Java

Cool: http://www.ibm.com/developerworks/library/j-robots/index.html

Getting Started

Download and install Java, if you need to.

Hello World

This will walk through compiling and running a "Hello world" example in Java.

The program

Here's a simple Java "Hello world" program:

public class HelloWorld {
    public static void main(String[] args) { 
        System.out.println("Hello world!");
    }
}

Compiling

To compile this program, use the javac command:

$ javac HelloWorld.java

This will create a HelloWorld.class file, which is the Java executable.

Running

To run a Java executable, use the java command, and pass it the executable's filename without the extension:

$ java HelloWorld
Hello, world!

Measuring Performance of Java Code

See Java/Profiling and Java/Timing

Easily Reading from Input Files

Like many components of Java, it's easy to get tripped up with the simple stuff, since there are many ways to do it. Here's the canonical way:

Scanner s = new Scanner(new BufferedReader(new InputStreamReader(System.in)));

This allows you to pass input file contents on the command line like so:

$ java MyProgram < data.txt

See Java/Input Files

Using Jar Files

If you have a jar file that contains class definitions and you wish to use it, you can do the following:

• To check contents of the jar, use the jar utility - it works a lot like tar
• jar xf - expands (shows contents of) jar

Compile

To compile code mysource.java with a jar file called org.example.jar:

On Linux/Mac:

javac -cp '.:org.example.jar' mysource.java

On Windows:

javac -cp .;org.example.jar mysource.java

After this, you obtain the bytecode file mysource.class

Run

Now run the bytecode file:

java -cp '.:org.example.jar' mysource

java -cp .;org.example.jar mysource

Libraries

awesome-java repo on github: https://github.com/akullpp/awesome-java

Immutables in Java (similar notation to javascript/d3): https://immutables.github.io/

libgdx for 3d games in Java: https://libgdx.badlogicgames.com/

• 3d graphics: https://github.com/libgdx/libgdx/wiki/3D-Graphics

stanford core NLP in java: https://stanfordnlp.github.io/CoreNLP/

legion of the bouncy castle (lightweight crypto in Java): https://www.bouncycastle.org/java.html

Selenium web browser test suite: http://docs.seleniumhq.org/

jsoup crawler and html parser https://jsoup.org/

twitter4j twitter client: http://twitter4j.org/en/index.html

crawler4j another crawler and html parser https://github.com/yasserg/crawler4j

Apache Shiro for session management (large enterprise or small mobile) https://shiro.apache.org/

Assorted Basic Features

File Reading/Writing

There's a particular way to allow for easy reading and writing to a file so that you can say:

$ java MyProgram < in.txt > out.txt

Reading input from file

To read from an input file:

Scanner s = new Scanner(new BufferedReader(new InputStreamReader(System.in)));

Exceptions

Common Java Exceptions

First, if you want to compile your own list of Java exceptions, each package in the Java standard API will list a package summary on its package summary page. This has an "Exceptions summary" at the bottom that gives a brief summary of all the exceptions defined in a Java package.

For java.io.* the page is here: http://docs.oracle.com/javase/7/docs/api/java/io/package-summary.html

That being said, here are the most common:

Method parameters:

• IllegalArgumentException

Arrays:

• ArrayIndexOutOfBoundsException
• NullPointerException

Collections:

• NullPointerException
• ClassCastException (if not using diamond notation <>)

Type casting:

• ClassCastException

File IO (java.io package):

• IOException
• FileNotFoundException
• EOFException
• ParseException

Everybody:

• NullPointerException
• IllegalArgumentException (UNCHECKED EXCEPTION)

Serialization:

• java.io.ObjectStreamException

Threads:

• InterruptedException
• SecurityException
• IllegalThreadStateException

Assorted Advanced Features

Generics

Generics involve designing a data structure or class that takes an object of arbitrary type.