one map
  33. {
  34. 1
  35. =
  36. User
  37. {
  38. name
  39. =
  40. ‘jay‘
  41. ,
  42. age
  43. =
  44. 26
  45. },
  47. 2
  48. =
  49. User
  50. {
  51. name
  52. =
  53. ‘fany‘
  54. ,
  55. age
  56. =
  57. 25
  58. }}
  59. {
  60. 1
  61. =
  62. User
  63. {
  64. name
  65. =
  66. ‘test‘
  67. ,
  68. age
  69. =
  70. 26
  71. },
  73. 2
  74. =
  75. User
  76. {
  77. name
  78. =
  79. ‘fany‘
  80. ,
  81. age
  82. =
  83. 25
  84. }}
  从运行结果看出，对cloneMap修改，userMap没有被改变，所以是深度复制

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关于Java Map应该掌握的问题

如何把一个Map转化为List

伪代码如下：

示例代码：

运行结果：

2、如何遍历一个Map

通过entrySet+for实现遍历

实例代码：

实例代码：

运行结果：

3、如何根据Map的keys进行排序

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

实例代码：

使用SortedMap+TreeMap+Comparator实现

实例代码：

运行结果：

4、如何对Map的values进行排序

实例代码：

运行结果：

5、如何初始化一个静态/不可变的Map

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

7、如何创建一个空map

8、有关于map的复制

例子一，使用=复制一个map：

运行结果：

例子二，使用hashmap的clone复制：

运行结果：

例子三，通过putAll操作

运行结果：

那么，如何实现深度复制呢？

运行结果：