CSCI 2310: Complete Java Programs

• Contents • Test •

The main method

A Java program contains a method called main that is run automatically when the program starts. The main method is similar to the Execute part of a Cinnameg program.

The heading for main is required to be as follows.

  public static void main(String[] args)

The argument args is an array of strings. We will encounter arrays a little later. For now, you can safely ignore args. Just be sure to write the heading exactly as shown.

The form of a program

To write a program, choose a name for the program. Let's say you have chosen to call the program Rabbit. Create a file called Rabbit.java. In that file, write the program in the following form. (We will discuss imports later.)

  put imports here

  public class Rabbit
  {
    public static void main(String[] args)
    {
      ... say what main does
    }

    ... any additional method definitions
  }

It is important that the name that you write in the program matches the name of the file. If you decide to call the program Lizard, then it must be in a file called Lizard.java. The case of letters matters. It will not work to put a program called Lizard in a file called lizard.java.

Java does not care how you order your method definitions. It allows a method to use other methods that are defined after it in the program.

For example, here is a complete java program that displays a table of factorials. It would be placed in a file called Factorial.java.

  public class Factorial
  {
    public static void main(String[] args)
    {
      long n = 1;
      while(n <= 10)
      {
        System.out.println(n + "! = " + factorial(n));
        n++;
      }
    }

    public static long factorial(long n)
    {
      if(n == 1) return 1;
      else return n * factorial(n-1);
    }
  }

Compiling and running a Java program

To compile and run a Java program called Rabbit.java directly (not using these web pages) on Linux, do the following steps.

Compile the program using command
```
  javac Rabbit.java
```
If there are errors, you will be told. If not the compiler will be silent, and will create a file called Rabbit.class that is the translated version of your program.
After the program compiles successfully, run it using command
```
  java Rabbit
```
Notice that you do not write Rabbit.java, just Rabbit.

If your program is called Frog, replace Rabbit by Frog.

Summary

A Java program starts with imports (if you need them) and then defines a class. Write the methods inside the class.

One of the methods is called main. It must have heading

  public static void main(String[] args)

Do not try to write one method inside another method. Write the methods side by side, inside the class.