Definition   A COMMAND is a program that performs an action (other than returning a value).

Definition   An OPERATOR is a program that returns a value.

EXAMPLES:  

In the context of the Circle class....

• area () is an operator
  
• shrink (_) is a command
  

In the context of the IO class....

• print (< int >) is a command
  
• read_int is an operator
  

The operator computes the real average of two integers.

double average (int x,  int y)     int sum = x + y;     double ave = sum / 2.0;     return ave;

 

Using the operator...  

double a = average (2,  5);

 

The Execution....  

average (2,  5)

1. parameters are ``passed.'' This means bindings are established from the ``formal parameters'' in the definition to the ``actual parameters'' in the application.
   • x -- 2
     
   • y -- 5
     
   
   
2. The body of the method is executed in the context of the parametric bindings.
   • sum -- 7
     
   • ave -- 3.5
     
   
   

PROBLEM  

Read two real numbers.   Interpret the first as the radius of a circle and the second as the side of a square.   Display the average value of the areas of the corresponding circle and square.  

?   Class -- AverageCS     {         static public void main (String args [])         {         double r = IO.read_double ();         double s = IO.read_double ();         double acs = averageCS (r,  s);         IO.println (``The average area is ``+         acs + ``square units.'');         {         static private double averageCS (double radius,         double side)         {         Circle c = new Circle (radius);         Square s = new Square (side);         double ave = (c.area () + s.area ()) / 2.0;         return ave;         }   }

 

Example of a command definition  

Command which takes one integer and displays both the predecessors and the successor.  

static private void printPS (int x)   {         IO.println (x - 1);         IO.println (x + 1);   }

 

Example use...  

static public void main (String args [])   {         printPS (5);         printPS (10);   }