Intro

A container is an object that can hold other Java objects. Java Collections Framework(JCF) Provides a generic container for Java developers, starting with JDK 1.2, with the following advantages:

• Reduced programming difficulty
• Improve program performance
• Improve interoperability between APIs
• Make learning things easier
• Reduce the difficulty of designing and implementing related APIs
• Increase program reusability

You can only put objects in a Java container. For basic types (int, long, float, double, etc.), you need to wrap them as object types (Integer, long, float, double, etc.) before you can put them in the container. In many cases unpacking and unpacking can be done automatically. This incurs additional performance and space overhead, but simplifies design and programming.

Collection

容器主要包括 Collection 和 Map 两种，Collection 存储着对象的集合，而 Map 存储着键值对(两个对象)的映射表。

Set

TreeSet
Red-black tree based implementation. Supports ordered operations, such as finding elements by a specific range.
However, it's efficiency is not as good as HashSet. A HashSet, which has a lookup time complexity of O(1), while a TreeSet has a lookup time complexity of O(logN).

HashSet
Hash-based implementation, support fast lookup, but do not support ordered operations. It also loses information about the order in which elements are inserted, which means that Iterator traversing the HashSet yields an indeterminate result.

LinkedHashSet
As efficient as HashSet, and it also maintains the order of insertion by using bidirectional linked list.

List

ArrayList
Based on dynamic array implementation, support random access.NOT thread safe.

Vector
Similar to ArrayList, but thread safe.

LinkedList
Based on bi-directional linked list implementation, only sequential access. It can insert & remove quickly.
Linked list can be used as a stack, queue & two-way queue.

Queue

LinkedList
We can use it to implement two-way queue

PriorityQueue
Based on heap. We can use it to implement priority queue

Map

TreeMap
Based on Red-black tree.

HashMap
Based on hashtable. NOT thread safe.

HashTable
Similar to HashMap, but it is thread safe -- multiple threads can write to Hashtable at the same time without inconsistent data.
This class seems to be deprecated? We'd better not to call it.

To achieve thread safety for Hash stuff, try use ConcurrentHashMap. What's more, ConcurrentHashMap is more effective because of segmenting locking (分段锁).

LinkedHashMap
Using bidirectional linked list to maintain orders of elements.