Intro to Java

I am learning how to do Java via Udemy, so let’s get started. BTW, I use Mac OSX.

SET UP

1) Download the Java SE JDK latest version at Oracle.

• Accept License Agreement
• Double click to install
• Open a terminal shell and type java -version to see if it’s in.
• It should be in a path that looks like this /Library/Java/JavaVirtualMachines/
• open .bash_profile in terminal shell by open ~ .bash_profile
• add export JAVA_HOME='/usr/libexec/java_home' to .bash_profile, so JAVA_HOME is always set. (Note that ' ... ' between /usr/libexc… are actually back-ticks)

2) Download the Eclipse IDE for Java Developers from Eclipse.

• Click ‘Eclipse IDE for Java Developers’ to install
• Open finder, and double click on Eclipse application to fully install.
• Move it into Applications from Downloads.

All SET and now the first exercise!

Java is an object oriented language (OOP). Objects in Java are called “classes”. Objects are instances of Classes. Classes are in other word templates or files.

The whole process looks like filename.java –> javac (compiler) –> .class file (byte codes) –> Outputs (Java Virtual Machines i.e. Window, Mac, and Linux). I think by now, everybody knows the first exercise is always “Hello World!”, yep we are doing that.

Create a file named Hello.java in a new folder exercises, and fill these codes inside then save it.

package exercises;

public class Hello {
  public static void main(String[] args) {
    System.out.print("Hello, ");           
    if (args.length == 0) {
      System.out.println("World!");
    } else {
      System.out.println(args[0] + "!");
    }
  }
}

package is pinpointing to a folder that holds the file, declares path
public lets know that anyone can access it
static means main is not needed to run it
void declares that this method doesn’t return a value
main is just a name of the method
System is a pre-defined class that Java provides us, has useful methods and variables
out a static variable for “output” within System
println is a method short for “print a line”

No worries, I don’t understand most of these yet.

The whole set of codes inside brackets after main is called a method. The parameter has two arguments String[] and args. I am not sure why there’re square brackets by String. It’s associated with array. Give me time, I am new to Java.

System.out.println obviously prints “Hello “ when it is being run, and there are conditional statements. if that says if args.length is 0 or in another word, empty, then it is “World!”, printing out “Hello World!” in complete. else operates if args isn’t empty, and that args is printed out instead of “World!” and then is added an exclamation to it.

Open terminal shell and follow step by step below:

• cd to the exercises directory
• ls to see what files are stored in the exercises directory, probably just a Hello.java file
• javac Hello.java compiling the file to .class file
• Type ls again, and should see Hello.class, meaning it’s working
• java Hello and should see “Hello World!” printed out
• java Hello Bob and then should see “Hello Bob!” printed out