BAT面試必問HashMap原始碼分析

1

2

3

4

5

6

7

static   final   int   hash( Object   key) {

   int   h;

   // key.hashCode()：返回雜湊值也就是hashcode

   // ^ ：按位異或

   // >>>:無符號右移，忽略符號位，空位都以0補齊

   return   (key ==  null ) ?    : (h = key.hashCode()) ^ (h >>>  16 );

}

1

2

3

4

5

6

7

8

static   int hash(int h) {

     // This function ensures that hashCodes that differ only by

     // constant multiples at each bit position have a bounded

     // number of collisions (approximately 8 at default load factor).

     h ^= (h >>>  20 ) ^ (h >>>  12 );

     return   h ^ (h >>>  7 ) ^ (h >>>  4 );

}

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

21

22

23

24

25

26

27

28

public   class   HashMap<K,V>  extends   AbstractMap<K,V>  implements   Map<K,V>, Cloneable, Serializable {

     // 序列號

     private   static   final   long   serialVersionUID = 362498820763181265L;   

     // 預設的初始容量是16

     static   final   int   DEFAULT_INITIAL_CAPACITY =  1   <<  4 ;  

     // 最大容量

     static   final   int   MAXIMUM_CAPACITY =  1   <<  30 ;

     // 預設的填充因子

     static   final   float   DEFAULT_LOAD_FACTOR =  .75f;

     // 當桶(bucket)上的結點數大於這個值時會轉成紅黑樹

     static   final   int   TREEIFY_THRESHOLD =  8 ;

     // 當桶(bucket)上的結點數小於這個值時樹轉連結串列

     static   final   int   UNTREEIFY_THRESHOLD =  6 ;

     // 桶中結構轉化為紅黑樹對應的table的最小大小

     static   final   int   MIN_TREEIFY_CAPACITY =  64 ;

     // 儲存元素的陣列，總是2的冪次倍

     transient   Node<k,v>[] table;

     // 存放具體元素的集

     transient   Set<map.entry<k,v>> entrySet;

     // 存放元素的個數，注意這個不等於陣列的長度。

     transient   int   size;

     // 每次擴容和更改map結構的計數器

     transient   int   modCount;  

     // 臨界值 當實際大小(容量*填充因子)超過臨界值時，會進行擴容

     int   threshold;

     // 填充因子

     final   float   loadFactor;

}

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

21

22

23

24

25

26

27

28

29

30

31

32

33

34

35

36

37

38

39

// 繼承自 Map.Entry<K,V>

static   class   Node<K,V>  implements   Map.Entry<K,V> {

        final   int   hash; // 雜湊值，存放元素到hashmap中時用來與其他元素hash值比較

        final   K key; //鍵

        V value; //值

        // 指向下一個節點

        Node<K,V> next;

        Node( int   hash, K key, V value, Node<K,V> next) {

             this .hash = hash;

             this .key = key;

             this .value = value;

             this .next = next;

         }

         public   final   K getKey()        {  return   key; }

         public   final   V getValue()      {  return   value; }

         public   final   String toString() {  return   key +  "="   + value; }

         // 重寫hashCode()方法

         public   final   int   hashCode() {

             return   Objects.hashCode(key) ^ Objects.hashCode(value);

         }

         public   final   V setValue(V newValue) {

             V oldValue = value;

             value = newValue;

             return   oldValue;

         }

         // 重寫 equals() 方法

         public   final   boolean   equals(Object o) {

             if   (o ==  this )

                 return   true ;

             if   (o  instanceof   Map.Entry) {

                 Map.Entry<?,?> e = (Map.Entry<?,?>)o;

                 if   (Objects.equals(key, e.getKey()) &&

                     Objects.equals(value, e.getValue()))

                     return   true ;

             }

             return   false ;

         }

}

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

static   final   class   TreeNode<K,V>  extends   LinkedHashMap.Entry<K,V> {

         TreeNode<K,V> parent;   // 父

         TreeNode<K,V> left;     // 左

         TreeNode<K,V> right;    // 右

         TreeNode<K,V> prev;     // needed to unlink next upon deletion

         boolean   red;            // 判斷顏色

         TreeNode( int   hash, K key, V val, Node<K,V> next) {

             super (hash, key, val, next);

         }

         // 返回根節點

         final   TreeNode<K,V> root() {

             for   (TreeNode<K,V> r =  this , p;;) {

                 if   ((p = r.parent) ==  null )

                     return   r;

                 r = p;

        }

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

21

22

23

24

25

26

27

// 預設建構函式。

public   More ...HashMap() {

     this .loadFactor = DEFAULT_LOAD_FACTOR;  // all   other fields defaulted

  }

  // 包含另一個“Map”的建構函式

  public   More ...HashMap(Map<?  extends   K, ?  extends   V> m) {

      this .loadFactor = DEFAULT_LOAD_FACTOR;

      putMapEntries(m,  false ); //下面會分析到這個方法

  }

  // 指定“容量大小”的建構函式

  public   More ...HashMap( int   initialCapacity) {

      this (initialCapacity, DEFAULT_LOAD_FACTOR);

  }

  // 指定“容量大小”和“載入因子”的建構函式

  public   More ...HashMap( int   initialCapacity,  float   loadFactor) {

      if   (initialCapacity <  )

          throw   new   IllegalArgumentException( "Illegal initial capacity: "   + initialCapacity);

      if   (initialCapacity > MAXIMUM_CAPACITY)

          initialCapacity = MAXIMUM_CAPACITY;

      if   (loadFactor <=    || Float.isNaN(loadFactor))

          throw   new   IllegalArgumentException( "Illegal load factor: "   + loadFactor);

      this .loadFactor = loadFactor;

      this .threshold = tableSizeFor(initialCapacity);

  }

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

21

22

23

24

final   void   putMapEntries(Map<?  extends   K, ?  extends   V> m,  boolean   evict) {

     int   s = m.size();

     if   (s >  ) {

         // 判斷table是否已經初始化

         if   (table ==  null ) {  // pre-size

             // 未初始化，s為m的實際元素個數

             float   ft = (( float )s / loadFactor) +  1 .0F;

             int   t = ((ft < ( float )MAXIMUM_CAPACITY) ?

                     ( int )ft : MAXIMUM_CAPACITY);

             // 計算得到的t大於閾值，則初始化閾值

             if   (t > threshold)

                 threshold = tableSizeFor(t);

         }

         // 已初始化，並且m元素個數大於閾值，進行擴容處理

         else   if   (s > threshold)

             resize();

         // 將m中的所有元素新增至HashMap中

         for   (Map.Entry<?  extends   K, ?  extends   V> e : m.entrySet()) {

             K key = e.getKey();

             V value = e.getValue();

             putVal(hash(key), key, value,  false , evict);

         }

     }

}

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

21

22

23

24

25

26

27

28

29

30

31

32

33

34

35

36

37

38

39

40

41

42

43

44

45

46

47

48

49

50

51

52

53

54

55

56

57

58

59

60

61

62

63

64

65

66

67

68

69

70

71

public   V put(K key, V value) {

     return   putVal(hash(key), key, value,  false ,  true );

}

final   V putVal( int   hash, K key, V value,  boolean   onlyIfAbsent,

                    boolean   evict) {

     Node<K,V>[] tab; Node<K,V> p;  int   n, i;

     // table未初始化或者長度為0，進行擴容

     if   ((tab = table) ==  null   || (n = tab.length) ==  )

         n = (tab = resize()).length;

     // (n - 1) & hash 確定元素存放在哪個桶中，桶為空，新生成結點放入桶中(此時，這個結點是放在陣列中)

     if   ((p = tab[i = (n -  1 ) & hash]) ==  null )

         tab[i] = newNode(hash, key, value,  null );

     // 桶中已經存在元素

     else   {

         Node<K,V> e; K k;

         // 比較桶中第一個元素(陣列中的結點)的hash值相等，key相等

         if   (p.hash == hash &&

             ((k = p.key) == key || (key !=  null   && key.equals(k))))

                 // 將第一個元素賦值給e，用e來記錄

                 e = p;

         // hash值不相等，即key不相等；為紅黑樹結點

         else   if   (p  instanceof   TreeNode)

             // 放入樹中

             e = ((TreeNode<K,V>)p).putTreeVal( this , tab, hash, key, value);

         // 為連結串列結點

         else   {

             // 在連結串列最末插入結點

             for   ( int   binCount =  ; ; ++binCount) {

                 // 到達連結串列的尾部

                 if   ((e = p.next) ==  null ) {

                     // 在尾部插入新結點

                     p.next = newNode(hash, key, value,  null );

                     // 結點數量達到閾值，轉化為紅黑樹

                     if   (binCount >= TREEIFY_THRESHOLD -  1 )  // -1 for 1st

                         treeifyBin(tab, hash);

                     // 跳出迴圈

                     break ;

                 }

                 // 判斷連結串列中結點的key值與插入的元素的key值是否相等

                 if   (e.hash == hash &&

                     ((k = e.key) == key || (key !=  null   && key.equals(k))))

                     // 相等，跳出迴圈

                     break ;

                 // 用於遍歷桶中的連結串列，與前面的e = p.next組合，可以遍歷連結串列

                 p = e;

             }

         }

         // 表示在桶中找到key值、hash值與插入元素相等的結點

         if   (e !=  null ) {

             // 記錄e的value

             V oldValue = e.value;

             // onlyIfAbsent為false或者舊值為null

             if   (!onlyIfAbsent || oldValue ==  null )

                 //用新值替換舊值

                 e.value = value;

             // 訪問後回撥

             afterNodeAccess(e);

             // 返回舊值

             return   oldValue;

         }

     }

     // 結構性修改

     ++modCount;

     // 實際大小大於閾值則擴容

     if   (++size > threshold)

         resize();

     // 插入後回撥

     afterNodeInsertion(evict);

     return   null ;

}

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

21

22

public   V put(K key, V value)

     if   (table == EMPTY_TABLE) {

     inflateTable(threshold);

} 

     if   (key ==  null )

         return   putForNullKey(value);

     int   hash = hash(key);

     int   i = indexFor(hash, table.length);

     for   (Entry<K,V> e = table[i]; e !=  null ; e = e.next) {  // 先遍歷

         Object k;

         if   (e.hash == hash && ((k = e.key) == key || key.equals(k))) {

             V oldValue = e.value;

             e.value = value;

             e.recordAccess( this );

             return   oldValue;

         }

     }

     modCount++;

     addEntry(hash, key, value, i);   // 再插入

     return   null ;

}

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

21

22

23

24

25

26

27

28

public   V get(Object key) {

     Node<K,V> e;

     return   (e = getNode(hash(key), key)) ==  null   ?  null   : e.value;

}

final   Node<K,V> getNode( int   hash, Object key) {

     Node<K,V>[] tab; Node<K,V> first, e;  int   n; K k;

     if   ((tab = table) !=  null   && (n = tab.length) >    &&

         (first = tab[(n -  1 ) & hash]) !=  null ) {

         // 陣列元素相等

         if   (first.hash == hash &&  // always check first node

             ((k = first.key) == key || (key !=  null   && key.equals(k))))

             return   first;

         // 桶中不止一個節點

         if   ((e = first.next) !=  null ) {

             // 在樹中get

             if   (first  instanceof   TreeNode)

                 return   ((TreeNode<K,V>)first).getTreeNode(hash, key);

             // 在連結串列中get

             do   {

                 if   (e.hash == hash &&

                     ((k = e.key) == key || (key !=  null   && key.equals(k))))

                     return   e;

             }  while   ((e = e.next) !=  null );

         }

     }

     return   null ;

}

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

21

22

23

24

25

26

27

28

29

30

31

32

33

34

35

36

37

38

39

40

41

42

43

44

45

46

47

48

49

50

51

52

53

54

55

56

57

58

59

60

61

62

63

64

65

66

67

68

69

70

71

72

73

74

75

76

77

78

79

80

final   Node<K,V>[] resize() {

     Node<K,V>[] oldTab = table;

     int   oldCap = (oldTab ==  null ) ?    : oldTab.length;

     int   oldThr = threshold;

     int   newCap, newThr =  ;

     if   (oldCap >  ) {

         // 超過最大值就不再擴充了，就只好隨你碰撞去吧

         if   (oldCap >= MAXIMUM_CAPACITY) {

             threshold = Integer.MAX_VALUE;

             return   oldTab;

         }

         // 沒超過最大值，就擴充為原來的2倍

         else   if   ((newCap = oldCap <<  1 ) < MAXIMUM_CAPACITY && oldCap >= DEFAULT_INITIAL_CAPACITY)

             newThr = oldThr <<  1 ;  // double threshold

     }

     else   if   (oldThr >  )  // initial capacity was placed in threshold

         newCap = oldThr;

     else   {

         signifies using defaults

         newCap = DEFAULT_INITIAL_CAPACITY;

         newThr = ( int )(DEFAULT_LOAD_FACTOR * DEFAULT_INITIAL_CAPACITY);

     }

     // 計算新的resize上限

     if   (newThr ==  ) {

         float   ft = ( float )newCap * loadFactor;

         newThr = (newCap < MAXIMUM_CAPACITY && ft < ( float )MAXIMUM_CAPACITY ? ( int )ft : Integer.MAX_VALUE);

     }

     threshold = newThr;

     @SuppressWarnings ({ "rawtypes" , "unchecked" })

         Node<K,V>[] newTab = (Node<K,V>[]) new   Node[newCap];

     table = newTab;

     if   (oldTab !=  null ) {

         // 把每個bucket都移動到新的buckets中

         for   ( int   j =  ; j < oldCap; ++j) {

             Node<K,V> e;

             if   ((e = oldTab[j]) !=  null ) {

                 oldTab[j] =  null ;

                 if   (e.next ==  null )

                     newTab[e.hash & (newCap -  1 )] = e;

                 else   if   (e  instanceof   TreeNode)

                     ((TreeNode<K,V>)e).split( this , newTab, j, oldCap);

                 else   {

                     Node<K,V> loHead =  null , loTail =  null ;

                     Node<K,V> hiHead =  null , hiTail =  null ;

                     Node<K,V> next;

                     do   {

                         next = e.next;

                         // 原索引

                         if   ((e.hash & oldCap) ==  ) {

                             if   (loTail ==  null )

                                 loHead = e;

                             else

                                 loTail.next = e;

                             loTail = e;

                         }

                         // 原索引+oldCap

                         else   {

                             if   (hiTail ==  null )

                                 hiHead = e;

                             else

                                 hiTail.next = e;

                             hiTail = e;

                         }

                     }  while   ((e = next) !=  null );

                     // 原索引放到bucket裡

                     if   (loTail !=  null ) {

                         loTail.next =  null ;

                         newTab[j] = loHead;

                     }

                     // 原索引+oldCap放到bucket裡

                     if   (hiTail !=  null ) {

                         hiTail.next =  null ;

                         newTab[j + oldCap] = hiHead;

                     }

                 }

             }

         }

     }

     return   newTab;

}

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

21

22

23

24

25

26

27

28

29

30

31

32

33

34

35

36

37

38

39

40

41

42

43

44

45

46

47

48

49

50

51

52

53

54

55

56

57

58

59

60

61

62

63

64

65

66

67

68

69

70

package   map;

import   java.util.Collection;

import   java.util.HashMap;

import   java.util.Set;

public   class   HashMapDemo {

     public   static   void   main(String[] args) {

         HashMap<String, String> map =  new   HashMap<String, String>();

         // 鍵不能重複，值可以重複

         map.put( "san" ,  "張三" );

         map.put( "si" ,  "李四" );

         map.put( "wu" ,  "王五" );

         map.put( "wang" ,  "老王" );

         map.put( "wang" ,  "老王2" ); // 老王被覆蓋

         map.put( "lao" ,  "老王" );

         System.out.println( "-------直接輸出hashmap:-------" );

         System.out.println(map);

         /**

          * 遍歷HashMap

          */

         // 1.獲取Map中的所有鍵

         System.out.println( "-------foreach獲取Map中所有的鍵:------" );

         Set<String> keys = map.keySet();

         for   (String key : keys) {

             System.out.print(key+ "  " );

         }

         System.out.println(); //換行

         // 2.獲取Map中所有值

         System.out.println( "-------foreach獲取Map中所有的值:------" );

         Collection<String> values = map.values();

         for   (String value : values) {

             System.out.print(value+ "  " );

         }

         System.out.println(); //換行

         // 3.得到key的值的同時得到key所對應的值

         System.out.println( "-------得到key的值的同時得到key所對應的值:-------" );

         Set<String> keys2 = map.keySet();

         for   (String key : keys2) {

             System.out.print(key +  "："   + map.get(key)+ "   " );

         }

         /**

          * 另外一種不常用的遍歷方式

          */

         // 當我呼叫put(key,value)方法的時候，首先會把key和value封裝到

         // Entry這個靜態內部類物件中，把Entry物件再新增到陣列中，所以我們想獲取

         // map中的所有鍵值對，我們只要獲取陣列中的所有Entry物件，接下來

         // 呼叫Entry物件中的getKey()和getValue()方法就能獲取鍵值對了

         Set<java.util.Map.Entry<String, String>> entrys = map.entrySet();

         for   (java.util.Map.Entry<String, String> entry : entrys) {

             System.out.println(entry.getKey() +  "--"   + entry.getValue());

         }

         /**

          * HashMap其他常用方法

          */

         System.out.println( "after map.size()：" +map.size());

         System.out.println( "after map.isEmpty()：" +map.isEmpty());

         System.out.println(map.remove( "san" ));

         System.out.println( "after map.remove()：" +map);

         System.out.println( "after map.get(si)：" +map.get( "si" ));

         System.out.println( "after map.containsKey(si)：" +map.containsKey( "si" ));

         System.out.println( "after containsValue(李四)：" +map.containsValue( "李四" ));

         System.out.println(map.replace( "si" ,  "李四2" ));

         System.out.println( "after map.replace(si, 李四2):" +map);

     }

}