J2SE synthesis: Two types of Java container class list and set analysis

The container class can greatly improve programming efficiency and programming capabilities, and in Java2, all containers are redesigned by Sun's Joshua Bloch to enrich the functionality of the Container class library.

The purpose of the Java2 container class library is "Save Object", which is divided into two categories:

Collection----A set of independent elements, which are usually subject to some sort of rule. The list must maintain an element-specific order, and set cannot have duplicate elements.

MAP----A pair of "key-value pairs" objects, whose elements are pairs of objects, the most typical application is the data dictionary, and there are other extensive applications. In addition, a map can return a set of all its keys and all its values, or a set consisting of its key pairs, and can also extend the multidimensional map as an array, so long as each "value" of the key value pair in the map is a map collection.

1. iterators

An iterator is a design pattern that is an object that traverses and selects objects in a sequence, and developers do not need to understand the underlying structure of the sequence. Iterators are often referred to as "lightweight" objects because the cost of creating them is small.

The iterator function in Java is simpler and can only be moved in one Direction:

(1) using the method iterator () requires the container to return a iterator. The first time the iterator next () method is invoked, it returns the first element of the sequence.

(2) Use Next () to get the next element in the sequence.

(3) Use Hasnext () to check if there are any elements in the sequence.

(4) Remove the newly returned elements of the iterator using remove ().

Iterator is the simplest implementation of the Java iterator, with more functionality for the list design listiterator, which can traverse the list in two directions or insert and delete elements from the list.

2.List Method of function

List (interface): Order is the most important feature of the list; it ensures that the order of elements is maintained. List adds a number of methods to collection that allow you to insert and remove elements in the middle of the list (only recommended for linkedlist use). A list can generate Listiterator, using it to traverse the list in two directions, or to insert and delete elements from the list.

ArrayList: A list implemented by an array. It allows fast random access to elements, but it is slow to insert and remove elements in the middle of the list. Instead of inserting and deleting elements, listiterator should only be used to traverse the ArrayList backwards, because it is much larger than the linkedlist overhead.

LinkedList: Optimized sequential access, the insertion and deletion of the list is not expensive, random access is relatively slow (can be replaced by ArrayList). It has method AddFirst (), AddLast (), GetFirst (), GetLast (), Removefirst (), Removelast (), these methods (not defined in any interface or base class) Enables LinkedList to be used as stacks, queues, and bidirectional queues.

3.Set Method of function

Set (interface): Each element that is stored in a set must be unique because the set does not save repeating elements. The object that joins the set must define the Equals () method to ensure the uniqueness of the object. Set has exactly the same interface as collection. The set interface does not guarantee the order of elements to be maintained.

HashSet: Set designed for quick lookup. The object that is stored in the HashSet must define HASHCODE ().

TreeSet: Keep order set, bottom is tree structure. Use it to extract ordered sequences from set.

Linkedhashset: Has a hashset query speed and internally uses a linked list to maintain the order of elements (in the order of insertion). So when you use iterators to traverse set, the results are displayed in the order in which the elements are inserted.

HashSet uses hash functions to sort elements, which are specifically designed for quick queries; TreeSet The data structure of the red-black tree to sort elements; Linkedhashset uses hashes to speed up query speed, while using linked lists to maintain the order of elements, Makes it appear that the elements are saved in the order in which they were inserted. Note that when building your own class, the set needs to maintain the order in which the elements are stored, so implement the comparable interface and define the CompareTo () method.