Java goes wild, lesson 1

Java goes wildLesson 1: Introduction to Java

Thierry Wasylczenko

After this course ...You will:

• Know what is the JDK/JRE

• Be able to set up your development environment

• Know how to compile and run a Java program

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Agenda• Bascis

• How does all of this work?

• Developer’s environment

3

BasicsSuccess is neither magical nor mysterious. Success is the natural

consequence of consistently applying the basic fundamentals.

Jim Rohn“4

Basics• Source code is inside files with .java extension

• The source code is compiled into .class files

• The class files are executed by a strange thing (covered later)

5

A Java file

public class Foo {

public static void main(String[] args) {

System.out.println("Hello World!");

}

}

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How do I get class files?• By compiling all Java files

$ cd /location/of/java/files

$ javac Foo.java

$ ls

Foo.class Foo.java

$

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How do I run it?• With the java command

$ java Foo

Hello World!

$

• Yes you DON’T put the .class extension when you run a class !

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Java Archive• aka JAR

• A collection of compiled Java classes

• Package of a Java program

• Can be runnable

java -jar Foo.jar

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That'sfreaking

AWESOME !

That’s awesome dude. But how

does all of this work?

You“11

How does all of this work?It is not the beauty of a building you should look at; it's the construction

of the foundation that will stand the test of time.

David Allan Coe“12

The source code• Plain text file

• File extension: .java

• Rules for naming each file (covered later)

13

The bytecode• Result of compilation

• Something in between the source code and the machine’s language

• Interpreted by the Java Virtual Machine

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The Java Virtual Machine• aka the JVM

• Kind of a black box

• Interprets the Java bytecode

• Is available on major platforms

• "Compile once, run everywhere"

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A little schemaSourcecode

Sourcecode

JVMByte code

JVMByte code

turns on

turns on

java

java

javac

javac

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Well man, you’re talking about

JVM, javac, bytecode and cool

stuff. But how do I set them up?

You“

17

Developer's environmentWhen people think about computer science, they imagine people with

pocket protectors and thick glasses who code all night.

Marissa Mayer“18

Java Development Kit• aka JDK

• Provides tools

• javac: java compiler

• javadoc: generates the documentation

• ...

• Includes a Java Runtime Environment

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Java Runtime Environment• aka JRE

• Provides tools:

• java: for running Java applications

• ...

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JAVA_HOME• An environment variable

• Points to the JDK installation

• Required by some tools such as:

• IDEs

• build tools (maven, gradle, ...)

• ...

• Used to update the PATH variable

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JAVA_HOME: example

# Unix

export JAVA_HOME="/path/to/jdk"

export PATH=$JAVA_HOME/bin:$PATH

REM Windows

set JAVA_HOME="C:\path\to\jdk"

set PATH=%JAVA_HOME%\bin;%PATH%

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Avoid spaces in the path of your

JAVA_HOME.

JAVA_HOME

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JAVA_HOME: test the setupTo test the setup you can execute the following command:

$ java -version

java version "1.8.0_11"

Java(TM) SE Runtime Environment (build 1.8.0_11-b12)

Java HotSpot(TM) 64-Bit Server VM (build 25.11-b03, mixed mode)

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What does JRE stand for?

• Java Runtime Embeded

• Java Routine Exclusive

• Java Runtime Environment

Start Stop

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What is the correct command to

execute a Java program?

• java Foo.java

• java Foo

• java Foo.class

Start Stop

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1. Set the JAVA_HOME in your

environment

2. Verify your setup

Practice #1

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Practice #2Given the following code:

public class Foo {

public static void main(String[] args) {

System.out.println("Hello World!");

}

}

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1. Copy it in your favorite editor

2. Save the file as Foo.java

3. Compile it

4. Run it

Practice #2

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Appendix

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Appendix• Java Code conventions

• Not maintained since 1997, may not be accurate

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