What are the differences between commonly used implementation classes of collections?

#1. What is a collection

Collection is the embodiment of object-oriented reusability , Generally, we refer to research objects as elements; the totality of some elements is called a set, also referred to as a set.

2. Characteristics of set elements

(1) Deterministic: Let A be a given set , x is a specific object, then it is either an element of A, or it is not an element of A. One and only one of the two situations must be true. (2) Mutuality: An element in a given set means that it belongs to this Different individuals (objects) of a set, therefore, the same element should not appear repeatedly in the same set. (3) Disorder: Generally, the order between elements is not considered, but when expressing special sets such as sequences, Usually written in the customary order of the number axis from small to large

collection collection implementation class

ArrayList Implementation method: Array implementation Version: 1.2 Time: Released in December 1998 Lightweight (fast running) Thread unsafe (commonly used)
Vector Implementation method: Array implementation Version: 1.0 Time: Released in May 1995 Heavyweight (slow running) Thread safety
LinkedList implementation method: Linked list implementation
Array: continuous space, fast query speed, direct access to subscripts, slow addition and deletion, addition and deletion need to move the subsequent data backward or forward
linked list: Node association Query speed is slow Use the previous one to find the next Add and delete Quick linked list increase Directly point the previous one to the new data and the next one also points to the new data

Vector implementation Class Stack Stack LIFO (Last In First Out) query less addition and deletion more

3. How to implement a stack in java

a. Array b. LinkedList c. Stack class

Use LinkedList, why not use Stack to implement the class? Because Stack inherits the wrong Inherited from Vector, Vector provides the add method, but the stack can only pop and push and cannot be inserted from the middle, making the stack unsafe. Stack inherits Vector, so Stack has many methods that should not exist.

 1 //用java创建一个栈 2 public class AStack { 3 public static void main(String[] args) { 4     Stacks s = new Stacks(); 5     s.push("lin"); 6     s.push("hello"); 7     s.push("hou"); 8     s.push(123); 9     System.out.println(s.pop());10     System.out.println(s.pop());11     System.out.println(s.pop());12     System.out.println(s.pop());13 }14 }15 class Stacks{16     //变继承复用为主核复用17     private LinkedList list = new LinkedList();18     public void push(Object o){19         list.addFirst(o);20     }21     public  Object pop(){22         Object o = list.removeFirst();23         return o;24     }25 }

Map collection

Element: key-value pair (key-value pair)

Features: The key content is unique and the corresponding value is repeatable and unordered

Usage: Find value through key

##Example: Dictionary word - explanation Website username - password

Main method:

1.put (Object k, Object v): Put the key-value pair k-v into the Map. If k already exists, the new v will replace the old v

2.get(Object k): Returns the v corresponding to k in the Map

3.remove(Objeck k): Delete the key value where k is located Right

4.containsKey(Object k): Determine whether K exists in the Map

5.containsValue(Object v): Determine whether v exists in the Map

Map 3 types of traversal

1. Value traversal

1 　　Collection c = map.values();2    Iterator i =  c.iterator();3      while(i.hasNext()){4        System.out.println(i.next());5     }

##2. Key traversal

1         Set s = map.keySet();2         Iterator i =  s.iterator();3         while(i.hasNext()){4             Object k = i.next();5             Object v = map.get(k);6             System.out.println(k+"+++"+v);7         }

3. Key value traversal

Map.Entry object, encapsulation Got a key and a value

1 　　　　　Set set = map.entrySet();2         Iterator i = set.iterator();3         while(i.hasNext()){4             Map.Entry me = (Map.Entry)i.next();5             System.out.println(me.getKey());6             System.out.println(me.getValue());7         }

Implementation class:

1.HashMap: 1.2 Allows using null as key or value

2.Hashtable: 1.0 does not allow null as key or value
3.Properties: Hashtable subclasses both key and value String is generally used for reading configuration files

##4.TreeMap: Implements SortcdMap (sub-interface of Map) to automatically sort keys

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