Objectives

1. Learn about the syntax and semantics of the HelloWorld program.

Notes

• Apologies
  • This is going to take a while.
  • And is going to include a bunch of terms.
    • I expect you to eventually learn these terms.
    • But not on the first pass.
  • And has a bunch of new ideas
    • We will come back and examine most of these later.
• Our program

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

• Comments
  • Comments are elements you can add to your program that have no impact on the program!
  • They are notes to those reading the program.
  • Syntactically we can create comments in three different ways.
    1. Single line comments
       • Start with //
       • Go to the end of the current line.
       • But are only one line long
       • They can start anywhere
    2. Multi line comments
       • start with /*
       • End with */
       • Can not contain */
    3. Javadoc comments
       • start with /**
       • End with */
       • Used by a program called javadoc to create documentation.
       • We will probably not use these.
  • I am going to add comments as I write hello world to explain what I am doing.
  • We will revisit comments in the future
  By the way I searched for javapoint java comments to find a reference.
• class HelloWorld {
  • Java is an object oriented programming languages
    • Therefore everything needs to be an object.
    • The word class declares an object.
    • class is a keyword or reserved word.
      • It has special meaning, IE it declares or defines an object.
  • Here is a list of Java reserved words
    • You will learn these eventually
    • You need this for the next step.
    • I found this by searching javapoint java reserved words
  • HelloWorld is the class name.
    • It must not be a reserved word. (syntax)
    • It must start with a letter (syntax)
    • It Should start with an upper case letter (style)
    • It is something called an identifier.
    • Until further notice, you will only have one class and the class name must be the same as the file name (syntax, and getting started)
  • Identifiers
    • Are words "made up" by the programmer.
    • They are not reserved words. (syntax)
    • They can contain only: (syntax)
      • letters (A-Z, a-z)
      • numbers (0-0)
      • The underscore _
      • The dollar sign $
    • It can not start with a number (syntax)
    • It should be meaningful in the context (style)
  • I found these by searching javapoint identifiers
  • {
    • This along with the matching } mark the body or contents of the class.
  • You will always have this line
    • Only the class name will change.
• public static void main(String[] args) {
  • This declares a main method.
  • public, static and void are reserved words
    • public means that any class can access this method
    • static is outside the scope of our discussion right now, but we need it
    • void means that this method returns nothing. Don't worry about that.
  • main tells the program where to "start" or begin.
    • Every program needs a main (syntax)
    • Sort of like the "go" square in Monopoly. (semantics)
    • main is a standard identifier
      • We use standard because we must have it.
      • But it is not a reserved word.
  • The items inside the parenthesis () indicate the arguments to main.
    • In this case it is an array or collection of objects ([])
    • Of data typeString
    • Called args.
  • String is a Java class that supports a collection of letters
    • More on this later.
  • Finally the { starts the code block for the main routine.
  • Again, for this class you will ALWAYS have this line.
• System.out.println("Hello World!");
  • System is a Java class that supports interaction with the Operating system
  • System.out is a subclass that supports interaction with the output system
  • System.out.println() is a function that prints a line to the output.
  • "Hello World!" is called a string literal
    • string because it is a collection of letters.
      • Actually it is a collection of characters. (! and space are not letters
      • The " mark the contents and are not part of the string.
    • literal because it is literally in the program.
  • This line prints out the greeting.
  • It is actually the body of the main routine.
• }
  • This closes the code block for the main routine
  • Notice that indentation stops for main.
• }
  • This closes the code block for the HelloWolrd class
  • Notice the indentation for the class stops.
• So every program needs

  class ClassName {
      public static void main(String[] args) {
          // the code body
      }
  }