Map collection as well as its implementation class source code analysis HashMap

Double row Collections Framework: Map

1. Common implementation class structure

|----Map:双列数据，存储  key-value  对的数据   ---类似于高中的函数：y = f(x)
   |----HashMap:作为Map的主要实现类；线程不安全的，效率高；存储null的key和value
          |----LinkedHashMap:保证在遍历map元素时，可以照添加的顺序实现遍历。
                原因：在原的HashMap底层结构基础上，添加了一对指针，指向前一个和后一个元素。
                对于频繁的遍历操作，此类执行效率高于HashMap。
   |----TreeMap:保证照添加的key-value对进行排序，实现排序遍历。此时考虑key的自然排序或定制排序
                  底层使用红黑树
   |----Hashtable:作为古老的实现类；线程安全的，效率低；不能存储null的key和value
          |----Properties:常用来处理配置文件。key和value都是String类型


  HashMap的底层：数组+链表  （jdk7及之前)
                数组+链表+红黑树 （jdk 8)

2. [problem]

1. The realization of the principle underlying HashMap?
2. HashMap and Hashtable similarities and differences?
3. CurrentHashMap and Hashtable similarities and differences? (Temporarily not speak)

3. The storage structure is understood:

Map the key: disorderly, unrepeatable, use the Set stores all of the key -> class key where you want to override the equals () and hashCode () (to HashMap for example)
value in the Map: disorderly, reproducible, using the Collection store all value -> where the class value to rewrite equals ()

A key-value pair: key-value constitute an Entry object.

Map of entry: disorderly, unrepeatable, use the Set stores all entry

Icon:

4. Common methods

Add: put (Object key, Object value )
Delete: remove (Object key)
Modify: put (Object key, Object value )
query: get (Object key)
Length: size ()
traversal: keySet () / values () / entrySet ()
Clear Map:
map.clear (); // not null = Map
System.out.println (map.size ()); // 0
System.out.println (Map);} // {

Contains key: containsKey (Object key)
contains the value: containsValue (Object value)

5. Memory Structure:

5.1 HashMap implementation principles in jdk7:

HashMap map = new HashMap():
  在实例化以后，底层创建了长度是16的一维数组Entry[] table。
  ...可能已经执行过多次put...
  map.put(key1,value1):
      - 首先，调用 key1 所在类的 hashCode() 计算key1 哈希值，此哈希值经过某种算法计算以后，得到在Entry数组中的存放位置。
          + 如果此位置上的数据为空，此时的 key1-value1 添加成功。 ----情况1
          + 如果此位置上的数据不为空，(意味着此位置上存在一个或多个数据(以链表形式存在)),比较key1和已经存在的一个或多个数据的哈希值：
              = 如果key1的哈希值与已经存在的数据的哈希值都不相同，此时key1-value1添加成功。----情况2
              = 如果key1的哈希值和已经存在的某一个数据(key2-value2)的哈希值相同，继续比较：调用key1所在类的equals(key2)方法，比较：
                       > 如果equals()返回false:此时key1-value1添加成功。----情况3
                       > 如果equals()返回true:使用value1替换value2。

补充：关于情况2和情况3：此时key1-value1和原来的数据以链表的方式存储。

在不断的添加过程中，会涉及到扩容问题，当超出临界值(且要存放的位置非空)时，扩容。默认的扩容方式：扩容为原来容量的2倍，并将原的数据复制过来。

5.2 HashMap in jdk8 and jdk7 achieved in terms of the underlying differences:

1. new HashMap (): the bottom layer is not create an array of length 16
2. Is the underlying array jdk 8: Node [], rather than the Entry []
3. When the first call to put () method to create the underlying array of length 16
4. jdk7 substructure only: + array list. jdk8 the underlying structure: Array + + red-black tree list.

• When the formation of the list, at sixes and sevens (jdk7: new elements point to the old elements .jdk8: old elements point to the new element)
• When the number of data elements on a certain position in the array index is present as a list> and the current length of the array 8> 64, then the data on the position of the index used to store red-black tree.

5.3 HashMap properties typical attributes of the underlying Description:

• DEFAULT_INITIAL_CAPACITY: default capacity of HashMap, 16
• DEFAULT_LOAD_FACTOR: HashMap default load factor: 0.75
• threshold: expansion threshold, capacity = fill factor: 16 0.75 => 12
• TREEIFY_THRESHOLD: Bucket in the chain length is greater than the default value, is converted to red-black tree: 8
• MIN_TREEIFY_CAPACITY: the minimum hash table capacity of the tub is Node tree: 64

The principle underlying the 5.4 LinkedHashMap (understand)

LinkedHashMap same underlying HashMap configuration used, since LinkedHashMap inherited HashMap.
Difference is: inside LinkedHashMap provides Entry, replacing the HashMap Node.

6. TreeMap use

TreeMap added to the key-value, the key requirement by the object must be created in the same class
as sort key according to: natural order, custom ordering

Read the configuration file 7. Properties

//Properties:常用来处理配置文件。key和value都是String类型
public static void main(String[] args)  {
    FileInputStream fis = null;
    try {
        Properties pros = new Properties();

        fis = new FileInputStream("jdbc.properties");
        pros.load(fis);//加载流对应的文件

        String name = pros.getProperty("name");
        String password = pros.getProperty("password");

        System.out.println("name = " + name + ", password = " + password);
    } catch (IOException e) {
        e.printStackTrace();
    } finally {
        if(fis != null){
            try {
                fis.close();
            } catch (IOException e) {
                e.printStackTrace();
            }

        }
    }