Java Notes-11

本文介绍了如何使用Java的Collections类创建线程安全的集合及只读集合,并探讨了Properties类的应用,包括配置数据的存储、读取及XML格式的保存与加载。

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Summary:Synchronized and Unsynchronized Collections, Properties
-In many applications, there is no need for synchronization, so the Collections API doesnot provide it by default. Instead, you can create a synchronized version of any collection

using the following methods of the  Collections class:

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  1. public static Collection synchronizedCollection(Collection c)  
  2. public static Set synchronizedSet(Set s)  
  3. public static List synchronizedList(List list)  
  4. public static Map synchronizedMap(Map m)  
  5. public static SortedSet synchronizedSortedSet(SortedSet s)  
  6. public static SortedMap synchronizedSortedMap(SortedMap m)  
-For example, the following shows how to create a threadsafe  List

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  1. List list = new ArrayList();  
  2. List syncList = Collections.synchronizedList(list);  
-This is important, so remember this! Although synchronized collections are threadsafe, the  Iterator s returned from them are not.

 

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  1. synchronized(syncList) {  
  2. Iterator iterator = syncList.iterator();  
  3. // do stuff with the iterator here  
  4. }  
-The  java.util.concurrent.ConcurrentHashMap class is part of the concurrency utilities package and provides a  Map that performs well under multithreaded access. 

-You can use the  Collections class to create read-only versions of any collection

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  1. public static Collection unmodifiableCollection(Collection c)  
  2. public static Set unmodifiableSet(Set s)  
  3. public static List unmodifiableList(List list)  
  4. public static Map unmodifiableMap(Map m)  
  5. public static SortedSet unmodifiableSortedSet(SortedSet s)  
  6. public static SortedMap unmodifiableSortedMap(SortedMap m)  

-The  java.util.Properties class is a specialized hash table for strings. Properties are generally used to hold textual configuration data.

-Any string values can be stored as key/value pairs in a  Properties table. However, the convention is to use a dot-separated naming hierarchy to group property names into
logical structures. 

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  1. Properties props = new Properties();  
  2. props.setProperty("myApp.xsize""52");  
  3. props.setProperty("myApp.ysize""79");  

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  1. String xsize = props.getProperty( "myApp.xsize" );  

You can save a  Properties table to an  OutputStream using the  save() method. The property information is output in a flat ASCII format. 

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  1. props.save( System.out, "Application Parameters" );  

-The  load() method reads the previously saved contents of a  Properties object from an  InputStream 
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  1. FileInputStream fin;  
  2. ...  
  3. Properties props = new Properties()  
  4. props.load( fin );  

-The  Properties class also contains  storeToXML() and  loadFromXML() methods. These operate just like the  save() and  load() methods but write an XML file like the following

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  1. <?xml version="1.0" encoding="UTF-8"?>  
  2. <!DOCTYPE properties SYSTEM "http://java.sun.com/dtd/properties.dtd"  
  3. >  
  4. <properties>  
  5. <comment>My Properties</comment>  
  6. <entry key="myApp.ysize">79</entry>  
  7. <entry key="myApp.xsize">52</entry>  
  8. </properties>  

-The  java.lang.System class provides access to basic system environment information through the static  System.getProperties() method. This method returns a  Properties table that contains system properties. 



 Next:The Preference API

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