When and when not using main method in Java

The basic thing to remember is that there is a big difference between programs and classes. Both start off the same:

public class ClassName { . . . }

but programs have a main() method, classes do not. You can compile and run programs. You can compile classes, but you can't run them.

PezDispenser is a class:

public class PezDispenser {
   public static final int MAX_PEZ = 12;

   private String mCharacterName; private int mPezCount;

   public PezDispenser(String characterName){
      mCharacterName = characterName; mPezCount = 0;
   }

   public boolean isEmpty(){
    return mPezCount == 0;
   }  
   . . . 
}

There is no main() method. It is not runnable.

Example is a program:

public class Example {

   public static void main(String[] args) {
       // Your amazing code goes here...
       System.out.println("We are making a new Pez Dispenser.");
       PezDispenser dispenser = new PezDispenser ("Yoda");
       . . . 
   }
}

You can run this program. In the main() method a PezDispenser object named dispenser is being created. This line of code would cause an error if the PezDispenser class wasn't available (in the same directory, ...).

In order to call a method that is in a class you need to create an object or instance of the class first, and then call the method on the object.

PezDispenser p = new PezDispenser("...");
c.isEmpty();  //assumes PezDispenser has a isEmpty() method.

Here the general rule for calling methods is objectName.methodName

Some folk also made a distinction between programs, classes and modules, with the latter being classes with static methods and no main() method. An example of a "module", in this sense, is the Math class. The general rule for calling methods on modules is ClassName.methodName. E.g., Math.sqrt(24.0)

Hope this helps.