Mar 10, 2020 Notes

IT 212 -- Mar 10, 2020

Review Questions

Explain what these classes or interfaces are from the java.sql module:
```
DriverManager  Connection  Statement  ResultSet
```
DriverManager: that manages the database driver that is used to connect to a database.
The static getConnection method creates a connection object that implements the Connection interface.
DriverManager.getConnection is an example of the Factory pattern.

Connection: an interface that specifies about 58 required methods for all objects that manage the connection between the database and Java.
The Connection method that we used in the JDBC Example is createStatement.

Statement: an interface that specifies about 54 required methods. The objects that implement this interface execute and manage SQL statements.
The Statement methods that we used in the JDBC Example were executeUpdate and executeQuery, which returns a ResultSet object.

ResultSet: an interface that specifies about 205 methods for navigating through the table resulting from an SQL query.
We used these ResultSet methods: next, getString, getInt, getFloat.
Look at the LoadTransactions and DisplayTransactions classes in the JDBC Example.

Multithreading

A thread is a concurrent, independent path of execution within a program.
By default, the Java Virtual Machine starts the main thread, which runs the main method of the startup class.
To improve usability, put a time consuming operation, such as downloading a large file, into a thread that is different than the main thread.
To start a thread, which is independent of the main thread:
1. Define a class A that implements the Runnable interface. The required method of Runnable is run.
  The run method contains the work that is to be done in a concurrent thread.
2. Define a thread that will execute the run method of the class A:
```
Thread t = new Thread(new A( ));
```
3. Start the thread:

The instance methods of the Thread class:

Method	Meaning
getName	Obtain thread’s name, if any
getPriority	Obtain thread’s priority
isAlive	Determine if a thread is still running
join	Wait for a thread to terminate
sleep	Suspend a thread for a period of time
start	Start a thread by calling its run method

Look at the Multithreading Example Project.
In the main thread, use this statement to cause the main thread to wait until the thread t terminates before continuing:
```
t.join( );
```

Some Java Related Technologies

Java Swing

Abstract Window Toolkit (AWT) 1995 by Sun. AWT is Java's platform-independent windowing, graphics, and user-interface widget toolkit.
Internet Foundation Classes (IFC) 1996. IFC was introduced by Netscape Communications Foundation as a Java graphics library.
Swing 1997. Sun and IFC announced their intention to merge IFC with other graphics libraries to form Java Foundation Classes, which were later renamed to Swing.
Some Java Alternatives to Swing
1. SWT (Standard Widgit Toolkit) 2003. Developed at IBM as an alternative to Swing. Currently maintained by Eclipse.
2. GWT (Google Web Toolkit) 2006. Open source set of tools for maintaining applications with a JavaScript front end and a Java back end.
3. Spring MVC 2003. Framework for developing web applications that uses inversion of control and dependency injection.
4. Java FX 2007 by Sun. Platform for creating rich Internet applications.
5. Vaadin 2009 by Vaadin Ltd. Open source web framework for creating web applications. The front end is developed with Java tools but rendered with GWT. The Finnish word "vaadin" has a double meaning. It means "female reindeer" and "I insist."
6. Tapestry 2004 by Apache. A web framework that emphasizes simplicity, ease of use, and developer productivity. Here are some deployment files from a Hello, World application.
See the SwingGui Example.

The Kotlin Language

Reference: Wikipedia, Kotlin (Programming Language), Accessed Mar 2, 2018.
https://en.wikipedia.org/wiki/Kotlin_(programming_language).
Kotlin was developed by a team of Jet Brains programmers based in St. Petersburg, Russia.
It was named after Kotlin Island, near St. Petersburg, as Java was named after the Indonesian island Java (... or was Java named after coffee ☺).
Kotlin is strongly typed, but uses duck typing when possible.
Kotlin is designed to
- be an industrial-strength language,
  run on the Java Virtual Machine,
  be fully interoperational with Java code,
  be a "better" language than Java.
Kotlin is primarly used for Android development. It ships with IntelliJ and Android Studio.
Look at the KotlinProj Example Project.

Class files and the JVM

Find the compiled class file Person.class from the Employee Hierarchy example in the classes2 examples file.
The class file contains binary byte code that executes on the Java Virtual Machine, JVM.
Use the disassembler to reverse engineer the class file:
```
> javap Person.class
> javap -c Person.class
```
Here are the the options for javap.
The JVM is responsable for loading .class files, checking them, and executing the opcodes that they contain.
The JVM also manages the garbage collector, which reclaims blocks of memory on the heap that is no longer being referenced.
Although the JVM was designed to execute compiled Java code, it also executes code from other JVM languages. Here are the best known examples:
- Groovy (Similar to Ruby) Clojure (Lisp dialect)
- Scala (functional language) JRuby JPython
- Rhino (implementation of JavaScript) Kotlin (see above)

JAR Files and Ant

A package is collection of related Java classes.
The class files in a package can be stored in compressed format as a JAR (Java Archive) file, which simplifies and speeds up deployment.

To create a JAR file:

> jar -cvf file.jar file1.class file2.class

A jar file is simply a compressed file containing .class files in zipfile format.
The JAR file format is used for other purposes:
WAR (Web Application Archive) Stores XML files, Java classes, JavaServer Pages and other objects for Web Applications.

RAR (Resource Adapter Archive) Store XML files, Java classes and other objects for J2EE Connector Architecture (JCA) applications.
Not to be confused with the RAR (Rochal Archive) compressed file format.

EAR (Enterprise Archive) Stores composite Java archives which combine XML files, Java classes and other objects including JAR, WAR and RAR Java archive files for Enterprise Applications.

SAR (Service Archive) Is similar to EAR. It provides a service.xml file and accompanying JAR files.

APK (Android Application Package) A variant of the Java archive format, which is used for Android applications.
Ant is software for automatically performing tasks related to compiling and manipulating Java source code and class files, including creating JAR files.
Ant files store their directions in XML format.
Example: Running Ant from IntelliJ
Ant files can also be created, edited, and run in IntelliJ. Try out this example:
1. Create a Java project named TestAnt.
2. Create a new file named build.xml:
  Select the project, then select main menu File >> New >> File. Enter build.xml as the filename.
3. Enter this text for build.xml. Ant build files are written in XML format.
```
<?xml version="1.0" encoding="UTF-8"?>

<project name="Hello World" default="Hello" basedir="."> 

    <property name="HelloText" value="Hello Message"/>
    <property name="WorldText" value="World Message"/>

    <target name="Hello">
        <echo>${HelloText}</echo>
    </target>

    <target name="World">
        <echo>${WorldText}</echo> 
    </target>

</project>
```
4. Save the build.xml file.
5. Select the Ant Build Window on the right.
6. Click the + Icon, which is the leftmost Ant Build Icon,
  select build.xml, and click OK to add this file to the list of Ant build files.
  You will see the build file targets Hello and World.
7. Click the Ant Build Run Button (green triangle) to run the build file using the default target (Hello). Check the Messages window on the bottom to see the results of the build (Ant build completed with one warning).
8. You can also run the targets individually or together by selecting the ones you want to run and clicking the Run Button.
Example: Running Ant from a Command Prompt Window
1. Download and install Apache Ant from ant.apache.org.
2. Unzip build.zip to obtain build.xml, and place it in a folder.
3. Follow the directions in the comments.
4. You can also run the build.xml file using IntelliJ.
Other build tools for Java that are modern alternatives to Ant:
Maven Gradle Leiningen
Android Studio uses Gradle build files.
Gradle files tend to be short because they are designed with the CoC principle (Convention over Configuration).
Directions for getting started with Gradle in an IntelliJ project:
https://www.jetbrains.com/help/idea/getting-started-with-gradle.html.
Build tools can help with version management and perform checks for code quality.
Jenkins is a tool for managing plugins.

Android Apps

Before December 2014, Android phone applications were developed using Eclipse.
After December 2014, they are developed using Android Studio, which is built on top of IntelliJ.
Android Studio is a free download. It comes with its own copy of Java.
Android applications are written in Java or Kotlin. Android uses Gradle instead of Ant to control software builds.
Directions for building a simple Android application.
Android Studio versions are named after sweets. Here is the Android Studio version history.
An Android Activity corresponds to a page of the app.
When Android launches an app, the default Activity is launched.
An Intent is a message sent to the operating system telling it to launch an Activity.
Activity and Intent, are Java classes.

Java Server Pages (JSP)

Java Server Pages (JSP) is a server-side technology similar to PHP, Ruby on Rails, and ASP.Net.
JSP uses Java as its server-side language.

Iterator Pattern

Remember that an iterator returns its items sequentially until it is out of items.
To create your own iterator you need to supply next and hasNext methods.
remove is an optional method. It removes an item from the underlying collection.
Look at the iterator module in the Patterns1 Example.