Java

Install openjdk

See http://openjdk.java.net/install/. On Ubuntu, I can use

sudo apt-get install openjdk-7-jdk

Some projects written in Java

• FastQC The code uses Java 2D graphics APIs in awt like BasicStroke, Color, Dimension, Graphics, Graphics2D, RenderingHints and javax.swing.JPanel. It also uses java.util.Vector.

Tutorial/Books

• Oracle Tutorial
• Head First Java
• Murach's Java Programming
• Interactive Java Tutorial
• Introduction to programming in Java
• Package, classpath
• codecademy

Compile a simple Java program

javac HelloWorldApp.java # generate Example.class; bytecode version of the program
java HelloWorldApp   # run bytecode in Java Virtual Machine

Get a hello world program from http://docs.oracle.com/javase/tutorial/getStarted/cupojava/unix.html.

Note that the file name can not be arbitrary. It should match with the class name. For the above example, if we rename <HelloWorldApp.java> to <example.java>, we will get an error when we run java example on the command line.

Another example that requires command line argument.

// Sqrt.java
public class Sqrt { 
    public static void main(String[] args) { 

        // read in the command-line argument
        double c = Double.parseDouble(args[0]);
        double epsilon = 1e-15;    // relative error tolerance
        double t = c;              // estimate of the square root of c

        // repeatedly apply Newton update step until desired precision is achieved
        while (Math.abs(t - c/t) > epsilon*t) {
            t = (c/t + t) / 2.0;
        }

        // print out the estimate of the square root of c
        System.out.println(t);
    }

}

$ javac Sqrt.java
$ java Sqrt 9
3.0
$ java Sqrt 9.5
3.0822070014844885

Set the Java version for a project

To change the Java version for a project if a newer version becomes available: right click on a project, then select properties

• Libraries - Java Platform - change the version from the drop-down list. I only see JDK 1.7.

If you want to use the new features of that version and you don't need the project to run under earlier versions of Java.

• Sources - Source/Binary format - change the version from the drop-down list. I can see a lot of choices.

Java Programming

Main method

Main method has to be declared within a class declaration.

public class InvoiceApp
{
  public static void main(String[] args)
  {
    System.out.println("Welcome to my app");
  }
}

Create objects of numerical variables, string, from a class

int scoreCounter = 1;
double unitPrice = 15.9;
String firstName = "Joe";
String lastName = "Wang";
String name = firstName = " " + lastName;
Scanner sc = new Scanner(System.in);
double subtotal = sc.nextDouble(); // get a double entry from the console
Date now = new Date();
String currentDate = now.toString(); // convert the date to a string

Common packages

• java.lang
• java.text
• java.util
• java.io
• java.sql
• java.applet
• java.awt
• java.awt.event
• javax.swing

Two ways to create an object from the Scanner class.

With an import statement

import java.util.Scanner;
Scanner sc = new Scanner(System.in);

Without an import statement

java.util.Scanner sc = new java.util.Scanner(System.in);

Console for input/output

Use the System.out object to print output to the console (println() is a method). Here, System.out refers to an instance of the PrintStream class. Because this object is created automatically by Java, you don't have to include code that creates it in your program.

System.out.println("Total: " + total);

Use the Scanner class to read input from the console

import java.util.Scanner
String name = sc.next();
int count = sc.nextInt();
double subtotal = sc.nextDouble();
String cityName = sc.nextLine();

Control statement

== 
!= 
equals(String)
equalsIgnoreCase(String)
if () { } else {}
while () { }

Swing

Hello World Example

• http://docs.oracle.com/javase/tutorial/uiswing/start/compile.html

For example, create a new subdirectory 'start' and put HelloWorldSwing.java there. Then we can build and run the swing program by

javac start/HelloWorldSwing.java # Or javac HelloWorldSwing.java if we are in start directory
java start.HelloWorldSwing

Quit Button Example

• http://zetcode.com/tutorials/javaswingtutorial/

Note that it is necessary to create the directory com/zetcode according to package statement in java code. Also the filename must be consistent with the class name.

mkdir com
mkdir com/zetcode
nano com/zetcode/QuitButtonExample.java
javac com/zetcode/QuitButtonExample.java
java com/zetcode/QuitButtonExample