The difference between several confusing concepts such as Array,vertor,arraylist,hashable,hashmap

An array can hold object and basic data types, but you must specify the size of the array when you create it, and you cannot change it.

Vertor is the object that is put. Vertor one-dimensional, hashmap/hashtabe two-dimensional; vertor/arraylist use index for retrieval, Hashmap/hashtabe use key as the search; Hashmap,arraylist is not synchronous, means that they are faster; Hashtable,vertor is synchronous and applicable to threads; Hashtale key cannot be null for Null,hashmap key and values;

Performance: When accessing and updating elements through an index, the implementation of vectors and ArrayList has excellent performance because it does not

There is a cost other than the scope check. Inserting elements and deleting elements is always an array copy.

The essence of LinkedList is a doubly linked list. ArrayList and vectors are usually more than linkedlist and synchronous seals

The LinkedList after loading has better performance.

Hashmap,arraylist is not synchronous, it is a lightweight application and should be better in performance.

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1, first look at the picture: Collection├list│├linkedlist│├arraylist│└vector│└stack└set Map├hashtable├hashmap└weakhashmap 2, Collec The tion interface Collection is the most basic collection interface, and a Collection represents a set of Object, the Collection element (Elements). Some collection allow the same elements while others do not. Some can sort and others can't. The Java SDK does not provide classes that inherit directly from collection, and the Java SDK provides classes that inherit from collection, such as list and set. All classes that implement the collection interface must provide two standard constructors: a parameterless constructor is used to create an empty collection, and a constructor with a collection parameter is used to create a new collection. This new collection has the same elements as the incoming collection. The latter constructor allows the user to copy a collection. How do I traverse every element in the collection? Regardless of the actual type of collection, it supports a iterator () method that returns an iteration that uses the iteration to access each element of the collection one at a time. The typical usage is as follows: Iterator it = Collection.iterator (); Gets an iterator while (It.hasnext ()) {Object obj = It.next ();//Gets the next element} two interfaces derived from the collection interface are list and set.

The list interface list is an ordered collection, using this interface to precisely control where each element is inserted. The user is able to access the elements in the list using an index (where the element is positioned in the list, similar to an array subscript), similar to an array of java. Unlike the set mentioned below, the list allows the same elements. In addition to the iterator () method, which has the collection interface prerequisites, the list also provides a listiterator () method that returns a Listiterator interface, compared to the standard iterator interface. Listiterator has a number of add () methods that allow you to add, delete, set elements, and traverse forward or backward. The common classes that implement the list interface are Linkedlist,arraylist,vector and stacks.

The LinkedList class LinkedList implements the list interface, allowing null elements. Additionally LinkedList provides an additional Get,remove,insert method at the first or the tail of the LinkedList. These operations make the LinkedList available as a stack (stack), queue, or two-way queue (deque). Note LinkedList does not have a synchronization method. If multiple threads access a list at the same time, you must implement access synchronization yourself. One workaround is to construct a synchronized list:list list = collections.synchronizedlist (new LinkedList (...) when the list is created;

The ArrayList class ArrayList implements a variable-sized array. It allows all elements, including null. ArrayList is not synchronized. Size,isempty,get,set method run time is constant. But the Add method cost is the allocated constant, and adding n elements requires an O (n) time. Other methods run at a linear time. Each ArrayList instance has a capacity (capacity), which is the size of the array used to store the elements. This capacity automatically increases as new elements are added, but the growth algorithm is not defined. When you need to insert a large number of elements, you can call the Ensurecapacity method before inserting to increase the capacity of the ArrayList to improve insertion efficiency. Like LinkedList, ArrayList is also unsynchronized (unsynchronized).

Vector vectors are very similar to ArrayList, but vectors are synchronous. The iterator created by the vector, although the same interface as the iterator created by ArrayList, but because the vector is synchronous, when a iterator is created and is being used, another thread changes the state of the vector (for example, Some elements have been added or removed, Concurrentmodificationexception will be thrown when the iterator method is called, so the exception must be caught.

The Stack class stack inherits from the vector and implements a last-in-first-out stack. The stack provides 5 additional ways to make the vector available as a stack. The basic push and pop methods, and the Peek method to get the stack top element, the empty method tests if the stack is empty, and the search method detects the position of an element on the stack. Stack has just been created as an empty stack.

The set interface set is a collection that contains no duplicate elements, that is, any two elements E1 and E2 have E1.equals (E2) =false,set have a maximum of one null element. Obviously, the constructor of a set has a constraint that the passed-in collection parameter cannot contain duplicate elements. Note: Variable objects (Mutable object) must be handled with care. If a mutable element in a set changes its state, causing Object.Equals (Object) =true will cause some problems.

Map interface Note that map does not inherit the collection interface, and map provides a key-to-value mapping. A map cannot contain the same key, and each key can only map one value. The map interface provides views of 3 collections, and the contents of the map can be treated as a set of key sets, a set of value collections, or a set of key-value mappings. The Hashtable class Hashtable inherits the map interface, implementing a key-value mapped hash table. Any object that is not empty (non-null) can be either a key or a value. Add data using put (key, value), take out the data using get (key), the time overhead for these two basic operations is constant. The Hashtable adjusts performance through the initial capacity and load factor two parameters. Normally the default load factor 0.75 is a good way to achieve a balanced time and space. Increasing the load factor can save space but the corresponding lookup time will increase, which will affect operations like get and put. A simple example of using Hashtable is to put the three-to-one in the Hashtable, with their key being "single", "Two", "three": Hashtable numbers = new Hashtable (); Numbers.put ("One", New Integer (1)); Numbers.put ("n", New Integer (2)); Numbers.put ("Three", New Integer (3)); To take out a number, such as 2, with the corresponding Key:integer n = (Integer) numbers.get ("two"); System.out.println ("both =" + N); Because an object that is a key will determine the position of its corresponding value by calculating its hash function, any object that is a key must implement the Hashcode and Equals methods. The hashcode and Equals methods inherit from the root class object, and if you use a custom class as key, be quite careful, as defined by the hash function, if two objects are the same, i.e. obj1.equals (OBJ2) =true, Their hashcode must be the same, but if the two objects are different, their hashcode is not necessarily different, if the hashcode of two different objects is the same, this phenomenon is called a conflict, the conflict causes the time overhead of manipulating the hash table, so as far as possible the defined hasThe Hcode () method can speed up the operation of the hash table. If the same object has different hashcode, the operation of the hash table will have unexpected results (expecting the Get method to return null), to avoid this problem, you need to keep in mind one: to replicate both the Equals method and the Hashcode method, rather than write only one of them. The Hashtable is synchronous.

The HashMap class HashMap is similar to Hashtable, except that HashMap is non-synchronous and allows NULL, which is null value and null key. , but when HashMap is treated as collection (the values () method can return collection), its iteration sub-operation time overhead is proportional to the capacity of HashMap. Therefore, if the performance of the iterative operation is quite important, do not set the initialization capacity of the hashmap too high or load factor too low.

The Weakhashmap class Weakhashmap is an improved hashmap, which implements a "weak reference" to key, which can be recycled by GC if a key is no longer referenced externally.

Summary if involved in stacks, queues and other operations, should consider using the list, for the need to quickly insert, delete elements, should use LinkedList, if you need to quickly randomly access elements, you should use ArrayList. If the program is in a single-threaded environment, or if access is done only in one thread, it is more efficient to consider non-synchronous classes, and if multiple threads may operate on a class at the same time, the synchronized classes should be used. Pay special attention to the operation of the hash table, and the object as key should correctly replicate the Equals and Hashcode methods. Try to return the interface rather than the actual type, such as returning a list instead of ArrayList, so that if you need to change ArrayList to LinkedList later, the client code does not have to be changed. This is for abstract programming.

    synchronous vectors are synchronous. Some of the methods in this class ensure that the objects in the vector are thread-safe. The ArrayList is asynchronous, so objects in ArrayList are not thread-safe. Because the requirements of synchronization affect the efficiency of execution, it is a good choice to use ArrayList if you do not need a thread-safe collection, which avoids unnecessary performance overhead due to synchronization.     data growth from an internal implementation mechanism both ArrayList and vectors use arrays to control the objects in the collection. When you add elements to both types, if the number of elements exceeds the current length of the internal array, they all need to extend the length of the internal array, and vector automatically grows the array length by default, ArrayList is the original 50%. So in the end you get this collection that takes up more space than you actually need. So if you're going to save a lot of data in a collection then there are some advantages to using vectors, because you can avoid unnecessary resource overhead by setting the initialization size of the collection.     usage patterns in ArrayList and vectors, it takes the same amount of time to find data from a specified location (through an index) or to add or remove an element at the end of a collection, as shown in O (1). However, if an element is added or removed elsewhere in the collection, the time spent will grow linearly: O (n-i), where n represents the number of elements in the collection, and I represents the index position at which the element is incremented or removed. Why is that? It is assumed that all elements after the first and second elements of the collection will perform the operation of the displacement. What does all this mean?         This means that you just look for elements in a particular location or only add and remove elements at the end of the collection, either with a vector or ArrayList. If this is another operation, you might want to choose a different collection operation class. For example, does the Linklist collection class take the same amount of time to add or remove elements from any location in the collection? O (1), but it is slow to use an element in the index-O (i), where I is the position of the index. It's also easy to use ArrayList because you can simply use an index instead of creating an iterator object. Linklist also creates objects for each inserted element, all of which you have to understand that it also brings additional overhead.         Finally, in the book Practical Java, Peter Haggar suggests using a simple array instead of a vector or a ArrayList. This is especially true for programs with high efficiency requirements. Because the use of arrays (array) avoids synchronization, additional method calls, and unnecessary reallocation of space operations

Array,vertor,arraylist,hashable,hashmap The difference between several confusing concepts