Our First Java Program

The Hello World Program

The classic first program a programming writes in any new programming language is known as the Hello World Program. Basically, it is a short program where by the words “Hello World!” are printed on the screen. We will use this program to understand the basic anatomy of a Java class.

HelloWorld.java

public class HelloWorld

{
     public static void main(String[] void)
     {
          // Display a greeting in the console window
          System.out.println(”Hello World!”);

     }
}

Output: Hello World!

Classes

The Puzzle As A Whole

In Object Oriented Programming (OOP), a class is a file that encompasses the many attributes and capabilities of a certain entity. For example, each class has variables and methods.

Let’s take a look at our code…

public class HelloWorld
{
     …

}

  • The keyword class must be included between the access specifer and the class name.
  • Access Specifiers
    • Classes that are public can be accessed from any other class
    • Classes that are protected can only be accessed from other classses in the same package
    • Classes that are private cannot be accessed from anywhere. This is usually pointless, unless you are using inner classes. More on this during later sessions.
  • Nomenclature
    • Must begin with a letter (A-Z)
    • Cannot contain symbols, except underscore “_”
    • Java language is case-sensitive, ie. helloWorld is not the same as HelloWorld
    • Cannot be the same as special keywords reserved in Java
    • Usually, camel notation is used for class names, ie. HelloWorld

Objects

Different Interpretations of the Same Puzzle

In Object Oriented Programming (OOP), data is stored and carried out by objects. Objects are instances of a specific Class. Objects are created and instantiated by the new keyword new.

Let’s take a look at our code…

HelloWorld myProgram = new HelloWorld();

  • Objects contain the same properties as their parent classes
  • Multiple objects can be created from a single class, each with different information stored within
  • Objects are created using the keyword new
  • Object values are stored as variables, and follow the same nomenclature
  • Note: Objects are stored in memory; all objects created while a program is running are destroyed when the program halts

Variables/Fields

The Attributes of Every Java Program

A variable is a structure that is assigned to hold data and is accounted for by a unique name. In Java, fields are variables that are created outside of methods, at the beginning of the class.

Let’s take a look at our code…

public class HelloWorld
{
     private String myString = “Value”;
}

  • Only fields have access specifiers; variables don’t
    • Fields that are public can be accessed from any other class
    • Fields that are protected can only be accessed from other classses in the same package
    • Fields that are private can only be accessed from within their own class.
  • Nomenclature
    • Must begin with a letter (A-Z)
    • Cannot contain symbols, except underscore “_”
    • Java language is case-sensitive, ie. myWorld is not the same as MyString
    • Cannot be the same as special keywords reserved in Java
    • Usually, camel notation is used for variable names; however, with the first word in all lower-case, ie. myString
  • Primitive Types
    • boolean - true/false
    • int - an integer value ranging from -2,147,483,648 to 2,147,483,467
    • double - a decimal value ranging from NEG INF to POS INF
    • char - an integer representation of characters from the Unicode encoding system
    • String - a set of consecutive character values
  • Declaration vs Instantiation
    • Declare - a variable is declared by typing its type and variable name.
    • Instantiate - a variable is instantiated or assigned to a specific value, only when an assignment operator is used.

Methods

Actions Speak Louder Than Fields

In Object Oriented Programming (OOP), methods are used to access and manipulated data stored in variables. Methods are also known functions, routines, and procedures. The signature of a method contains four elements - access specifer, return type, method name, and parameters.

public class HelloWorld
{
     public void incrementCounterByValue(int increment)
     {
          newValue = oldValue + increment;

     }
     public int getNewValue()
     {
          return newValue;

     }
}

  • Mutators vs Accessors
    • Mutator methods are called upon to mutate or update values stored in that class as varibles.
    • Accessor methods are called upont to access certain variables and return their data.
  • Access Specifiers
    • Methods that are public can be accessed from any other class
    • Methods that are protected can only be accessed from other classses in the same package
    • Methods that are private can only be accessed from within their own class.
  • Nomenclature
    • Must begin with a letter (A-Z)
    • Cannot contain symbols, except underscore “_”
    • Java language is case-sensitive, ie. helloWorld is not the same as HelloWorld
    • Cannot be the same as special keywords reserved in Java
    • Usually, camel notation is used for method names; however, with the first word in all lower-case, ie. myMethod
  • Return Value/Statement
    • Every method must have a return value.
    • The return value of accessor methods is the Class Type of the value they return. ie, public String myMethod… if a String is returned
    • The return value of mutator methods that do not return any value is void

Main Method
The main method is a very important method in Java. Every program can have many classes working together; however, each program can have a maximum of one main method. This is because when a program is run, the Operating System calls the main method.

When trying to execute a Java program, you type:

java MyProgram

This signified that MyProgram is only class in this program with the main method.

  • public - the main method must always be public because it has to accessed by the OS, which is outside the boundaries of the class
  • static - static is a very confusing term for beginners and often skipped by professors in introductory courses. It basically means that this method will only exist in the class and not object of this class. More on this in later sessions.
  • void - void, of course, is a return type and signifies that this is a mutator method that does not return any values when called
  • main - main is simply the name of the method. Remember, Java is case-sensitive, so Main will not work
  • (String[] args) - args is the parameter of this method. String[] signifies the data type of the parameter. This method accepts an array of Strings, separated by a space, as the parameter.

Comments

The Invisible Instructions

Comments are very important in software development. Whether you’re programming in Java, PHP, C#, or BASIC, it is very important to write code that is readible not only by yourself, but by others you may work with. Comments
allow you to leave notations and notes regarding the structure of your program and any clarifications you may want to make.

Let’s take a look at our code…

public class HelloWorld
{
     // Display a greeting in the console window
}

  • There are two notations for comments:
  • Remember, any segment of code contained within a comment is not read by the compiler
  • // Comment
    • Any line that begins with // in considered a comment
  • /* Comment */
    • Any segment of code beginning with /* and ending with */ is considered a comment