| Class are declaration template for a potential objectClasses are instantiate to become an object | |||
| Class Declaration | keyword class Myclass [extends Superclass] [implements Interface]{ class variablesdeclarationsstatic initializeinstance variablesconstructorsclass methodsmethodsinstance methods} |
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| A class signature:public abstract class MyClass extends BaseClass implements MyInterface, OtherInterface | |||
| class name | Can start with any alphanumeric character including $ and _ | ||
| Exam Tip | class 199 {} in file 199.java will compileclass $1 {} will compileclass! {} will compile | ||
| Can contain numbers after 1st char. | |||
| Source Files | source file: *.java contains class declaration.byte code file: *.class is what the compiler generates. | ||
| compiler: javacJVM: java | |||
| Only one public class or interface per file. Must be same name as file. | |||
| Exam Tip | Cannot have both public class and interface in a file. | ||
| Identifiers Variables | Case sensitive language | ||
| Must start with letters, $ or _ | |||
| Can contain numbers after 1st char. | |||
| From the unicode character set | |||
| Comments | Java comments are:// – Single line/* */ – blocked | ||
| Declarations | Introduce class interface, methods, package or variables. | ||
| Declaration order:package (optional)import optional)class declarationvarmethods | |||
| package | Organize and group classes together | ||
| Visibility | Similar function and services are placed in a package for organization | ||
| Specify access restriction to variable and methods. Hence visibility.import other classes into current code. | |||
| No package declaration is (default package) | |||
| Package java.lang.* is imported automatically. | |||
| Only one package statement can be in file. | |||
| Makes it easier to use common names | |||
| Package reflect directory structure | |||
| classpath define root directorysubclasses begin off of classpath | |||
| package com.sun.java define a subdirectory from the classpath as:../com/sun/java | |||
| Ex.Classpath=c:mypackageSource as:package trades.quotes; for file Trade.javaplace code in c:mypackagetradesquotesTo run file the entire package name must be used:java trades.quotes.Trade | |||
| java -d | The “javac -d” can compile classes and generate the directory structure from the package statement. | ||
| Exam Tip | Note that the Trade.java file must be in the tradesquotes directory.As specified by package description: package trades.quotes; | ||
| Java API Packages | java.appletjava.awtjava.awt.datatransferjava.awt.eventjava.awt.image
java.awt.peer java.beans java.io java.lang java.lang.reflect java.net java.sql java.text java.util java.util.zip |
Applet class, sound, browser interactionGUI componentClipboardGUI event handlersHandling Images
Platform specific component Creates reusable software components. IO classes and interfaces, Sockets, Files and Streams Default standard classes and interfaces for Java programs Dynamic discovery of class components Communication via Internet/Intranet SQL and database Manipulation of numbers, dates, characters and string Manipulation of: dates, time, random number.. JAR files |
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| java.rmijava.rmi.dgc | Remote Method Invocation (Distributed Programming) | ||
| java.security | Security package | ||
| import | All public classes in our classpath is accessible to our program. | ||
| To use a class in package other than the one in which it is defined, you must first import that class. | |||
| By using the import keyword we don’t have to provide the fully qualified name for each class. | |||
| We can import entire package:import java.util.*; | |||
| We can import an individual class:import java.util.Date; | |||
| java.lang.* is imported automatically. | |||
| Exam Tip | import java.*;Will compile, but will try to import classes within java directory. Not the java API’s. | ||
| If import is not used, the full name of the class must be used:java.applet.AudioClip; | |||
| Keywords and Reserved Words | There are 48 java keywords to be memorized. Pg 21 of Java 2 Certification book. | ||
| Exam Tip | const goto are keywords not used Java yet.final, finally are keywords.null, true, false are reserved words but not keywords.(??I don’t know. Some say these are literal values, some say not??) | ||
| Applet | Java programs that run in a browser. Used on the Web. | ||
| An applet must extend the java.awt.Applet class. | |||
| //Packages usually usedimport java.awt.Applet;import java.awt.Graphics;import java.awt.event.*;public class xxx extends Applet {……} | |||
| <applet code=”x.class” codebase=”.” width=232 height=232> </applet> | |||
| public void init() | Called once after applet is loaded and before anything as been shown on the screen. | ||
| public void start() | Called when applet is downloaded and becomes visible after been hidden. | ||
| public void paint(Graphics g) | Overridden method from the java.awt.Component class. Used to render (draw) the applet. | ||
| public void stop() | Called when the applet is hidden. | ||
| public void destroy() | Called by JVM when applet is about to be permanently disposed. | ||
| Java Programs | Run by calling main() method.Don’t have to extend any classes. | ||
| main() | Main method is used in java programs. | ||
| Exam Tip | Signature | public static void main(String arg[]) {} or,static public void main(String arg[]) {} | |
| arg | Arguments start at index 0. | ||
| Using main() | |||
| Exam Tip | java myprogram var1 var2var1 is argument 0.The myprogram is not var1. | ||
| Exam Tip | public void main(String arg[]) method will compile, but will not start a java program. | ||