我們都知道C語言有格式化輸出printf()可以進行格式化輸出這一功能，其格式如 printf("[%d %f]\n",x,y);其中d%、f%稱為占位符，也叫做格式修飾符，他們說明了插入數據的位置也說明了將插入數據的類型(d表示整數，f表示浮點數)以及如何進行格式化。 <br> Java SE5之后推出了格式化輸出功能，System.out.formart()可以用于PrintStream與PrintWriter對象。<br> 在java中所有新的格式化功能都是由java.util.Formatter類處理。 在java中如果需要控制輸出字符串長度、空格已經對齊方式需要使用格式修飾符，其語法如下： %[argument_index$][flags][width][.precision]conversion

內容包含：目的;數據和它們的屬性；創建數組；訪問數組元素；修改數組元素。

說起Java，人們首先想到的是Java編程語言，然而事實上，Java是一種技術，它由四方面組成:Java編程語言、Java類文件格式、Java虛擬機和Java應用程序接口(JavaAPI)。它們的關系如下圖所示：圖1??Java四個方面的關系運行期環境代表著Java平臺，開發人員編寫Java代碼(.java文件)，然后將之編譯成字節碼(.class文件)。最后字節碼被裝入內存，一旦字節碼進入虛擬機，它就會被解釋器解釋執行，或者是被即時代碼發生器有選擇的轉換成機器碼執行。從上圖也可以看出Java平臺由Java虛擬機和Java應用程序接口搭建，Java語言則是進入這個平臺的通道，用Java語言編寫并編譯的程序可以運行在這個平臺上。

第2講Java編程基礎Java的基本數據類型(重點)Java的關鍵字、標識符、常量、變量Java的運算符、表達式、語句Java程序的流程控制Java程序的基本結構及常用輸入輸出格式(重點)Java數組的聲明、創建和使用(重點)教學目標熟悉Java的基本數據類型；熟悉Java的關鍵字、標識符的命名規則，掌握常量、變量的使用方法；熟悉Java的運算符，理解表達式、語句的構成；掌握Java程序的流程控制；熟悉Java程序的基本結構及常用輸入輸出格式；掌握Java數組的聲明、創建和使用。

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This book is a valuable reference for learning about regular expressions in Java, and focuses on the use of regular expressions in the Java language.

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單例對象（Singleton）是一種常用的設計模式。在Java應用中，單例對象能保證在一個JVM中，該對象只有一個實例存在。正是由于這個特點，單例對象通常作為程序中的存放配置信息的載體，因為它能保證其他對象讀到一致的信息。例如在某個服務器程序中，該服務器的配置信息可能存放在數據庫或文件中，這些配置數據由某個單例對象統一讀取，服務進程中的其他對象要獲取這些配置信息，只需訪問該單例對象即可。

JSTL標簽庫的使用是為類彌補html表的不足，規范自定義標簽的使用而誕生的。在告別modle1模式開發應用程序后，人們開始注重軟件的分層設計，不希望在jsp頁面中出現java邏輯代碼，同時也由于自定義標簽的開發難度較大和不利于技術標準化產生了自定義標簽庫。

一、JSP EL語言定義EL（Expression Language）目的：為了使JSP寫起來更加簡單。表達式語言的靈感來自于ECMAScript和XPath表達式語言，它提供了在JSP中簡化表達式的方法。它是一種簡單的語言，基于可用的命名空間（PageContext屬性）、嵌套屬性和對集合、操作符（算術型、關系型和邏輯型）的訪問符、映射到Java類中靜態方法的可擴展函數以及一組隱式對象。EL提供了在JSP腳本編制元素范圍外使用運行時表達式的功能。腳本編制元素是指頁面中能夠用于在JSP文件中嵌入Java代碼的元素。它們通常用于對象操作以及執行那些影響所生成內容的計算。

EL表達式規定為：eval-expression和 literal-expression,同時EL表達式支持Compositeexpressions,很多EL表達式（eval-expressions和 literal-expressions）被聚合在一起。 EL表達式被解析成數值表達式和方法表達式。其中，取值表達式去引用一個值，而方法表達式則是關聯一個方法。一旦被解析之后，表達式能夠被優化地計算一次或多次。

主要內容TCP協議Socket編程1UDP協議Socket編程2Socket編程實例3JavaMail郵件編程4Socket通訊網絡上的兩個程序通過一個雙向的通訊連接實現數據的交換，這個雙向鏈路的一端稱為一個Socket。Socket通常用來實現客戶方和服務方的連接。兩類傳輸協議TCP(Transport Control Protocol)面向連接的能夠提供可靠的流式數據傳輸的協議。類似于打電話的過程。URL,URLConnection,Socket,ServerSocket等類都使用TCP協議進行網絡通訊。UDP(UserDatagramProtocol)非面向連接的提供不可靠的數據包式的數據傳輸的協議。

類概念現實世界：實體=屬性+動作計算機世界：數據（信息）：持久化編程世界：類是描述具有相同屬性與動作的實體