Introduction

Java is a class-based programming language. It is an interpreted language. It compiles to bytecode instead of machine language. Applications are portable between platforms without recompiling. Java apps are compiled to bytecode and run on a Java Virtual Machine (JVM).

Main article: http://introcs.cs.princeton.edu/java/11hello/

Terminology

java: Java’s runtime is named java
javac: The compiler for Java
javadoc: Java’s documentation builder
jar: Java’s archive builder
Package: Package in Java is a mechanism to encapsulate a group of classes, sub packages and interfaces. Packages are used for: Preventing naming conflicts. A Java package organizes Java classes into namespaces, providing a unique namespace for each type it contains.
Class: A distinct unit of programming. In Java, all code is defined in classes. Create applications by combining classes together. Enables you to create your code in small chunks, which makes it easy to debug.
```
  public class Main {
      public static void main(String[] args){
          System.out.println("Hello from Java!");
      }
  }
```
Interface: A contract. It defines a set of methods with specific signatures. Any class that implements this interface must implement those defined methods. It’s a guarantee that those methods and other members of the class will be present.
```
  public interface Product {
      String getType();
      String getSize();
      String getPrice();
  }
```
Method: Reusable code blocks. These are code blocks you can call from multiple places in an application. Methods are always declared as members of a class.
```
  public static void main(String[] args){
      for (int i = 0; i < months.length; i++){
          System.out.println(months[i]);
      }
```

Runtime architecture:

  +-------------------------------------+
  | Custom-compiled bytecode            |
  +-------------------------------------+
  | Core runtime and addition libraries |
  +-------------------------------------+
  | Java Virtual Machine (JVM)          |
  +-------------------------------------+
  | Operating system                    |
  +-------------------------------------+

Class with Method Example

A method is a reusable code block. It can be called from multiple places in an application.

In this example, the method is named main. It is called automatically as the class Main2 is started. This method loops through a list of months and displays them individually.

public class Main2 {

	private static String[] months =
		{"January", "Febrary", "March"};

	public static void main(String[] args){
		for (int i = 0; i < months.length; i++){
			System.out.println(months[i]);
		}
	}
}

Do this to run the code:

Compile Main2.java by using the Java compiler:
```
 $ javac Main2.java
```
After completion, a file named Main2.class is created.
Run the Main2 program:
```
 $ java Main2
 January
 Febrary
 March
```

Hello World

Create a Java program:

 vim HelloWorld.java

and put this in it:

 public class HelloWorld {

     public static void main(String[] args) {
         System.out.println("Hello, World");
     }

 }

Use javac to compile the Java program:
```
 javac HelloWorld.java
```
A class file (the computer language version) will be created:
```
 HelloWorld.class
```
Execute the program:
```
 $ java HelloWorld
 Hello, World
```

Installation

Do this:

$ pkg install openjdk8

The OpenJDK project provides a native open-source implementation of the Java® SE Platfo.

This OpenJDK implementation requires fdescfs(5) mounted on /dev/fd and procfs(5) mounted on /proc. Add the following lines in /etc/fstab:

fdesc	/dev/fd		fdescfs		rw	0	0
proc	/proc		procfs		rw	0	0

Is Java installed?

$ which java
/usr/bin/java

What version?

$ java -version
openjdk version "17.0.5" 2022-10-18 LTS
OpenJDK Runtime Environment Microsoft-6841604 (build 17.0.5+8-LTS)
OpenJDK 64-Bit Server VM Microsoft-6841604 (build 17.0.5+8-LTS, mixed mode, sharing)

JAVA_HOME

Many Java applications must know the location of a $JAVA_HOME directory. If not, you’ll run into a JDK version mis-match problem like this:

ERROR: JAVA_HOME is set to an invalid directory: /Library/Java/JavaVirtualMachines/jdk1.8.0_282-msft.jdk/Contents/Home

Please set the JAVA_HOME variable in your environment to match the
location of your Java installation.

Use echo $JAVA_HOME to display the current path:

$ echo $JAVA_HOME
/Library/Java/JavaVirtualMachines/jdk17.0.5-msft.jdk/Contents/Home

Use brewin to install the new JDK version:

$ brewin engtools install lnkd-java-8u282-msft
[in] => Please enter your password if prompted. 
Password:
[in] => Xcode Command Line Tools found.
[in] => Checking /System/Volumes/Data/export
[in] => Checking for missing or outdated artifacts
     |=> Artifacts installed: 674
     |=> All installed artifacts up to date.
[in] => 
 [in] => Progress: [########################################################################################################] 1/1 

Confirm it’s installed:

$ ls /Library/Java/JavaVirtualMachines
jdk1.8.0_121.jdk	jdk1.8.0_282-msft.jdk	jdk11.0.13.8-msft.jdk	jdk17.0.5-msft.jdk

Set the new path for JAVA_HOME:

$ export JAVA_HOME=/Library/Java/JavaVirtualMachines/jdk1.8.0_282-msft.jdk/Contents/Home

Echo the path to test it:

$ echo $JAVA_HOME
/Library/Java/JavaVirtualMachines/jdk1.8.0_282-msft.jdk/Contents/Home

The new Java version is available:

$ java -version
openjdk version "1.8.0_282"
OpenJDK Runtime Environment (Microsoft)(build 1.8.0_282-b08)
OpenJDK 64-Bit Server VM (Microsoft)(build 25.282-b08, mixed mode)
gmcmilla@gmcmilla-mn1 ~/eng-bootcamp/training-mp-gmcmilla-1912> $ 

APIs

Basics

For methods, it’s common to call them get when getting a variable and set when setting a variable.

Networking library. Java has built in.

API Education

How To Design A Good API and Why it Matters, by Joshua Bloch. See https://www.youtube.com/watch?v=aAb7hSCtvGw

https://www.linkedin.com/learning/learning-java-by-example/what-is-an-api?autoplay=true&u=0

Next Learning

https://www.linkedin.com/learning/java-essential-training-syntax-and-structure/welcome?u=104

http://www.java-made-easy.com/java-for-beginners.html http://javabeginnerstutorial.com/core-java/