'main'

A Minimal Java program

.. looks like this:

public class MyFirstProgram
{
    public static void main(String[] args)
    {
        System.out.println("Hello!");
        int sum = 2 + 2;
        System.out.println("The sum is " + sum);
        System.out.println("OK, I'm done. Bye!");
    }
}

Classes

Every Java program is built from classes. A class combines variables to hold data and methods which contain code. A class is defined like this:

public class NameOfTheClass
{
    // The content of the class
}

Class names usually start Uppercase, and may use CamelCaseNotation. Most classes are public, meaning other code can see them. In special cases, you create private classes, we'll get to that later.

Methods

Inside a class, you can define methods like this:

    public void doSomething()
    {
        System.out.println("I'm doing something");
    }

This method returns nothing ,void. Method names usually start lowercase(), and may_use_underscores() or switchToCamelCase() for longer names.
Methods end in parenthesis for their arguments. Empty parenthesis '()' declare that a this method takes no arguments. When you invoke it via doSomething(), it will print some text.

This method takes one argument, you would invoke it with a String like hello("Fred")

    public void hello(String name)
    {
        System.out.println("Nice to meet you, " + name);
    }

This method takes two numeric arguments, and returns a number:

    public int doMath(int a, int b)
    {
        int c = 2 * b;
        return a - 5*c;
    }

A _method _that takes arguments and returns a value is sometimes also called a function. You could use that as follows:

   int result = doMath(47, 123);
   System.out.println("The result is " + result);

A Java program starts by running the code in a special main method which must have this exact signature:

   public static void main(String[] args)
   {
   }