A simple program

We'll jump right into a simple program:

//file Hello.java
public class Hello {
   public static void main(String[] args) {
       String name = args[0];
       int age = Integer.parseInt(args[1]);
       if(args.length != 2) {
           System.out.println("Usage: Hello name age");
           System.exit(1);
       }
       System.out.println("Hello " + name + ". You are "
                           + age + " years old.");
   }
}

Compiling this program involves

   javac Hello.java

which creates a class file, and then

  java Hello Harry 44

runs the executable. There are a few things worth noting about this example:

• The program is wrapped inside of a class definition;
• A new type of basic variable - the String (an array of characters) - is available, with the command-line arguments captured as an array of strings;
• Printing out to standard output involves the System.out.println command, which prints a newline character at the end, with individual components to be printed joined with the + character;
• The Integer.parseInt(string) command can be used to convert a string to an integer;
• Generally, variables in Java can be declared when introduced, as compared to C/C++, which generally likes all variables declared near the beginning of a routine.

Note also the use of args.length to check for the length of the args array. This reminds us of an object.method syntax call to invoke a method, and indeed that is the case here - as one gets deeper into Java, one finds that many fundamental entities one works with have an underlying class structure.