HashTable,HashMap,Properties

Batkid 發(fā)布于2019-08-14 14:07 / 1132人閱讀

摘要：的區(qū)別和的區(qū)別一，他們繼承的父類不一樣繼承于抽象類繼承于抽象類二，線程不安全，線程安全如何將變成線程安全的，使用下面的方法線程不安全，非同步，一般效率相對(duì)高一些三，允許鍵值為不可以。

HashTable,HashMap,Properties 的區(qū)別

HashMap 和 HashTable的區(qū)別

一，他們繼承的父類不一樣

1,HashTable繼承于抽象類 Dictionary

  public class Hashtable extends Dictionary

2,HashMap 繼承于抽象類 java.util.AbstractMap

二，HashMap線程不安全，HashTable線程安全

HashMap:Note that this implementation is not synchronized.
如何將HashMap變成線程安全的，使用下面的方法：

 Map m = Collections.synchronizedMap(new HashMap(...));

線程不安全，非同步，一般效率相對(duì)高一些

三，HashMap 允許鍵,值為 null, HashTable 不可以。

 JDK：Hash table based implementation of the Map interface. This implementation provides all of the optional map operations, and permits null values and the null key. (The HashMap class is roughly equivalent to Hashtable, except that it is unsynchronized and permits nulls.) This class makes no guarantees as to the order of the map; in particular, it does not guarantee that the order will remain constant over time.
 JDK:Any non-null object can be used as a key and as a value.(鍵,值不能為空)

三，Properties 與 HashTable 的區(qū)別

Properties 是HashTable的子類

 public class Properties
extends Hashtable

并且是線程安全的，

JDK:This class is thread-safe: multiple threads can share a single Properties object without the need for external synchronization.

使用properties輸出到文件

資源配置文件

1,.properties

store(OutputStream out, String comments)

store(Writer writer, String comments)

2,.xml

storeToXML(OutputStream os, String comment)

storeToXML(OutputStream os, String comment, String encoding)

Properties pro 
          = new Properties();
    pro.setProperty("name", "mk");
    pro.setProperty("url", "www.baidu.com");
    //存儲(chǔ)到 d盤盤符
   1,存儲(chǔ)為.properties 文件
    pro.store(new FileOutputStream(new File("D://db.properties")), "db配置文件");
    //2,存儲(chǔ)為 .xml文件
//    pro.storeToXML(new FileOutputStream(new File("D://db.xml")), "db配置文件");
    //使用相對(duì)路徑 當(dāng)前的項(xiàng)目根目錄
//    pro.store(new FileOutputStream(new File("db.properties")), "db配置文件");

使用Properties讀取資源配置文件

load(InputStream inStream)

load(Reader reader)

loadFromXML(InputStream in)

Properties pro 
          = new Properties();
    //讀取絕對(duì)路徑
    pro.load(new FileReader("D://db.properties"));
    //讀取相對(duì)路徑
    pro.loadFromXML(new FileInputStream("D://db.xml"));

如果文件保存在bin里面，也是在我們工作中經(jīng)常碰到的一種情況

Properties pro 
          = new Properties();
    pro.load(Demo.class.getResourceAsStream("/project/db.properties"));
    pro.load(Thread.currentThread().getContextClassLoader().getResourceAsStream("project/db.properties"));