Java Enum Type

How do you represent enumerated constants in your java code? It probably will look something like this:
public static final int STATUS_ACTIVE = 0;
public static final int STATUS_PENDING = 1;
public static final int STATUS_CLOSED = 2;


This way of representation poses some problems. So in Java 5.0 Enum Type is introduced. Now you can write your code as,
public enum Status { ACTIVE, PENDING, CLOSED }
and refer to it by Status.ACTIVE, Status.PENDING and Status.CLOSED. The reserved word enum is used like class.


The links below are some discussions on what, how and when they should be used.

http://java.sun.com/j2se/1.5.0/docs/guide/language/enums.html
http://java.sun.com/docs/books/tutorial/java/javaOO/enum.html
http://www.ajaxonomy.com/2007/java/making-the-most-of-java-50-enum-tricks

Java nested classes

I usually get confused with nested classes so I'll note some important points here.

Nested classes
- is also a member of enclosing class like variables and methods
- can use all access modifiers like variables and methods

Types of nested classes
1) static
- like a static method, it is not associated to any instance of enclosing class and has access only to all the static members
- syntax: Outer.Inner i = new Outer.Inner();

2) non-static / inner
- has access to other members of enclosing class even if declared private
- like non-static methods, it cannot define any static members as static members only belong to the (outer) class
- syntax: Outer.Inner i = new Outer().new Inner();

3) local inner - declared and known only within the body of method/block

4) anonymous inner - no name and known only within the statement in w/c they are defined

Java modifiers summary

Modifier	Class	Interface	Inner Class	Inner Interface	Variable	Method	Constructor	Free-Floating Block
public	yes	yes	yes	yes	yes	yes	yes	no
protected	no	no	yes	yes	yes	yes	yes	no
none or package or default	yes	yes	yes	yes	yes	yes	yes	yes
private	no	no	yes	yes	yes	yes	yes	no
final	yes	no	yes	no	yes	yes	no	no
abstract	yes	yes/no	yes	yes/no	no	yes	no	no
static	no	no	yes	yes	yes	yes	no	yes
native	no	no	no	no	no	yes	no	no
transient	no	no	no	no	yes	no	no	no
volatile	no	no	no	no	yes	no	no	no
synchronized	no	no	no	no	no	yes	no	yes
strictfp	yes	yes	yes	yes	no	yes	yes	no

Points:

• all access modifiers can be used to classes and members except for protected and private which cannot be used to outer classes and interfaces
  


• free-floating block cannot have access modifiers. it can only use static or synchronized
  


• constructors can only use access modifiers and strictfp
  


• native is only for methods
  


• transient and volatile are only for variables. Transient indicates that it is not serializable. Volatile indicates that it can be modified simultaneously by many threads.
  


• synchronized can only be used on methods or free-floating blocks
  


• strictfp can't be used on variables or free-floating blocks
  
  Using strictfp ensures that you get the same result of floating-point expressions across multiple platforms. But it may also result to overflow or underflow hence the expression, "Write-Once-Get-Equally-Wrong-Results-Everywhere". If you don't use strictfp, JVM can calculate floating-point expressions however it want and thus could produce more accurate results.
  

Getting started with Java

1) download the Java EE Software Development Kit (SDK)
- download link: http://www.oracle.com/technetwork/java/javaee/downloads/index.html

What Java Do I Need?
You must have a copy of the JRE (Java Runtime Environment) on your system to run Java applications and applets. To develop Java applications and applets, you need the JDK (Java Development Kit), which includes the JRE.

What's the difference between J2SE and J2EE?
J2SE has access to all of the SE libraries. However, EE adds a set of libraries for dealing with enterprise applications such as Servlets, JSP and Enterprise
Javabeans.

2) install JDK
- Windows instructions/troubleshooting: http://java.sun.com/javase/6/webnotes/install/jdk/install-windows.html

Why shouldn't I install in "C:\Program Files\Java"?
Some apps (e.g. maven, plugins) uses your Java path without considering potential whitespace on the path causing "C:\Program Files\Java" to become "C:\Program" w/c leads to errors. So either use a path w/out whitespace or set you path to JAVA_HOME=C:\Progra~1\Java not JAVA_HOME=C:\Program Files\Java

3) update environment variables (NOT case-sensitive under Windows)
PATH
- defines the search paths for executable programs (with file extension of ".exe", ".bat" or ".com" for Windows systems) invoked from a command shell ("cmd.exe")
- allows the use of javac and java
- if not set, you need to specify the full path to the executable every time you run it, such as: C:\Program Files\Java\jdk1.6.0\bin\javac MyClass.java
- so add here the JDK binary (bin) directory (e.g., "c:\jdk1.6\bin")
- Note: The JDK binary directory should be listed before "c:\windows\system32" and "c:\windows" in the PATH. This is because some Windows systems provide their own Java runtime (which is often outdated) in these directories (try search for "java.exe" in your computer!).

CLASSPATH
- defines the directories and Java's jar-files for searching for the Java classes referenced in a Java program
- normally, no explicit CLASSPATH setting is required
- if not set, default is current working directory (since JDK 1.3)
- if set, include the current working directory '.'
- link: How Classes are found

JAVA_HOME
- needed for running Tomcat and many Java applications
- set here the JDK installation directory, e.g., "c:\jdk1.6

How to set environment variables in Mac/Unix
1. Set environment variables for your user in ~/.bash_profile (will affect bash shells only).
Create the file if it does not exist:

touch ~/.bash_profile

Open the file:

open ~/.bash_profile

Add environment variables in the file:

export JAVA_HOME=$(/usr/libexec/java_home)
export JRE_HOME=$(/usr/libexec/java_home)

2. Set temporary environment variables for the current bash shell. Just type the same command in 1. to the current bash shell.

Ref: http://www3.ntu.edu.sg/home/ehchua/programming/howto/environment_variables.html

Design Patterns

Definitions on this blog came from the book Head First Design Patterns.

• Strategy pattern - defines a family of algorithms, ancapsulate each one, and makes them interchangeable. Strategy lets the algorithm vary independently from clients that use it.



From http://www.ida.liu.se



• Observer pattern - defines a one-to-many dependency between objects so that when one object changes state, all its dependents are notified and updated automatically.
  - think Swing/GUI



From http://kur2003.if.itb.ac.id


Using Java's built-in Observer pattern (allows observer to pull data from observable)...


From http://kur2003.if.itb.ac.id




• Decorator pattern - attach additional responsibilities to an object dynamically. Decorators provide a flexible alternative to subclassing for extending functionality.
  - think Java I/O



From http://oreilly.com

Ref:
Head First Design Patterns by By Eric Freeman, Elisabeth Robson, Kathy Sierra, and Bert Bates

Web Application Basics

HTTP - Hypertext Transfer (or Transport) Protocol), is a connectionless protocol for communicating clients/browsers and web servers

TCP/IP - Transmission Control Protocol, allows communication between your application software. It is responsible for breaking data down into IP packets before they are sent, and for assembling the packets when they arrive.
-connection oriented
-used to make connections and exchange information to one another
IP - Internet Protocol, is responsible for sending the packets to the correct destination
- connectionless

URL -Uniform Resource Locator, is a full specification of a resource. It includes the protocol, host machine name (domain name or IP address), optional protocol number and resource location.
Three ranges of port numbers:

1. well-known ports - from 0 through 1023
   20 & 21: File Transfer Protocol (FTP)
   22: Secure Shell (SSH)
   23: Telnet remote login service
   25: Simple Mail Transfer Protocol (SMTP)
   53: Domain Name System (DNS) service
   80: Hypertext Transfer Protocol (HTTP) used in the World Wide Web
   110: Post Office Protocol (POP3)
   119: Network News Transfer Protocol (NNTP)
   143: Internet Message Access Protocol (IMAP)
   161: Simple Network Management Protocol (SNMP)
   194: Internet Relay Chat (IRC)
   443: HTTP Secure (HTTPS)
   465: SMTP Secure (SMTPS)
2. the registered ports - from 1024 through 49151
   - they can be registered to specific protocols by software corporations/companies or users
   - assigned by or registered to Internet Assigned Numbers Authority (IANA) (or by Internet Corporation for Assigned Names and Numbers (ICANN) before March 21, 2001[1])
3. dynamic or private ports - from 49152 through 65535
   - available for use by any application or just about anybody

HTML - is the principal language b/w the client and server that expresses content of webpages.

• DOM - is an interface to the browser and HTML/XML documents.


• DHTML
  Dynamic HTML is a term used to describe the combination of HTML, style sheets, and scripts that allow dynamic pages.


• Javascript - is the most common scripting language for browsers.


• Inversion of Control and Dependency Injection

Martin Fowler, who first coined Dependency Injection, considers it the same to Inversion of Control. He used Dependency Injection since he finds Inversion of Control too generic. However, other sources claim that Dependency Injection is just a form of Inversion of Control. With all the articles and blogs written about them, it is very easy to get confused. So I'll try to summarize their description here according to my own understanding and hopefully, it will be simple and easy to understand.

Inversion of Control is a general principle in which the flow of control is inverted, unlike the traditional sequential flow. It follows the "Hollywood principle" - "don't call us, we'll call you". Your objects don't call services directly. Instead, your objects expect to be called. It is the framework that manages the objects and services, and is aware of what to instantiate and invoke.

The main idea of Inversion of Control is to have none of your classes know or care how they get the objects they depend on.

Dependency Injection is a way of implementing Inversion of Control in which an external mechanism is supplied in order for your objects not to call services (other objects) directly and therefore not depend on them. The external mechanism is the one responsible in injecting the concrete implementation that your objects needed.


• Loose coupling - describes a relationship of two entities/objects where they can interact, but have very little knowledge of each other.

To r: inline css vs external css

What is robustness?

Disadvan of frames

Website-static

Webapp-dynamic

Get vs post- when to use get?

Session - uses cookies

Enabling technoligies:

Compiled modules-java servlets

Interpreted scripts-jsp

Other Pointers

• Weblogic Erroneous Handlers Error - this is caused by the jrockit version of jdk. To get the correct error message, use the Java jdk first. Source: http://ricksolutions.blogspot.hk/2009/03/fixing-erroneous-handlers-error.html
• checking tables on db2
select * from sysibm.systables where dbname='NACS01DB' and name like 'PC%'
• command for unzipping war files
jar xvf xxx.war
• Extracting the Contents a JAR File
  The basic command to use for extracting the contents of a JAR file is:
  

  jar xf jar-file [archived-file(s)] 

  Let's look at the options and arguments in this command:
  
  • The x option indicates that you want to extract files from the JAR archive.
  • The f options indicates that the JAR file from which files are to be extracted is specified on the command line, rather than through stdin.
  • The jar-file argument is the filename (or path and filename) of the JAR file from which to extract files.
  • archived-file(s) is an optional argument consisting of a space-delimited list of the files to be extracted from the archive. If this argument is not present, the Jar tool will extract all the files in the archive.
• character encoding - http://www.joelonsoftware.com/articles/Unicode.html
• DB2 Timestamp to java.util.Date or java.sql.Timestamp
  
  • DB2 format = yyyy-MM-dd HH:mm:ss.ZZZZZZ where Z = seconds since previos midnight (00000 - 86400), http://pic.dhe.ibm.com/infocenter/db2luw/v9r7/index.jsp?topic=%2Fcom.ibm.db2.luw.sql.ref.doc%2Fdoc%2Fr0007107.html
  • java.sql.Timestamp = yyyy-mm-dd hh:mm:ss.fffffffff where f = nanoseconds
  The ZZZZZZ part of DB2 timestamp is insignificant because it is just the total seconds since previos midnight.
  To convert to java.util.Date, use
  

  new SimpleDateFormat("yyyy-MM-dd HH:mm:ss").parse(dateStr)

  To convert to java.sql.Timestamp, use
  

  resultSet.getTimestamp([column name or column index])

• 
  

Ref:
http://technobuz.com/2011/04/run-commands-for-microsoft-applications/